Scientific Questions
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Topic: Scientific Questions
Posted By: sidofmillenium
Subject: Scientific Questions
Date Posted: 04/12/2008 at 4:15am
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Lets make this thread a place for all the scientific phenomenons we don't understand about table tennis...let me start rolling the wheel. 1. How is spin converted into speed? When I topspin, you block it, the rubber reverses the spin and the ball comes back as topspin....then why does the ball speeds up as well?(what exactly goes on that makes this occur?)
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Replies:
Posted By: anixon
Date Posted: 04/12/2008 at 4:29am
I have also pondered this question for centuries... I think it has to do with elasticity. When we think of the ball going into the rubber we think of it pushing into the sponge and then firing back perpendicular to the racket face... that's the obvious way to think about it and that makes sense so we'd think it's just go forward at a speed relative to the elasticity of the sponge and topsheet. However, this does nothing to explain spin reversal. But now think of the effect of the top spin. The topspin pulls the topsheet and sponge DOWN the face of the racket, PARALLEL to the racket face. So it builds up potential energy in the direction UP the racket face. When that energy is released. It snaps the ball in the opposite direction imparting topspin. VOILA. I "Think" that's how it works. ------------- Blades: Xiom Stradivarius - Sriver EL 2.1 Andro Super Core Kinetic OFF+ - Plasma 380 2.0 Stiga Allround Evolution - Palio CJ8000 2.2 |
SIDOFMILENIUM wrote:Posted By: TAKOYAK1
Date Posted: 04/12/2008 at 4:32am
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Great explanation anixon. maybe you could be a professer in TT someday |
Posted By: Jolan
Date Posted: 04/12/2008 at 4:42am
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Where did you see the blocked ball comes back with reverse spin ? Unless you use anti, or frictionless LP, the ball gets what the blocker puts in it. If he does a classic block, ball will be a normal drive. Almost all the spin is absorbed by the blocker inverted rubber. That's why I've always recommended to poor blockers to use non tacky/grippy rubber (alla chinese) but euro/jap (sriver/mark V). The more grippy the blocker rubber is, the more difficult it is to "absorb" opponent's spin. The block motion requires an excellent technic and timing and it's not given to anybody to master it. Learning to block with a tacky/chinese rubber isn't a bad idea either. If the rooky can fix that, he'll be a super blocker. I'm sorry, Sid, I'm not a scientist. I couldn't give you equations to prove what I say, sole solid experience. ------------- Joola Wyzaryz Freeze Vega pro 2.0mm Vega intro 2.0mm Blade collection : https://photos.app.goo.gl/PrgCu5ib5RnhVXTn9" rel="nofollow - https://photos.app.goo.gl/PrgCu5ib5RnhVXTn9 |
Posted By: sidofmillenium
Date Posted: 04/12/2008 at 4:53am
| Still....nobody has answered the question...I get how the spin rebounds...I don't get how the ball's speed increases after the impact...since the spin has rebounded....so has the speed...where is that extra speed coming from? |
Posted By: anixon
Date Posted: 04/12/2008 at 4:57am
Yeah, usually when people say reverse spin they talk about opposite of what the attacker gives. So if I send you topspin, reverse spin would send me underspin (This would be achieved with anti etc). But when the OP said reverse spin, he means if I send topspin I get topspin back. It's just a miscommunication, but you can tell by his description what he's talking about. ------------- Blades: Xiom Stradivarius - Sriver EL 2.1 Andro Super Core Kinetic OFF+ - Plasma 380 2.0 Stiga Allround Evolution - Palio CJ8000 2.2 |
Posted By: anixon
Date Posted: 04/12/2008 at 5:00am
Your own arm speed/wrist speed. Almost no block is truly a passive block. Even over the table blocks your probably quickly shifting your wrist forward to adjust the racket angle to compensate for the topspin, this forward motion would impart energy to the ball. And all the more if you used your arm. The blade and rubbers are usually designed to amplify these effects, so little energy is lost. So the ball seems to travel faster. Again, this is my own speculation, but it seems logical to me. ------------- Blades: Xiom Stradivarius - Sriver EL 2.1 Andro Super Core Kinetic OFF+ - Plasma 380 2.0 Stiga Allround Evolution - Palio CJ8000 2.2 |
Posted By: LachlanTan
Date Posted: 04/12/2008 at 5:05am
| From my current knowledge, I think it is because the top spinning ball grabs the rubber topsheet and due to the friction and impact, the ball grips it and pushes off(no the rebound power but spinning power) it is same with return a hoop, if you do it right it will come back to you |
Posted By: Pjotr
Date Posted: 04/12/2008 at 8:04am
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I think that if the block would be faster than the topspin loop, with a truly passive block (if that exists), then the spin would be less. You can of course not get more energy out than you put in. But I have to agree with Anixon that probably you are moving the racket (maybe just a little bit) wich will increase the speed. The spin will be reversed due to the 'gripping' of the rubber.
btw this is not only the case with spin - block. You can put more backspin to a ball when you get a backspin ball from your opponent.
This 'phenomenon' (if you can call it that) is one of the reasons why attackers often misread the effect on balls, returned from a defence player.
Example: a defence player with long pimples and 'normal' inverted rubber.
One would guess that he can put more backspin, chopping with his normal sheet....well no. It all depends on what spin the attacker puts on the ball. If the attacker puts topspin then the inverted rubber of the defence player will not 'grip' the ball as much (while chopping) so the spin will not inverted that much (of course if he is further away from the table the topspin will be less, allowing him to put more backspin anyway....) . If he chops with the pimples (or anti) the attacker will get back his own topspin as backspin. What an interesting game tt is :) ------------- Borko ST FH & BH: Falco-Boosted Samba |
Posted By: stiltt
Date Posted: 04/12/2008 at 9:55am
You read people very well! very nice anixon! ------------- /forum/topic91512_page1.html#1124698" rel="nofollow - sales - forum_posts.asp?TID=19315" rel="nofollow - feedback |
Posted By: sidofmillenium
Date Posted: 04/12/2008 at 1:25pm
| But what happens if I add no speed at all? Are you saying then the topspin ball would not accelerate after hitting my paddle?(I disagree) Lets look at it this way, a soft sponge absorbs more speed and imparts more spin, now, therefore, the block return is not as fast but a hard sponge wouldn't react the same way....even though I get it...I still going to ask for clarification...why? |
Posted By: gulca
Date Posted: 04/12/2008 at 7:18pm
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A flat ball hitting a surface (the table) will incorporate spin on contact. A spinning ball hitting a surface will change it's rotational velocity into translational velocity through contact. The bounce of a TT ball is about 0.8 on a table. Does it bounce the same on a piece of fast rubber paste to the table? The difference between hitting the ball with a solid surface and a elastic surface is the deformation of the surface and the ball. This deformation gives a spring like force and is a function of the speed of contact of both materials. To answer the OP's question, we need to understand anti-spin. You would expect a top spin from you into a inverted rubber block comes back with under spin like an anti spin. It is true to some extend. If there is no elasticity in the block, you will get the spin reversal from a block. However, since the block itself is a impulse collision on a grippy surface, we get a lot of rotational velocity changed into translational energy. So the ball gained speed on bounce. The translational energy from your top spin upon the surface changes into rotational velocity. This gives a top spin on bounce back to you. The speed gained from the bounce on the table gives you topspin on the way back to you. So you see a topspin back to you on a block of your topspin. To give a anti-spin kind of block with a grippy rubber, you need to reduce the impulse on contact. This would reduce the translational speed gained from elasticity. That would reduce the topspin gained from the ball on the table. Hope this answer your question. |
Posted By: sidofmillenium
Date Posted: 04/12/2008 at 11:32pm
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Nice first post! Gulca
Thanks for clarification Anixon!
Now to the topic, telling me that spinning ball changes into speeding ball through contact is not helping....coz I know that through observation...I want to know how.
