# Billiard Balls with English

Billiard Balls with "English"

I was just watching the classic episodes of The Mechanical Universe, and one scene explained how billiard balls rebound at right angles. I understand that.

But I wonder how http://en.wikipedia.org/wiki/Glossary_of_cue_sports_terms#English" causes the effects it does. Could someone explain it or point to some gentle information on the effect?

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Andrew Mason
Homework Helper

I was just watching the classic episodes of The Mechanical Universe, and one scene explained how billiard balls rebound at right angles. I understand that.

But I wonder how http://en.wikipedia.org/wiki/Glossary_of_cue_sports_terms#English" causes the effects it does. Could someone explain it or point to some gentle information on the effect?
English works because there is friction between the ball and the cloth on the table surface. The spin causes the cue ball to move in a curved path on the table after the collision. It does not affect the collision - that is determined by conservation of momentum and energy only.

AM

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rcgldr
Homework Helper

During collision, spin on the cue ball can result in a friction force from the felt, and that adds to the total force due to collision, which has some effect on the target balls velocity (speed and direction) after the collision. There's also some friction force from the felt on the target ball. I don't know the relative magnitude of the forces from the felt, but pool experts claim it's enough to require compensating for cue ball spin when they aim the cue ball.

Danger
Gold Member

I'm one of a very few local people who have cut an object ball over 90°; it is know in my area as a Danny-cut. Put a garbageload of opposite spin on the cueball and aim for the paint. There's a good chance of missing the thing entirely, but if you connect the effect is astounding. I won a trip to the World Finals in Las Vegas using that shot, about 20 years ago.

It does not affect the collision - that is determined by conservation of momentum and energy only.
AM
So that's different from a basketball (because the latter is squishy)? If I bounce a basketball with backspin, it does not reflect from the floor at the expected angle.

K^2

Yes. The fact that basketball deforms quite a bit means that collision time is significantly longer, and friction can transfer more momentum to the basketball.

Andrew Mason
Homework Helper

So that's different from a basketball (because the latter is squishy)? If I bounce a basketball with backspin, it does not reflect from the floor at the expected angle.
I was referring to the 90 degrees between the path of the object ball and the cue ball after collision. This is only the case in an elastic collision between two equal masses.

AM

Having played pool quite a bit in my time, I have learned that you can "throw" the object ball a little bit by putting either left or right spin on the cue ball. The effect is slight, but it can make a difference in a tight shot.

I envision it as happening because the spin of the cue ball at the point of contact will "nudge" the object ball in the opposite direction. The same thing happens when the cue ball strikes the rail with left or right spin. Except the rail is fixed and the friction is much higher, so the cue ball deflects at an angle. But the rail still experiences the force of the spin motion.

Andrew Mason
Homework Helper

Having played pool quite a bit in my time, I have learned that you can "throw" the object ball a little bit by putting either left or right spin on the cue ball. The effect is slight, but it can make a difference in a tight shot.

I envision it as happening because the spin of the cue ball at the point of contact will "nudge" the object ball in the opposite direction. The same thing happens when the cue ball strikes the rail with left or right spin. Except the rail is fixed and the friction is much higher, so the cue ball deflects at an angle. But the rail still experiences the force of the spin motion.
You are right that the spin of the cue ball can have a small effect on the direction of the object ball and of the cue ball (before the cue ball hits a rail - when it hits the rail, spin can have a significant effect). It could not have such an effect if there was no friction between the ball and the felt.

AM