The Myth of Top Spin???

[Neil]

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[mikepage]

I don't think there is any part of the train that is moving in the opposite direction it is travelling. The closest anypart of the wheel would be to moving backwards is when a point on the wheel is at 6 oclock. Then it stops moving down towards the ground and moves up and forward away from it.

My physics teacher explained this quite thouroughly, not convincing half the class. His example was as you are driving behind a car and it throws a rock toward your car, that rock is actully moving away from you and you drive into it. It cannot be thrown backwards because in relation to the ground there is NO part of the wheel that moves opposite of the direction of the wheel.

He had a wheel with a dot on it. We were asked to watch that dot and say when it moved backwards in realtion to the ground. It doesn't.

Alot to say about nothing.

I am curiouse what the original poster has to say about the train moving backwards?

Your mileage may vary, but I usually find that when I don't understand what Mr. Jewett is saying, it's because he's a step or two ahead of me.

He will agree there's no part of a moving car going backwards, and therefore as you said, a car can't kick a stone backwards (so long as it's not peeling out, that is).

But a train differs from a car in a significant way. There's the flat bottom of the train wheel that sits on the rail, and then there's a part of the wheel called the flange that sticks down lower on the inside of the rail (to keep the wheel from sliding out) In other words if the business part of the wheel is a circle of 40 inch diameter, then the wheel with the flange is a circle of, say, 46 inch diameter. The portion of the flange below the rail is moving west.

 

[Fragged]

That shot does not involve top spin but rather forward roll toward the rail that becomes "draw" when the CB rebounds off the rail in the opposite direction.

All the while, the CB is rotating in the same direction but begins to travel in the opposite direction.

That has nothing to do with "topspin."

Regards,
Jim

The shot started with Topspin so its a topsin shot-sorry.If you were a great draw shot maker but couldnt follow, you couldnt make the shot. and what do you get from normal roll into a rail a normal rebound.

 

[Neil]

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[3andstop]

And that's why water spins the other way in toilets below the equator when ya flush em too! Right?

 

[8-Baller]

An equal-weight cue ball bounces back from a rack because it's running into a greater mass than itself (more than one ball).

Errr.....that is wrong.....

Try this: line up 5 object balls in a straight line along a rail and strike one end with the cue ball with a stun shot..... guess what happens?


Donald

 

[DoubleA]

BUT... The argument is that the cb loses ALL foward momentum on a full hit. (which I believe it does) SO... the speed on approach has ZERO affect after impact. It stops dead in its tracks, and only moves forward or backward from any spin it retains.

ABSOLUTELY, assuming it stops dead.

 

[mikepage]

Don't mean to argue, but the speed of the surface of the cb can never be greater than the speed of the cue tip.

This is not true.

Not sure what people are refering to as "over spin". Would that be any thing like being to throw a ball faster than your hand can move?

No. it's a different issue. If you hit a cueball in the middle, the cueball starts out sliding, and then after a bit due to friction with the cloth it is rolling rather than sliding.

For a ball to be rolling, it has to have a spin that is just right for it's speed.

If you hit with a little bit of topspin, then the ball will still start out sliding, but it's a little closer to the rolling situation at the get go.

If you hit with more topspin --close to miscue-- you can give the ball all the spin it needs to be rolling right away.

If you're really good, you may be able to hit the cueball high enough that it has a wee bit more spin than it needs to roll--that's overspin--and it's a small effect if you can do it at all.

But the same concept of overspin becomes very important when a rolling cueball hits an object ball because it always has overspin right after the collision, and sometimes it has a lots of it.

 

[Neil]

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[8-Baller]

Drop a cb on the table and see if it bounces. It will.

Of course it bounces because the table is heavier than the cue ball....
A rack of balls is different.....unless at least ONE of the balls of the rack is heavier than the cue ball (assuming all other balls are of equal weight), then the cue ball will bounce back a little.


In a theoretically PERFECT rack of theoretically PERFECTLY equally weighted balls, a perfect direct hit (with no spin and no angle) on the apex of the rack will send only two balls moving at equal speeds precisely half of the speed of the cue ball.....and those two balls are located at the other corners of the triangle. ALL OTHER BALLS (including cue ball) WILL REMAIN MOTIONLESS. This is simple physics.