Here is my guess...tell me if I got it right, when a ball hits the rubber, the rubber deforms...and rebounds the ball(w/ little ball deformation as well...which results in additional speed....the more the spin, the greater the ball deforms) Now, when it hits the table...the ball deforms more b/c table doesn't absorb any impact...which is why it speeds up more!
Next question!
the more the sponge is pressed angularly...the greater the spin...since lighter the contact(more brush)....more we absorb the impact and leave a knuckle(dead) ball affect...then how is it possible to hit a really spinny slow shot...since more the sponge is pressed...the greater the speed as well!(I know what you are thinking....I used to think the same thing...the less the contact sound, higher the spin....but test it out yourself...greater the effort in spin(not just speed, but sponge pressing) more the spin!)
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Posted By: gulca
Date Posted: 04/13/2008 at 12:03am
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Ever seen how a ball robot work? It uses the rotational velocity to change the ball into translational velocity. That is how it spits out stationary balls. Now if you want to know the physics behind it, you would need to take some kinematic classes at a college. But to put things simply, it is the conservation of energy at play. A ball rotating will have inertia. Just like a moving object have mass. You need some force to change it's state following Newton's law. A rotating object touching a grippy surface will reduce it's rotational energy and this energy has to change into some other energies to conserve. These energies are translation and heat. Imagine a ball on a frictionless surface. You push it, it will translate and won't rotate. Imagine now we have friction. You push it, it will start to translate and with the grip to the surface, it creates a torque. This changes the translational force to rotational force. Same thing, if you put a spinning ball on a frictionless surface, it won't roll. It will spin in place. If you put it to a grippy surface, it will start to roll changing the rotational energy to translational energy. You are wrong. deformation and spin do not relate directly. Deformation gives you more speed after the bounce. Spin doesn't make it deform. A fast flat ball hitting the table does make it deform but that has little to do with the spin it gets. The translational force only changes to rotational force when the ball grips the table. The amount of grip depends on the surface area of contact. Deformation increases this surface area. You are right in deformation does increase the spin but not because of the reason you gave. |
Posted By: gulca
Date Posted: 04/13/2008 at 12:32am
Your question is not clear. Maximum amount of spin you can get out of a serve is related to how fast you can move your rubber speed. Assuming the rubber is infinitely grippy, you can generate in terms of rpm about 9000 rpm with a good swing. i'll get the equation later. |
Posted By: loop+loop
Date Posted: 04/13/2008 at 1:03am
| 9000 rpm? That's fast |
Posted By: gulca
Date Posted: 04/13/2008 at 1:18am
That's only theoritical rpm. The instantaneous velocity of a spinning ping pong ball at the surface set equal to the maximum speed of your swing comes to about that number. I think I can get roughly around 6m/s of arm swing speed. It might be faster but I don't have high speed video cam to capture my swing. Anyone with 60rpm camcorder might want to test this out. Many things lowers this number down. 1. not all swing speed changes to rotational speed of the ball on contact 2. grip is not infinite. 3. you are not hitting the perpendicular surface of the ball. 4. your contact point to the ball is not the optimum/max speed of your swing. 5. If it is underspin, the contact on the table twice before it reaches the opponent will lower the underspin. Top spin would spin more though. |
Posted By: tatan
Date Posted: 04/13/2008 at 2:26pm
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let me introduce TORQUE. when u put force on the ball the angle in which the contact was made and the direction of your arm determines the speed and and rotation count(spin) of the ball. when anything moves about some axis or base(door) it exerts torque(angular force).tt balls has a radius about 20mm and when it spins it exerts torque on anything which try to stop it in the direction of rotation. for grippy rubber: when we block topspin by using force or angle then the grip of the rubber also exerts torque on the ball in opposite direction of rotation of the ball(due to its grip which tries to stop the spin hence newtons third law).if our force is higher it goes back with the spin in opposite direction which means a topspin for my opponent too. a tacky rubber just catch the ball like adhesive.so easier to play with as u dont have to think abt the angle the applied force. lp,sp(due to surface area and less grip of material) and antispin (due to no grip and absorbing quality) just block the ball hence no spin riversal,just send the ball back with the same spin. u should understand that if a topspin is coming at your way and u send it back with same spin it becomes an underspin for ur opponent. ------------- 1)ZJ fl(89 gm) fh T05 red max bh T05 fx black max 2)TBS fl(89 gm) fh T05 black max bh GKI hybrid gx red max |
Posted By: tatan
Date Posted: 04/13/2008 at 2:33pm
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since a topspin exerts torque in a up->down motion on our rubber hence our rubber exerts torque on the ball in down->up motion which makes the ball fly.this is why block on a heavy topspin often goes outside. ------------- 1)ZJ fl(89 gm) fh T05 red max bh T05 fx black max 2)TBS fl(89 gm) fh T05 black max bh GKI hybrid gx red max |
Posted By: stiltt
Date Posted: 04/13/2008 at 2:36pm
Hi Torque
------------- /forum/topic91512_page1.html#1124698" rel="nofollow - sales - forum_posts.asp?TID=19315" rel="nofollow - feedback |
Posted By: gulca
Date Posted: 04/13/2008 at 2:51pm
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Actually the reason why a block on a heavy topspin often goes outside is due to 2 things.ni 1. As I explained, rotational velocity is converted to translational velocity. The ball comes back much faster after the the bounce on the rubber. This conversion depends on the grippiness of the rubber. The grippier, the more the translational speed. Thus an anti-spin or lp won't give you as much speed. 2. The angle which the ball bounce back is directed upwards for a incoming top spin. The spinning ball is trying to roll upwards but since the contact time is minimal, the bounce direction changes upwards. This gives the ball a higher trajectory angle on it's way back to you. Add 1 and 2, you get a ball that goes off the table. |
Posted By: sidofmillenium
Date Posted: 04/15/2008 at 10:28am
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Thanks...next question!
The more the sponge is pressed...greater the spin and speed! Then how is it possible to create a very low speed....but very spinny ball?
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Posted By: stiltt
Date Posted: 04/15/2008 at 10:43am
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We get a slow and spinny loop when:
the ball barely touches the wood; it sinks into the sponge and it's the faster arm speed while the ball is into the sponge that creates higher spin Since the stroke is quasi tangential, the contact with the wood is minimal then the speed of the ball is on the slower side.
------------- /forum/topic91512_page1.html#1124698" rel="nofollow - sales - forum_posts.asp?TID=19315" rel="nofollow - feedback |
Posted By: ffx-me
Date Posted: 04/15/2008 at 11:06am
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mechanics of loop drive -chemical energy in your arm and legs is converted into kinetic energy -you pass on a par off that energy into potential elastic energy in the sponge and rotatory kinetic energy. -the potential energy is converted into kinetic so the ball has 2 types of kinetic energy. -the spin of the ball grabs some air slowing the air down on the top of the ball making it so the air under the ball moves faster and therefore has a lower pressure and thus thus this creates a force downwards accelerating the ball. -when the ball hits the table energy is converted into elastic energy in the ball and a part of the rotatory energy is converted into kinetic energy as well as all the elastic energy. so the ball gains kinetic energy and loses spin and jumps forward ------------- -Primorac 2000, 802 + globe 999 quattro -Korbel, Cream MRS + 802-40 |
Posted By: sidofmillenium
Date Posted: 04/15/2008 at 12:11pm
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Posted By: ffx-me
Date Posted: 04/15/2008 at 12:45pm
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if you ht it flat on none of the energy is used to spin the ball so it goes faster and on the opposite side when you use almost no horizontal motion and tons of vertical motion most of the energy is converted in spin. But there must be horizontal mortion because you need to apply a normal force on the ball so there can be friction. the more that force is the more friction but there is a max because at a certain point the ball reaches the wood
------------- -Primorac 2000, 802 + globe 999 quattro -Korbel, Cream MRS + 802-40 |
Posted By: sidofmillenium
Date Posted: 04/15/2008 at 12:51pm
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nice ffx...I love hearing explanations in this language.....
