# Pool Players: Answer this collision question!

1. Feb 23, 2014

### gauss44

What is the physics explanation of why, when you play pool, and one ball slams into another ball, does the ball originally in motion become stationary and the other ball takes on all the velocity?

2. Feb 23, 2014

Not always.
You can use momentum to solve these kind of problems

3. Feb 23, 2014

### dauto

If the two balls have the same mass, the collision is head on, energy is conserved, and the cue ball wasn't spinning, the only way to conserve both energy and momentum is if the cue ball stops and the other ball takes all the velocity. Notice all the ifs.

4. Feb 23, 2014

### sophiecentaur

Newton's cradle is a more ideal example of the same thing happening. You can do the same thing with coins on a smooth table, when a moving one hits a stationary one.

In 'words', a fair explanation is that momentum and kinetic energy are both conserved in these near-ideal (elastic) collisions. The total momentum is (always) conserved so it is carried by just the moving ball, initially. In elastic collisions, no Kinetic Energy is lost and the parting velocity is the same as the approach velocity. For those two conditions to be met, the second ball must have all the velocity (momentum), so the first ball must be left stationary.

Spin and the nature of the pool table mean that the above rule of thumb is not always followed.
There is another useful rule of thumb in these situations and that is, the second ball will always travel away from the impact along the 'line of centres' of the two balls. That's because the only force on the balls is radial from the contact point.

5. Feb 23, 2014

### bahamagreen

Getting the cue ball to stop dead at collision is one of the primary techniques in pool. The way it is done is by ensuing that the cue ball is not spinning at contact with the target ball.

Hitting the cue ball, by striking high, middle, or low, will determine whether it spins forward or backward immediately after the strike. A backspin means that the ball is sliding and rotating backward before transitioning to rolling forward. The skillful player who wants to stop the cue dead at collision makes the cue ball's transition point between backspin and rolling forward coincide with the event of contact.

By adjusting this back and forth, he can also make the cue ball roll slightly forward or backward after contact to assume a desired resting position, and likewise for shots where the collision is offset to make these motions at an angle (except you can't do the dead stop on an angle collision, but you can control the resting position of the cue ball).

If you mark some stripes on the cue ball and hit it low, you will see that for the first part of its motion forward the cue ball is spinning backwards, then friction slows this until it is sliding without rolling for just an instant, then it begins to take a forward rolling motion.

The dead stop is when the cue hits the target when the cue is in the sliding non-rolling state of its motion.