Donald

 

[DoubleA]

This is not true.



No. it's a different issue. If you hit a cueball in the middle, the cueball starts out sliding, and then after a bit due to friction with the cloth it is rolling rather than sliding.

For a ball to be rolling, it has to have a spin that is just right for it's speed.

If you hit with a little bit of topspin, then the ball will still start out sliding, but it's a little closer to the rolling situation at the get go.

If you hit with more topspin --close to miscue-- you can give the ball all the spin it needs to be rolling right away.

If you're really good, you may be able to hit the cueball high enough that it has a wee bit more spin than it needs to roll--that's overspin--and it's a small effect if you can do it at all.

But the same concept of overspin becomes very important when a rolling cueball hits an object ball because it always has overspin right after the collision, and sometimes it has a lots of it.

I am not saying that you cannot make the cb spin, I am saying that neither the rotation of the ball nor the forward motion of the cb can be greater than the initial speed of the cue stick. (physics 101)

 

[8-Baller]

I am not saying that you cannot make the cb spin, I am saying that neither the rotation of the ball nor the forward motion of the cb can be greater than the initial speed of the cue stick. (physics 101)

You are forgetting about the compression effects of the tip. When accounting for tip compression....and subsequent spring-back, the cueball can actually move faster than the cue stick.

 

[DoubleA]

Your mileage may vary, but I usually find that when I don't understand what Mr. Jewett is saying, it's because he's a step or two ahead of me.

He will agree there's no part of a moving car going backwards, and therefore as you said, a car can't kick a stone backwards (so long as it's not peeling out, that is).

But a train differs from a car in a significant way. There's the flat bottom of the train wheel that sits on the rail, and then there's a part of the wheel called the flange that sticks down lower on the inside of the rail (to keep the wheel from sliding out) In other words if the business part of the wheel is a circle of 40 inch diameter, then the wheel with the flange is a circle of, say, 46 inch diameter. The portion of the flange below the rail is moving west.

It is called motion within motion or combined velocities. No part of the wheel is moving west, it is only moving east at a slightly smaller velocity.

 

[DoubleA]

You are forgetting about the compression effects of the tip. When accounting for tip compression....and subsequent spring-back, the cueball can actually move faster than the cue stick.

No it cannot. The tip compression actually absorbs energy and has the opposite effect.

 

[3andstop]

BUT... The argument is that the cb loses ALL foward momentum on a full hit. (which I believe it does) SO... the speed on approach has ZERO affect after impact. It stops dead in its tracks, and only moves forward or backward from any spin it retains.

Neil, yes it loses all forward momentum, but it doesn't lose all circular momentum. The ratio of forward momentum to circular momentum (i.e. 1 to 1 or 2 to 1 or whatever, is irrelevant.)

The speed of the circular momentum that remains is responsible for its continued forward movement regardless of the ratio.

 

[DoubleA]

Neil, yes it loses all forward momentum, but it doesn't lose all circular momentum. The ratio of forward momentum to circular momentum (i.e. 1 to 1 or 2 to 1 or whatever, is irrelevant.)

The speed of the circular momentum that remains is responsible for its continued forward movement regardless of the ratio.

ABSOLUTELY-----AGAIN.

 

[mikepage]

It is called motion within motion or combined velocities. No part of the wheel is moving west, it is only moving east at a slightly smaller velocity.

Part of the flange is actually moving west.

 

[Neil]

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[mikepage]

No it cannot. The tip compression actually absorbs energy and has the opposite effect.

8-baller is right.

The tip is storing energy like a spring does as it's compressed.

Halfway through the tip-ball contact , when the tip is fully compressed, the ball and stick are moving at the same speed. During the whole second half the ball is speeding up and the stick is slowing down. The tip is releasing it's stored energy during this part.

By the time the collision is over, the ball is going about 50% faster than the stick ever was.

 

[3andstop]

Part of the flange is actually moving west.

The entity is moving east. The same concept could be related to above and below the axle line as well as above and below the track base.

But I think, (not sure though) if a part of the train was actually moving west, on a long trip you might lose it.