(thanks everyone else as well...his explanation was very fancy...)
Ok...moving on to the next question...
How to hit the most powerful shot....basically...I like end the point when someone lobbing at once....similar to Waldner who never lets anyone lob....
Btw.....when we relax all of our muscles...to gain the maximum speed...but at the point of contact...we tighten them.(this is how a whip works)....I know by experience that this creates the maximum force in terms of speed...but would this also create the maximum force in terms of spin?)
There are 2 ways to do whip....one is to contract then relax...another is to stretch then relax....
P=W/T Therefore, using my whole body as one muscle and creating maximum force....the power lies in how quick can one relax all of his muscles......through my studying of martial arts method....we use both of the method at once to create the maximum force....first...we stretch all our muscles...and in that stretched position...we tighten them....then we relax all our muscles at once! But this method leaves the very basic force out of the equation...the force of moving our hand w/o any elasticity...the force of pulling one muscle while keeping its complementary muscle relaxed....can you guys think of unifying theory here?
Btw,
The harder the surface...the less the ground absorbs the force...therefore, more force is expressed through our upper body....
Thanks.
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Posted By: ffx-me
Date Posted: 04/15/2008 at 12:58pm
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I have no Idea about the body motions, I havent studied biology, my guess is the obvious one, do it quick and use your whole body
------------- -Primorac 2000, 802 + globe 999 quattro -Korbel, Cream MRS + 802-40 |
Posted By: ppgear
Date Posted: 04/16/2008 at 1:11am
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For the speed increase of topspin balls (if someone didn't already say this), I believe the topspin ball grips the table and jumps forward, increasing the speed. So you'll see a greater speed increase at each bounce. ------------- Arthur Lui Revspin.net - Table Tennis Equipment Reviews Top USATT Rating: 2131 |
Posted By: chu_bun
Date Posted: 04/16/2008 at 1:42am
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I believe the ball is slow down significantly after it hit the table. As a test, ask someone to hit a ball long and try to catch it with your blade. Not an easy task. Now ask him to hit the ball with the same speed but on the table this time. It will be much easier to block the ball. An top spin ball will bounce forward faster than a no spin or under spin ball thought. |
Posted By: ffx-me
Date Posted: 04/16/2008 at 9:38am
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I explained that its because there are 2 tipes of kinetic (mouvement) energy. there is rotation wich depends on the inertia moment of the ball Iweight distribution) and speed, and normal kinetic which depends on weight and mass. the ball's speed can be transforme is 2 speeds, vertical and horizontal. when the ball hits the table the table vibrates and therefore takes some of the vertical energy from the ball and it bounces less high. Also when it hits the table some of the spin energy is converted into horizontal kinetic energy and the ball accelerates ans looses spin
------------- -Primorac 2000, 802 + globe 999 quattro -Korbel, Cream MRS + 802-40 |
Posted By: sidofmillenium
Date Posted: 04/16/2008 at 8:21pm
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Nevermind, I got that question myself, release all our muscles meaning untightning it and giving all of my force a focus by simply following it through the muscle!(its ok if you don't get it...coz I do)
And yes, hitting the ball stiffly does increase the spin!
Anyways, seems like I am only person asking the question...ok, just one more....after that, you guys can take charge!
What makes the ball curve?(now, a lot of people have already explained this to me....but I still don't get it...so try to be fundamentally direct in your approach!)
Thanks.
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Posted By: ffx-me
Date Posted: 04/16/2008 at 8:26pm
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side spin, same principle as why the ball drops with topspin. When the ball travel through the air we can look at the ball as being still with the fluid, air, moving around it. the faster a gas moves the lower the pressure. if the ball travels and spins, one side spins with the air flow and there is no effect but on the otherside the ball spins in the opposite direction and grips a bit of air making the airflow slower on that side and therefore there is a lower pressure on the other side. naturaly there is a force applied on the ball towards the lowest pressure and the ball curves. Planes fly using the same principle.
------------- -Primorac 2000, 802 + globe 999 quattro -Korbel, Cream MRS + 802-40 |
Posted By: Mithrra
Date Posted: 04/17/2008 at 12:12pm
THis is bullsh*t... sorry... i dont agree with this....the principle of plane flying is totally not relevant to the spin of the ball...  ------------- Member of the Single Ply Hinoki Club *Non-Stop Changing Equipments!!!* HADO ST ASUKA 1 ply hinoki Accoustic FL AVX J-Power KKT Cho Tokusen-Tenergy05 |
Posted By: stiltt
Date Posted: 04/17/2008 at 12:27pm
he is actually right.
Let's think about regular topspin:
when the ball spins it creates a difference of atmospheric pressure between the top and the bottom of the ball. That creates a relative vacuum at the bottom of the ball and the ball falls into that vacuum.
It is the same thing when you do a sidespin; just rotate the system ball-paddle to the right (for a right handed forehand sidespin at the right of the ball) and you get the same resutls: there is a slight vacuum at the left of the ball since the sidespin was applied at the right of the ball and the ball curves to the left.
with underspin: that vacuum is at the top so the ball has a tendancy to go UP but gravity gets in the way and compensate and that's why we get a straight trajectory.
Did I get everything about right?
------------- /forum/topic91512_page1.html#1124698" rel="nofollow - sales - forum_posts.asp?TID=19315" rel="nofollow - feedback |
Posted By: sidofmillenium
Date Posted: 04/17/2008 at 2:27pm
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Nice...fatt....I have been struggling to get this since months...and you make it sound so simple!(did you major in physics?)
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Posted By: gulca
Date Posted: 04/17/2008 at 3:01pm
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The effect is bernoulli effect. It doesn't automatically mean a spinning ball will create the pressure difference that causes the ball to shift in its trajectory path. The ball needs to be moving in the same direction of the spinning ball. If the path of flight is the same as the axis of rotation, you do not see this effect. |
Posted By: sidofmillenium
Date Posted: 04/17/2008 at 3:52pm
Can you give us an example?
Alright...I got another one for you guys....why does a slower blade spins the ball more?(since the spin depends on how much a blade is able to flex....the thick soft blade should be same as slim soft blade....may be even greater since it won't absorb the spinning pressure!)
Thanks.
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Posted By: ffx-me
Date Posted: 04/17/2008 at 4:15pm
no its not bullsh*t  a plane flies because the path the air takes over the wing is longer (curved) so the air moves faster and there is a smaller pressure and there is a force upwards. with spin the ball grabs air on the side opposite to the direction, slowing it down making the air on the othe side have a smaller pressure and a force is created.------------- -Primorac 2000, 802 + globe 999 quattro -Korbel, Cream MRS + 802-40 |
Posted By: ffx-me
Date Posted: 04/17/2008 at 4:20pm
You have to look at it like a collision, you apply a force on the ball and there is an impulsion (force in a collision changing the quantity of movement). And, F=ma so in order to get more spin the contact time needs to be longer because the acceleration will be longer and we will have a greater jump in velocity (of spin, i dont know the term in english) Harder bats have shorter dwell times and brush the ball for a shorter time and therefore put less spin. Of cource if the blade flexes the time will be greater. Oh and an example of the spin that will not curve (doesnt oppose the movement) is topspin towards the side, ------------- -Primorac 2000, 802 + globe 999 quattro -Korbel, Cream MRS + 802-40 |
Posted By: stiltt
Date Posted: 04/17/2008 at 4:21pm
If I had a physics degree you would not have understood my answer
 I got answers from those 2 websites:
http://www.gregsttpages.com/articles/tt_physics_maths.html - http://www.gregsttpages.com/articles/tt_physics_maths.html
scroll down until you see the yello ball
 after the question: "Why does spin behave the way it does?"it's from Greg Lett's website; you know that guy who's running tt about dot com. That website is not much moving since he took over about tt dot com. He moved the article though:
http://tabletennis.about.com/od/basicconcepts/ss/spin_in_tt.htm - http://tabletennis.about.com/od/basicconcepts/ss/spin_in_tt.htm
http://tabletennis.about.com/od/beginnersguide/a/physics_mathsTT_2.htm - http://tabletennis.about.com/od/beginnersguide/a/physics_mathsTT_2.htm
Also this is really cool:
http://www.unc.edu/~sheng1/spin.htm - http://www.unc.edu/~sheng1/spin.htm
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Posted By: sidofmillenium
Date Posted: 04/17/2008 at 4:36pm
Hmm...I already visited those links...didn't help me...but your words did
Anyways...seems like the thicker a softer blade is(faster and heavier)...the spinnier it is b/c the less torque it absorbs...tell me if I am right....Therefore, the saying that slower the wood, faster it is, that is just a myth.(true in some cases obviously)
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Posted By: stiltt
Date Posted: 04/17/2008 at 5:11pm
the slower the blade the spinnier it is: YES.
