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sidofmillenium View Drop Down
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    Posted: 04/12/2008 at 4:15am

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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Post Options Post Options   Thanks (0) Thanks(0)   Quote anixon Quote  Post ReplyReply Direct Link To This Post Posted: 04/12/2008 at 4:29am
Originally posted by SIDOFMILENIUM SIDOFMILENIUM wrote:

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?)


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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TAKOYAK1 Quote  Post ReplyReply Direct Link To This Post Posted: 04/12/2008 at 4:32am

Great explanation anixon. maybe you could be a professer in TT somedayLOL

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jolan Quote  Post ReplyReply Direct Link To This Post Posted: 04/12/2008 at 4:42am
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sidofmillenium Quote  Post ReplyReply Direct Link To This Post 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?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote anixon Quote  Post ReplyReply Direct Link To This Post Posted: 04/12/2008 at 4:57am
Originally posted by jcdi jcdi wrote:

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.


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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote anixon Quote  Post ReplyReply Direct Link To This Post Posted: 04/12/2008 at 5:00am
Originally posted by SIDOFMILENIUM SIDOFMILENIUM wrote:

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?


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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote LachlanTan Quote  Post ReplyReply Direct Link To This Post 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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Pjotr Quote  Post ReplyReply Direct Link To This Post Posted: 04/12/2008 at 8:04am
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 :) 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stiltt Quote  Post ReplyReply Direct Link To This Post Posted: 04/12/2008 at 9:55am
Originally posted by anixon anixon wrote:

Originally posted by jcdi jcdi wrote:

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.


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.
 
You read people very well!  very nice anixon!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sidofmillenium Quote  Post ReplyReply Direct Link To This Post 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?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gulca Quote  Post ReplyReply Direct Link To This Post Posted: 04/12/2008 at 7:18pm
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sidofmillenium Quote  Post ReplyReply Direct Link To This Post Posted: 04/12/2008 at 11:32pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote gulca Quote  Post ReplyReply Direct Link To This Post Posted: 04/13/2008 at 12:03am
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.




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Post Options Post Options   Thanks (0) Thanks(0)   Quote gulca Quote  Post ReplyReply Direct Link To This Post Posted: 04/13/2008 at 12:32am
Originally posted by SIDOFMILENIUM SIDOFMILENIUM wrote:


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!)


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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote loop+loop Quote  Post ReplyReply Direct Link To This Post Posted: 04/13/2008 at 1:03am
9000 rpm? That's fast
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gulca Quote  Post ReplyReply Direct Link To This Post Posted: 04/13/2008 at 1:18am
Originally posted by loop+loop loop+loop wrote:

9000 rpm? That's fast


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.



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Post Options Post Options   Thanks (0) Thanks(0)   Quote tatan Quote  Post ReplyReply Direct Link To This Post Posted: 04/13/2008 at 2:26pm
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tatan Quote  Post ReplyReply Direct Link To This Post Posted: 04/13/2008 at 2:33pm
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stiltt Quote  Post ReplyReply Direct Link To This Post Posted: 04/13/2008 at 2:36pm
Originally posted by tatan tatan wrote:

let me introduce TORQUE.
 
Hi Torque
 
 
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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. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sidofmillenium Quote  Post ReplyReply Direct Link To This Post Posted: 04/15/2008 at 10:28am
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote stiltt Quote  Post ReplyReply Direct Link To This Post Posted: 04/15/2008 at 10:43am
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.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ffx-me Quote  Post ReplyReply Direct Link To This Post Posted: 04/15/2008 at 11:06am
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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sidofmillenium Quote  Post ReplyReply Direct Link To This Post Posted: 04/15/2008 at 12:11pm

Smile

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Post Options Post Options   Thanks (0) Thanks(0)   Quote ffx-me Quote  Post ReplyReply Direct Link To This Post Posted: 04/15/2008 at 12:45pm
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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sidofmillenium Quote  Post ReplyReply Direct Link To This Post Posted: 04/15/2008 at 12:51pm
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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Post Options Post Options   Thanks (0) Thanks(0)   Quote ffx-me Quote  Post ReplyReply Direct Link To This Post Posted: 04/15/2008 at 12:58pm
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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ppgear Quote  Post ReplyReply Direct Link To This Post Posted: 04/16/2008 at 1:11am
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.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote chu_bun Quote  Post ReplyReply Direct Link To This Post Posted: 04/16/2008 at 1:42am
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. 
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