Scientific Questions

debraj wrote: 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.

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.

debraj wrote: "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 floor, 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).]

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)

And Mithrra you may have a point, but it's all in good fun and knowledge is power baby.

ppgear wrote: esnift wrote: 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. 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.

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)

esnift wrote: ppgear wrote: esnift wrote: 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. 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. 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.

True, I forgot to factor in the torque. My bad.

what i would like to know is that trick where you throw the ball against the raquet and the ball would hop very strange on the table.

Basically what I was trying to ask is that earlier...someone mentioned that a ball w/ a slight backspin will travel farther than a straight hit shot...I am wondering why....

Here are another 2 questions for the serve masters:-

1. What type of spins can you serve with your racket facing up throughout the swing?
2. And how do they work (ie how should the swing go)?

Alright...forget about this question....when we hit a kill....why should we do follow through?  B/c it seems like the less follow through we do...the more intense and controlled the kill is....then why is follow through so popular?

SIDOFMILENIUM wrote: Alright...forget about this question....when we hit a kill....why should we do follow through?  B/c it seems like the less follow through we do...the more intense and controlled the kill is....then why is follow through so popular?

Why does a 45 degree backspin ball curves to the left when it is going toward your opponent?(b/c a 45 degree topspin ball does the same...it just doesn't travel as far...)

Why does thicker sponge adds speed?(since blade is harder than the sponge.....the less the sponge...the greater force blade must be implying on the ball.