I think it is the same idea than softer or harder sponge. When the sponge is softer, it's easier for the ball to sink into the sponge and the forearm moving while the ball is sinking into the sponge creates spin.
Same idea with a slower blade: the ball will stay with the blade and in the sponge a longer time instead of bouncing back right away so more spin is created.
By the way: you can still create a lot of spin with the faster blade; you just need to have a much higher arm speed to have the ball stay long enough with the paddle and in the sponge.
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Posted By: raptor3x
Date Posted: 04/17/2008 at 5:55pm
I'd just like to point out that the physics in the sections labeled "Why does spin behave the way it does?" and "Why a new ball is preferable to an old one" are completely wrong. Actually, I just read the entire article again and, while there are a few correct ideas, most of that article is complete garbage as far as physics goes. |
Posted By: chu_bun
Date Posted: 04/17/2008 at 6:14pm
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Not TT related, but sometimes ago I heard on the NPR (National Public Radio?), a guy has a new theory that explains why plane can fly based on Newton's third law "Action and reaction". According to him calculations based on Bernoulli's theory indicate that to produce enough lifting force a plan will have to go as fast as 300 - 400 miles/hrs before it can take off. His theory is that, the wings push the air and the air push back. So air goes down and plane go up, simple as that. He used a helicopter to prove his case. Anybody knows if this theory has been prove or disprove? |
Posted By: sidofmillenium
Date Posted: 04/17/2008 at 6:35pm
When you say its the same as softer or harder...don't you mean their width(sponge thickness) instead of degree b/c I am talking about same density blade...just different thickness....
Also...you say that when forearm is moving as the ball is sinking into the sponge...that creates more speed.....(I know you said spin...but I think you will agree that you might as well have said both)...well, I do not think that is true b/c in martial arts...while punching, a whip motion is used, where the arm is lose so it can gain maximum speed...then at the point of contact, it is tightened...so there is no recoiling effect...but then again, I also here other martial artists claiming that you should always pretend that your target is 2 feet farther than actuality so you can actually use your acceleration force, which is more powerful than max speed!
IDK here....you guys gotta solve this for me....
One more thing, how exactly ball staying at the blade for longer time creates greater spin? B/c spin is just angular speed...and therefore, similar to how the shorter the duration the ball spends on the blade, the faster the rebound....shouldn't it be same w/ spin?
Thanks.
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Posted By: stiltt
Date Posted: 04/17/2008 at 7:28pm
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there is the softness of the outter plies and the flex of the whole blade. both are important factors. if a 1-ply blade is made of very soft wood the ball will stay with the paddle longer and if that same paddle is thinner then it will bend and act like a spring (elasticity). the spinnier blade will be the thinnier made out of softer wood.
I need people like borko to comment because I am clearly reaching the limits of my knowledge of interaction between wood and the ball. ------------- /forum/topic91512_page1.html#1124698" rel="nofollow - sales - forum_posts.asp?TID=19315" rel="nofollow - feedback |
Posted By: stiltt
Date Posted: 04/17/2008 at 9:02pm
do you mean that that a very good player can make a very poor physicist ... et reciproquement? ------------- /forum/topic91512_page1.html#1124698" rel="nofollow - sales - forum_posts.asp?TID=19315" rel="nofollow - feedback |
Posted By: Mithrra
Date Posted: 04/17/2008 at 9:58pm
Why I say its bullsh*t is bcoz... plane has wings... does TT ball ever has wings? Plane uses the aerodynamic theory of physics to be able to fly. Even before picking up from land, it has to have the speed of 300-400miles/hour in order to fly... Does TT ball need to that kinda taking off process like a plane? And again..... the shape of an aeroplane vs the TT ball.... its totally different theory how it flies in hair....
In TT, how does a ball spin in air? Why does a couter looped ball is faster than the first ball driven? It alls comes back to the theory of conservation of energy. When the first person hits the ball with an amount of force, it will cause the ball to accelerate, changing the energy force into kinetic force as soon as the ball leave the blade, accelerating until the highest point before the ball starts to decrease in kinetic energy while gravity force starts to pull it down again. Now at the highest peak of the ball from ground, the ball itself pocess the highest potential energy = mgh( where m=mass, g=gravity, h= height from ground). If it is counter looped by the opponent, in order for the opponent to hit the ball and drive it back, he needs the amount of force to overcome the kinetic energy + potential energy of the ball at the highest peak. Since the ball at its peak has the highest level of potential energy, it will be added to the driving force of the second hitter thus, it results in force of user + potential energy of the ball. Once the ball leave the blade again, these two energy being added together and changes to kinetic energy. This is where the ball will accelerate again but with a greater speed this time since it has its potential energy from the first person who hit the ball. So.... as the number of rounds of counter looping increases, due to theory of conservation of energy, the energy changes into greater kinetic force which mades the ball harder to be driven back due to speed and momentum factor this time......
All right.... this is my first part of physics theory for TT ball driving.... Stay tune to the second part where I will talk about the spin of the ball.
------------- Member of the Single Ply Hinoki Club *Non-Stop Changing Equipments!!!* HADO ST ASUKA 1 ply hinoki Accoustic FL AVX J-Power KKT Cho Tokusen-Tenergy05 |
Posted By: gulca
Date Posted: 04/17/2008 at 10:43pm
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Mithrra, Your conservation of energies are explained incorrectly. Comparing a ball with a plane doesn't make sense. Shape of the plane is design to have maximum efficiency flying forward. That doesn't mean that a sphere ball cannot share the same effect of lift. ------------------- imagine a stationary ball in midair spinning without any gravity. How does one tell which side is up and which side is down? How do you apply force? Which side is faster? Slower? There is no difference if the ball isn't moving in some direction. A ball traveling along it's spinning axis will have no Bernoulli effect. I believe it is a pure corkscrew spin traveling towards the opponent will give this effect (ignoring gravity). |
Posted By: Mithrra
Date Posted: 04/17/2008 at 11:31pm
Gulca,
Your statement stationary ball is contradictory to the ball spining. If a ball is stationary, if will not spin. Oh well, you sure can tell how the ball spins base on the direction of the ball spining, namely : sideways, top, backwords or front= topspin. Without force how can you spin the ball?
"imagine a stationary ball in midair spinning without any gravity. " That is playing TT in space. Certainly, you can't play TT in space since everything is floating up there and there will not be any acceleration effect when you hit on it in space. Imagine playing TT in water.....Due to the gravitional effect of the earth, there exist force to opposing the gravitational effect of the ball in midair. Due to this ball having potential energy in midair, then can it accelerate in speed when a force is hit on it.
