Can a pool ball come off a cushion faster than it hits it?

[adr147]

Physics question help required. Can a pool ball come off a cushion faster than it hits it? - something in my distant memory tells me this breaks one of Newtons laws or something like that.

 

[PeroK]

Physics question help required. Can a pool ball come off a cushion faster than it hits it? - something in my distant memory tells me this breaks one of Newtons laws or something like that.

Yes, if there is sufficient spin involved. Imagine you play a screw shot so that the ball almost stops before it hits the cushion. It will rebound off the cushion faster than it was moving when it hit the cushion.

There are other possibilites: if someone shakes the table slightly(!) Things like that.

 

[adr147]

Where does the extra energy come from?

 

[PeroK]

Where does the extra energy come from?

The kinetic energy associated with the spin.

 

[adr147]

I see, now is that a theoretical possibility or does it actual happen in practice?

 

[PeroK]

I see, now is that a theoretical possibility or does it actual happen in practice?

If you can generate enough sidespin or backspin, you should be able to make it happen.

 

[anorlunda]

With enough backspin can't you make the ball move forward, stop, then move backward without hitting anything? That should make it clearer how backspin can be converted to linear motion.

 

[CPW]

There are other KE of rotation + KE of translation examples to elucidate the above explanations to your question. Consider the rolling down an incline of a sphere vs hoop vs solid disc. Thinking of that example helps me understand how a billiard ball's spin prior to collision with the cushioned wall could be related to a faster rebound translational speed than the translational speed prior to the collision.

Thanks for the interesting question and discussion.

 

[HallsofIvy]

If you hit a ball with backspin, directly at a cushion, when it hits the side, its forward momentum is converted into backward momentum (with a little lost to friction since the cushion is not "perfectly elastic") but the backspin can also be converted into linear backward momentum so that the ball comes off the side faster than it hit it.