Spinning of the ball itself comes from another theory : theory of circular motion of the TT ball. Of course, the shape of the object flying in air plays the key roll in determining its movement in air. For example : Flying a kite and trying to fly a stone tied on to a string. So can you fly a stone although it is being tied to a string? The TT ball shape is totally different from an aeroplane. How can it share the same theory of aerodynamics of the aeroplane?
------------- Member of the Single Ply Hinoki Club *Non-Stop Changing Equipments!!!* HADO ST ASUKA 1 ply hinoki Accoustic FL AVX J-Power KKT Cho Tokusen-Tenergy05 |
Posted By: kenneyy88
Date Posted: 04/17/2008 at 11:56pm
| magnus effect? |
Posted By: raptor3x
Date Posted: 04/18/2008 at 12:06am
Uh, this one just hurts. I should do a point by point with this but I only have five minutes so I'll just hit the major issues. - Lift is generated in both wings and in table tennis balls in essentially the same manner; by turning the incoming flow. This is as simple as newton's 2nd law. The manner in which they turn the flow is different, but the method of lift generating is the same in all cases. - Aircraft most certainly do not need to travel 300-400 mph to lift off, unless maybe you're talking about the F-104 Starfighter; not sure about that one's take off speed. Edit: The takeoff speed for the F-104 is around 230mph if anybody was interested. -Your hypothesis involving potential energy is so wrong it's not even funny. You need to look at conservation of momentum (linear & angular) and the impulses involved. "Energy force" makes my head hurt. - Conservation of Energy/Momentum/Mass/Charge/whatever are not theories; they are laws. A law is an observation that we have found to be true and to which we have found no exceptions. Big difference. |
Posted By: gulca
Date Posted: 04/18/2008 at 12:08am
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Mithrra, You mixed up energies, force, velocity, spin and gravity. Please do not confuse others with your own confusion. My description of a stationary ball spinning is just an example to show there is no top nor bottom left or right. You do have the axis of rotation. But other than that, you cannot say if the ball is spinning left or right, top or bottom, underspin or topspin. The ball needs to be moving in some direction. Bring this up with any physics professor or any science major. They will sort you out. But please, do not spread false scientific explanations to the less educated. |
Posted By: Mithrra
Date Posted: 04/18/2008 at 1:13am
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Gulca, Can you explained which part are you confused? Which part of it is false explanation? This is getting to be more interesting -Your hypothesis involving potential energy is so wrong it's not even funny. You need to look at conservation of momentum (linear & angular) and the impulses involved.
Please correct which part you think is wrong.....
------------- Member of the Single Ply Hinoki Club *Non-Stop Changing Equipments!!!* HADO ST ASUKA 1 ply hinoki Accoustic FL AVX J-Power KKT Cho Tokusen-Tenergy05 |
...... I like you to proof that "I am spreading false scientific explanation".... Well this is all part of learning isn't it?Posted By: gulca
Date Posted: 04/18/2008 at 1:52am
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Mithrra I'll let others have a say on your explanation. I do agree learning your mistake is progress. However spreading confusion to others while you learn isn't nice. I apologize if I seem to be attacking you. I am not. Find some physics teacher and have them listen to your explanation. If they agree with you, then I must be the confused one. |
Posted By: esnift
Date Posted: 04/18/2008 at 2:50am
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Mithra, What a noble stance you took to being corrected. I'll have to agree with gulca and raptor3x on this one, your understanding of physics principles are pretty far off base. Firstly a ball spinning in the air and an airplane wing are governed by exactly the same principle. The first thing we need to understand is that swiftly moving fluids (air and water behave as fluids) exert less pressure than do slowly moving fluids. Please don't try to argue that because it comes straight out of my physics book. If you need proof of that get a straw, put it into a cup of water and blow across the top of it, not directly into it or at a downward angle, and the water in the straw will rise because the pressure is less just above the straw when compared to the rest of the water in the cup. That may seem like an odd comparison but it's the same governing principle. Someone tried to explain it earlier but was a little mixed up on some parts. Going back to a spinning ball traveling in a certain direction: The side of the ball that is spinning against the direction of travel slows down the flow of air while the other side increases the flow. In a loop for example, the top of the ball is spinning against the direction of travel while the bottom is traveling with it, but faster than the rate of travel of course. As we learned above, swiftly moving fluids exert less pressure and since the air under the ball on a loop is traveling faster it exerts less pressure on the ball and causes it to flow in a downward direction. This is the same reason why a backspin does not drop as quickly and will actually go up if you give it enough backspin. Relating that to airplanes, air has to go faster to travel the curved top of the wing than air that is passing under the flat bottom side. Faster moving air = less pressure = principle of lift. This can be a difficult concept at first because it seems counter intuitive to how we naturally want to see things, but I assure you this is correct. ------------- YE Hurricane III Tenergy 05fx |
Posted By: esnift
Date Posted: 04/18/2008 at 3:37am
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As for other comments made, the law of conservation of energy and the law of conservation of momentum have been skewed as well. with regards to conservation of energy: - gravitational potential energy (PE) is given by mass*force of gravity(9.8m/s^2 on Earth)*height. Symbolized mgh - Translational Kinetic energy = 1/2*mass*velocity squared. Symbolized 1/2mv^2 - Rotational Kinetic energy will be more complicated to understand on paper but is derived from moment of inertia (like mass but has to do with how easy it is to start something spinning)*angular velocity squared. Symbolized I*omega^2 (sorry i don't have the greek symbol on my keypad) If we account for work done from non conservative forces (things like friction, sound and heat where energy is dissipated) energy is conserved in a system which can be show by this equation: PE before + KE(rotational) before + KE(translational) before + work before = PE after + KE(rotational) after + KE(translational) after + work after. Stay with me now. This simply means that if you have a ball and drop it from a cliff, the potential energy that it had at the top of the cliff, given by mgh, is directly proportional to its kinetic energy and thus velocity (remember KE(translational)=1/2mv^2). Work would account for energy lost due to wind resistance and also explains terminal velocity) This is a lot of info to try and explain in a couple words so it would probably be better to take a class or talk to a physics professor in person. The point I'm trying to make here is that when you put work into the system of the ball by hitting it with your paddle, it is translated into rotational kinetic energy, translational kinetic energy, and for lobbers, potential energy (which is actually insignificant because the mass of a ping pong ball is so small, PE=mgh) You could also take into account the potential spring energy stored in the sponge and rubber of your paddle but is not really necessary in this explanation. now that i think about it i don't really want to take the time to explain conservation of momentum (mass1*velocity1=mass2*velocity2), or newtons second law (Sum of the forces = mass*acceleration). I can't remember the specific points that were mentioned earlier and don't want to take the time to go back and read all the posts but if you have a question or something, go ahead and I'll do my best. ------------- YE Hurricane III Tenergy 05fx |
Posted By: ppgear
Date Posted: 04/18/2008 at 9:04am
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In a thread like this which really takes a physics degree in order to speak with authority, I'm reminded of the old saying: "It is better to remain silent and be thought a fool, than to open your mouth and remove all doubt. " ------------- Arthur Lui Revspin.net - Table Tennis Equipment Reviews Top USATT Rating: 2131 |
Posted By: Mithrra
Date Posted: 04/18/2008 at 1:54pm
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" To remain silent forever is a forever fool,
To be humble & honest to ask is a wise man's deed" ------------- Member of the Single Ply Hinoki Club *Non-Stop Changing Equipments!!!* HADO ST ASUKA 1 ply hinoki Accoustic FL AVX J-Power KKT Cho Tokusen-Tenergy05 |
Posted By: sidofmillenium
Date Posted: 04/18/2008 at 2:02pm
| but u weren't asking....but telling others how they are b.s.ing.... |
Posted By: gulca
Date Posted: 04/18/2008 at 3:44pm
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I'll say he was not asking in the 1st few posts he made but he did started to ask after that. So all in all it started bad but went back on track. So let's put this thing behind us. We're here to discuss table tennis, not to put anyone on trial. |
Posted By: sidofmillenium
Date Posted: 04/18/2008 at 7:15pm
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yeah....to put things in perspective...we almost ehanged him...didnt we...? No pun intended....
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Posted By: Mithrra
Date Posted: 04/18/2008 at 10:31pm
alright...... sorry guys......  ------------- Member of the Single Ply Hinoki Club *Non-Stop Changing Equipments!!!* HADO ST ASUKA 1 ply hinoki Accoustic FL AVX J-Power KKT Cho Tokusen-Tenergy05 |
Posted By: sidofmillenium
Date Posted: 04/19/2008 at 12:16am
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Ok...back to the questions...
W=fd then how come pushing a wall is tiring?
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Posted By: Mithrra
Date Posted: 04/19/2008 at 12:18am
What is W=fd???? and why push the wall/?? ------------- Member of the Single Ply Hinoki Club *Non-Stop Changing Equipments!!!* HADO ST ASUKA 1 ply hinoki Accoustic FL AVX J-Power KKT Cho Tokusen-Tenergy05 |
Posted By: ppgear
Date Posted: 04/19/2008 at 12:59am
When you push the wall, since the wall doesn't move, it's actually your body that gets displaced. For example, you lean against the wall, then push. You end up pushing your body away from the wall, instead of the wall away from your body. ------------- Arthur Lui Revspin.net - Table Tennis Equipment Reviews Top USATT Rating: 2131 |
Posted By: sidofmillenium
Date Posted: 04/19/2008 at 1:12am
| But when happens when I lean to my toes...then my body doesn't get displaced either.... |
Posted By: ppgear
Date Posted: 04/19/2008 at 1:21am
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Even if your feet never move from their spot on the ground, your upper body moves. So if you stand up straight, then lean over towards the wall in front of you. When you push against the wall, your feet won't move, but your upper body will move, from leaned-over-position to upright or further depending how far you push. So it will be your upper body that has been moved, even if your feet stayed planted on the ground. It's like doing a push-up but on a slope instead of flat ground. Your feet stay planted, but you move your upper body. ------------- Arthur Lui Revspin.net - Table Tennis Equipment Reviews Top USATT Rating: 2131 |
Posted By: Mithrra
Date Posted: 04/19/2008 at 2:15am
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How does this affect Table Tennis? ------------- Member of the Single Ply Hinoki Club *Non-Stop Changing Equipments!!!* HADO ST ASUKA 1 ply hinoki Accoustic FL AVX J-Power KKT Cho Tokusen-Tenergy05 |
Posted By: esnift
Date Posted: 04/19/2008 at 2:22am
I understand what ur saying sido. The answer is actually in your question. In physics the definition of work is a force multiplied by a change in distance. This is different from what we commonly think of as work: exertion of energy to accomplish a task, or even just accomplishing a task. The problem with physics isn't that concepts are hard per se, just that you kind of have to rewire your thinking. So if you are pushing against a wall without moving yourself or the wall, you haven't done any work in the physics definition. You have expended energy but energy doesn't factor into the equation W=fd. by the way i'm really not trying to sound pompous or offend anybody with my last posts. Just trying to be helpful. Spread the wealth of knowledge ------------- YE Hurricane III Tenergy 05fx |
Posted By: sidofmillenium
Date Posted: 04/19/2008 at 5:14am
| Alright....moving on, I have this understanding(tell me if I am right) that rebound is directly dependent on elasticity...when a ball is coming, and we move our paddle backward at the point of contact, it makes the rebound slower....b/c the ball doesn't sink as much....Therefore, when a ball is spinning, if a ball theoretically had only spin, no speed(I guess ball would have to travel at the speed of light angularly for that) Anyways, assuming this as practical, wouldn't stiffer the surface, better the spin? B/c currently, the reason what happens at stiff surface like table is that when a topspin bounces, friction slows the spin down, but it cannot give it back again like a spring b/c the ball leaves early...but how come it gains speed? B/c the ball deforms b/c of angular friction...and it is the undeformation which gives ball acceleration as a topspin ball after a bounce...what do you guys think? |
Posted By: sidofmillenium
Date Posted: 04/19/2008 at 7:39pm
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I'll take silence as agreement....alright, next question... I earlier asked the question that if W=fd then how come I get tired when I push a wall? You guys answered that there is a difference between physics work and biological work...well if that is true then how come when I sit long on a weak chair...it breaks?(I obviously performed work on it...but for sometime, when I weakened it...it didn't count as work...how come?
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Posted By: esnift
Date Posted: 04/19/2008 at 8:42pm
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Sorry I can only get to my computer in the afternoon so I didn't have time to respond to your first post. I'll answer the last one first then go back and take another look at the first one. The work that is performed on the chair comes from acceleration due to gravity multiplied by your mass (or in other words your weight). Time isn't a factor in work, where time comes in is the definition of power. Power is equal to the work performed divided by time required for that work to happen (P=W/T=fd/T=fv (force times velocity=power because d/T= velocity)). So the less time it takes to do something, the more power is required. The SI unit for power is watts but can be converted to horse power etc. So the work performed on the chair whether it takes one minute or a year, is the same, but the power in each scenario is different depending on the time it took. ------------- YE Hurricane III Tenergy 05fx |
Posted By: esnift
Date Posted: 04/19/2008 at 10:07pm
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The first question has to do a little more with conservation of momentum and newton's second law. Momentum=mass times velocity. When you hit a ball with a paddle, the ball exerts the same amount of force on the paddle as the paddle does on the ball but in opposite direction. When the two objects collide, the elasticity of the sponge and rubber comes into play but so does the momentum of the paddle and the momentum of the ball. Ok, this is kinda hard explaining this process and trying to introduce these definitions at the same time but i'm doing my best. So the ball goes into the sponge a certain amount according to it's own speed and mass as well as the speed of the paddle. I will assume that the sponge-rubber system behaves like a spring in the sense that how far you push into it is directly proportional to the force required to make it go that far. The spring potential energy is given by 1/2 * a constant labled k (this constant is a measure of the stiffness of the spring, or how easily it is compressed) * change in distance squared. symbolized: PE (spring) = -1/2kx (it is defined as a negative value because the force of the spring pushing back, the restoring force, is considered the positive value) This is probably way more information than you wanted, but thinking through this helps me prepare for my finals ;) So it gets a little more complicated factoring in the conservation of momentum upon impact and energy dissipated by the sponge not acting exactly as a spring and converting kinetic energy into heat and sound would have to be taken into account as well. The principle of conservation of momentum after a collision would tell us that the mass of one object times its velocity is equal to the mass of the second object time its velocity. So if you increase the mass of one object, its speed will decrease and vice versa. So what is really happening when you move your paddle back at the point of contact is decreasing the momentum of the paddle or even causing it to have momentum in the same direction as the ball (that is important because velocity is regarded as a vector where direction is just as important as amplitude, speed is not the same as velocity in physics). So yes that will affect how far the ball will go into the sponge but the main change is caused by the change in momentum of the paddle. It's kinda hard to stay on track here, but the ball deformation isn't where the spin is generated, but the force of friction is the main force to consider here. Friction force is given by multiplying the coefficient of friction (tells us how much two surfaces stick together) symbolized by the greek letter mew, multiplied by the normal force. The normal force is the opposing force that keeps the ball from going through the paddle. To understand normal force better, consider a mug on a table. There is a force being exerted on the mug equal to its weight. This comes from newton's second law: sum of the forces = mass times acceleration. When acceleration = 0 this is a situation called equilibrium. This means that the sum of the forces is equal to zero; so in the mug situation the normal force is equal to the weight of the mug (weight=mass*force of gravity) Holy crap the more in depth I go the more stuff there is to explain, this is rather exhausting. So friction comes into play because the component of the force that is perpendicular to the axis of rotation of the ball (that's a mouth full) is known as a torque and is what causes the ball to spin. So more than anything, good stroke technique is what will develop more spin, and I believe professionals and coaches would agree with me on that point. On the point about the ball bouncing on the table, it is more of a perfectly elastic collision where not as much energy is absorbed through deformation (like billiard balls). Not really the crucial point though. The ball traveling through the air has rotational kinetic energy as well as translational kinetic energy. If there were no friction between the table and the ball the ball wouldn't 'speed up' after impact; the only things that would affect its trajectory would be gravity and Bernoulli's principle (the one with fluids and air pressure etc) Since there is friction, at the moment of impact, the ball exerts a force on the table, mainly due to rotational KE because the ball is spinning faster than it is traveling. A component of that force will be in the up and down (x) direction, but the majority will be in the side to side (y) direction due to spin. This means that there has to be an equal but opposite force on the ball, which comes from friction force, and since the table is stationary (due to its much greater mass and static friction between it and the floor) and the ball is not, some of the rotational energy is converted to translational energy due to a torque that does not have an opposing force. well, I am exhausted from snowboarding all day and can't concentrate much more on this topic. Sorry that my explanation is a little hard to follow, but in reality that was a complicated situation when all aspects are considered. ------------- YE Hurricane III Tenergy 05fx |
Posted By: esnift
Date Posted: 04/19/2008 at 10:10pm
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wow i apologize, didn't realize that it i had written so much. Uh. . . read it if your interested but if not then don't bother and save yourself some time ------------- YE Hurricane III Tenergy 05fx |
Posted By: sidofmillenium
Date Posted: 05/28/2008 at 12:13am
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ok.....now, if a ball has spin and speed compared to a ball which has only spin....would the ball w/ only spin rebound w/ greater spin than the other one b/c speed will interfer w/ angular spin? Why? Thanks.
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Posted By: esnift
Date Posted: 05/28/2008 at 2:57am
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well it depends on the values that you assign to each. you would need to define the parameters more specifically. things like; is the object that the ball contacts stationary? do the balls have an equal amount of total kinetic energy? and what type of surface is the ball contacting? all take part in how the ball reacts upon impact. if the balls have equal total kinetic energy, the stationary ball is spinning fast enough to compensate for the speed + spin of the other ball. specify a little more and i'll do my best. ------------- YE Hurricane III Tenergy 05fx |
Posted By: sidofmillenium
Date Posted: 05/28/2008 at 3:19pm
| both balls have equal amount of spin...but one has more speed.....the blade is normal inverted in a stationary blocking position....would both balls come back w/ equal opposite spin?(or would speed of the ball interfer w/ the dwell time required for angular rebound)...How? |
Posted By: esnift
Date Posted: 05/28/2008 at 6:17pm
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I think were kinda getting lost in semantics here. First of all, if a spinning ball collides with a stationary high friction surface most of it's spin is going to be turned into speed. There is not some magic force that reverses spin. Changing incoming topspin to outgoing topspin comes from the force you apply and the angle of you racket. The force and principle to be considered here is actually torque. I may have said it in an earlier post but a torque is just a force applied about an axis of rotation. To get the idea of a torque correctly think about it like this; if you were to take an open door and push directly toward the hinges, the door wouldn't move and no torque has been applied (although you were applying a force), if you push in a manner perpendicular to the hinges all of the force is applied as a torque and works to close the door, if you were to grab the doorknob and push at a 45 degree angle towards the hinges part of the force you apply would be a torque and part wouldn't be (trigonometry would allow you to figure out how much) as only the component of the force perpendicular to the hinge goes into causing the door to rotate. Ok try to take what you can from that, because torque is a big part of how a ball reacts when it hits your paddle and why. In your scenario the angle of the block is a much bigger factor than the speed of the ball. changing the angle of the block will change the amount of torque applied by your paddle upon impact and dictate whether the ball's spin will reverse or stay the same or be entirely converted into speed leaving a fast dead ball. Most blocks against topspin are at a sharp downward angle. This is because without even moving the paddle much, the combination of forces at impact results in a torque without an opposing force causing a change in spin, creating an outgoing topspin. You can experiment with this concept by hitting a ball off your paddle with spin letting it fall back onto it for a second spinny bounce. Change the angle your paddle will come in contact with the ball and observe how the ball reacts differently. I'm not sure that angular rebound makes sense or what it would refer to so i cant really say much about that. ------------- YE Hurricane III Tenergy 05fx |
Posted By: debraj
Date Posted: 05/28/2008 at 7:48pm
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ahhh my favorite topic mechanics and materials !! here is my late entry responding to esnift.
"...... There is not some magic force that reverses spin. Changing incoming topspin to outgoing topspin comes from the force you apply and the angle of you racket....."
DISAGREE. a spinning ball will reverse the spin because it's getting a huge reverse torque from the high friction surface. Otherwise your topspins would comback to you as backspin... like it happens with LP opponent, where the surface can't impart reverse torque through friction.
While the degree of reverse torque will depend on the angle, the fact that it will havea reverse torque wont.
And i also think if you are having your racquet face down while blocking, you will convert a component of the incoming spin momentum from rotational kinetic energy to linear kinetic energy by increasing the spin of the ball.
so the forula would go:
Incoming Speed (X) + Incoming Spin (Y) = Outgoing speed (X + delta X) + Outgoing spin (Y - delta Y) + delta Z
where delta Z is energy lost in the encounter.
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Posted By: debraj
Date Posted: 05/28/2008 at 7:51pm
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now my additional query ?
what is the best way to pass the ball from one end of a large room to the players far away?
topspin it on air... or chop it on air ..or hit it without any spin at 45 degree angle? :)
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Posted By: ppgear
Date Posted: 05/28/2008 at 8:55pm
A chop-smack. I hit the ball about 15 degrees upward from the horizontal plane, and apply underspin. But not a sharp chop angle, it's a flat hit but with the paddle aimed upward a bit. Hard to explain. But the effect is that the ball travels very far, and you don't even have to hit it hard. Most people hit it with flat, hard contact, at a 45 degree angle upward from the horiztal, and it doesn't go far even when hit very hard. I'm always slightly annoyed when people do that to my ball because they're hitting it very hard and it might crack, whereas my method travels much further and you don't have to hit hard. The underspin applied makes the ball curve upward and travel further. ------------- Arthur Lui Revspin.net - Table Tennis Equipment Reviews Top USATT Rating: 2131 |
Posted By: sidofmillenium
Date Posted: 05/29/2008 at 12:57am
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But my question didn't even get answered....I asking if a ball is traveling w/ 5m/s of speed w/ 5m/s of angular speed....and if another ball was travelling with 1m/s of speed w/ 5m/s of angular speed....which was would return w/ more spin if the racket angle was same in both of them while blocking.....and why? |
Posted By: gulca
Date Posted: 05/29/2008 at 1:41am
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sido, It would depend on the surface of the racket. If the rubber is soft and grippy, then the faster ball would lose more spin due to more surface contact. If the rubber is hard and smooth, then the balls would have almost the same spin coming back. However, the faster ball bouncing back on the table would invoke more topspin than the slower ball. Two underspin balls would loose the underspins. Two topspin balls would gain spin. More for the faster ball on both cases. |
Posted By: debraj
Date Posted: 05/29/2008 at 1:48am
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great question... my logic says both will return with same spin which is dependent on friction of the racquet and mass of the ball. But there is a lower cut off limit depending on friction co-efficient, angle and ball mass. for eg if the cut off limit it 2 m/s,
then any ball above 2m/s (3, 4, 5 etc) incoming spin will return with exactly 2m/s returning spin. , but once yuo go below 2m/s then return spin will be a proportional function of incoming spin.
i will try deriving this mathematically i later post.
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Posted By: debraj
Date Posted: 05/29/2008 at 1:52am
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@gulca
wrong!
the higher friction rubber will reverse the spin more effectively and produce greater reverse torque. lower friction will reverse less effectively due to lower torque... long pip (low friction by nature) will not have enough friction to reverse the spin.. so may return a dead ball.... frictionless long pips will not reverse at all neither slow down the spin.. and return a backspin ball (i.e . original spin direction).
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Posted By: debraj
Date Posted: 05/29/2008 at 1:52am
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@ppgear great answer and explanation. :) |
Posted By: gulca
Date Posted: 05/29/2008 at 2:10am
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debraj, I disagree. Your reverse torque doesn't make much sense. Grippy surface first. The more grip the surface, the greater the spin is going to affect the direction of the bounce. It won't, and never will change the rotation/spin based on the gripping from the rotation. Drop a superspinning ball straight down onto a super grippy floor. What do you see? You never get any torque reversal. Ever. What you see is the change of the direction of the bounce towards the rotation of the ball. The speed of the ball would increase. The rotation would slow down. But never reversed. Now another experiment. Shoot a spinless ball at an angle towards the grippy floor. On the bounce, the ball would be spinning forward. It would loose a little of it's speed though. I hope someone would clarify both our argument. I don't want this to be another "mitthra" fight. :) Combine both cases, and you see the flaw in your explanation. |
Posted By: debraj
Date Posted: 05/29/2008 at 2:44am
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"Drop a superspinning ball straight down onto a super grippy floor. What do you see?"
Gulca, great logic. now would you do this?
spin a ball (preferably a racquetball to get enough friction on flor, and enough momentum as it happens with incoming tt shot) with forward spin.
... and throw it with some velocity in forward direction to the floor.
yes now you are mimicing exact condition (if you throw somewhat fast).
which way is the ballspining after dropping? ( you can put a mark on the ball to identify the rotational direction)
[what you just said is the condition of suboptimal friction condition, not enough to reverse the spin that i mentioned (<2m/s).]
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Posted By: debraj
Date Posted: 05/29/2008 at 2:47am
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think of it the tt ball while blocked is hitting the surface at an acute angle in the direction of spin.. just so you can compare with the rb experimnt. also logically, if there was no spin reversal.. wont a topspin ball when blocked, come back to you as a backspin? |
Posted By: esnift
Date Posted: 05/29/2008 at 3:20am
I'd have to disagree with you disagreeing  I think if you read my post with reference to sido's question it will make more sense, as the rubber surface was specified. though i understand that someone with a background if physics was probably screaming "what are you talking about, where is the normal force etc."I was simply trying to explain that the ball doesn't reverse spin because it hit something. if you hold your racket at a 90 degree angle to the ground to block an oncoming topspin shot before the ball hits its apex the spin won't be reversed but it will be slowed, and there will be a change in velocity (meaning speed and direction). Along the same wavelength if you tilted your racket beyond the 90 degree mark angling away from the table (i dont know why anyone would want to do that, just an example), even though the surface of the racket is high friction, there will not be any spin reversal, this will be a clear example of a velocity gain because the ball will continue traveling in the same direction but at a higher speed. That is where the angle of the racket comes into play which i was trying to explain to sido. by changing the angle of the racket you can increase or decrease the amount of force applied as a torque. So saying that there is always a reverse torque isn't exactly correct. there will always be a torque due to the unopposed friction force. ------------- YE Hurricane III Tenergy 05fx |
Posted By: Mithrra
Date Posted: 05/29/2008 at 3:51am
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I think TT is able skills and blade + rubber quality factor rather than debating about physics laws here...... ------------- Member of the Single Ply Hinoki Club *Non-Stop Changing Equipments!!!* HADO ST ASUKA 1 ply hinoki Accoustic FL AVX J-Power KKT Cho Tokusen-Tenergy05 |
Posted By: esnift
Date Posted: 05/29/2008 at 3:57am
I'm not sure where you are getting this automatic reversal idea. Gulca's example of dropping a spinning ball on the floor is what i was trying to illustrate with the paddle perpendicular to the floor example. I wish i could draw you a free body diagram to explain what's going on. the key thing going on here with a stationary block from a high friction paddle is that the only factors involved that make a difference are the moment of inertia of the ball which is very small, total kinetic energy (translational and angular), momentum, and torque. The main thing being torque. the same amount of torque is not applied in the same direction as you change the angle of the contact surface, this is why your bouncing a racket ball on the floor example will give a different result. it does not have to do with a "suboptimal friction condition" even though friction is related to velocity in this instance as it will change the amount of normal force exerted on the ball as friction force is derived from the coefficient of friction multiplied by the normal force. Take this example if you slice under you ball sending it vertical as with a backspin serve motion then allow it to drop onto the paddle while keeping the it parallel to the ground. the expected result if the paddle is flat will be a ball that now has a greater translational velocity in the x direction and no angular velocity. this adheres to you equation given above in that incoming speed and out going spin are negligible in that Xinitial = Y final + energy lost or 1/2 I*(omega squared) = 1/2 mass*(velocity squared) ------------- YE Hurricane III Tenergy 05fx |
Posted By: esnift
Date Posted: 05/29/2008 at 4:13am
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And you could argue that would be spin reversal but i look at it more in the sense of converting one type of energy into another (rotational to translational). now if you change the angle of the paddle against the direction of rotation it will reverse the rotation of the ball, not to equal the initial angular velocity however, and send the ball directly back up in the air. In this case enough torque was applied in the right direction to spin the ball the opposite way and send it up a small distance in the air. if you then angle the paddle with the rotation of the ball torque is applied which only slightly slows the ball in its spin but adds to the velocity. I hope you understand my example. both of you are mostly right and i'm trying my best to explain. not saying i'm always right but i am quite confident about this. ------------- YE Hurricane III Tenergy 05fx |
Posted By: esnift
Date Posted: 05/29/2008 at 4:14am
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And Mithrra you may have a point, but it's all in good fun and knowledge is power baby.
------------- YE Hurricane III Tenergy 05fx |
Posted By: ppgear
Date Posted: 05/29/2008 at 9:45am
Actually, when the ball hits a stationary object, it's physically impossible for the speed of the ball to increase after contact, because no extra force is applied. Only a moving paddle can increase the ball's speed. ------------- Arthur Lui Revspin.net - Table Tennis Equipment Reviews Top USATT Rating: 2131 |
Posted By: esnift
Date Posted: 05/29/2008 at 2:33pm
Sorry but that's incorrect  When the ball hits a stationary high friction surface there is an unopposed torque from static friction force and when the angle of the blade is complimentary with spin direction, that torque acts to propel the ball with greater speed but less spin. Essentially converting speed into spin, but not to exceed the total rotational and translational energy at the beginning minus work lost. Did you even try the experiment I outlined? I doubt it because the ball is clearly traveling at a higher translational velocity while spin has been greatly reduced or stopped depending on the scenario. What you may be thinking of is conservation of momentum. That is true that momentum in this scenario will not change but thats just reaffirming what I am saying because angular momentum(i) + linear momentum(i) = angular momentum(f) + linear momentum(f) ------------- YE Hurricane III Tenergy 05fx |
Posted By: esnift
Date Posted: 05/29/2008 at 2:50pm
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ok debraj, i read some of your posts again and realize what you mean by reverse torque, i think. When the ball hits the high friction rubber the unopposed torque coming form static friction force is always contrary to the direction of spin. so i see why you said "reverse torque." however, you can change how torque applied to the ball by changing the angle of the racket with relation to projectile velocity, spin, and gravity. I've been trying to think about a situation without gravity or velocity and in that instance angle of paddle would not matter (within a 2D plane). ------------- YE Hurricane III Tenergy 05fx |
Posted By: ppgear
Date Posted: 05/29/2008 at 2:54pm
True, I forgot to factor in the torque. My bad. ------------- Arthur Lui Revspin.net - Table Tennis Equipment Reviews Top USATT Rating: 2131 |