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I am doing a report on physics in pool (billiards) for my science class which is due this Friday. I was wondering if you could give me any information on how momentum (such as the conservation of momentum) and collision has to do with physics, and how knowing about how they are connected would help me in my pool game. Any information would be very helpful.
Question Date: 2000-03-24
Answer 1:

First, a part of physics is the study of the motion and collision of large (versus subatomic) particles such as pool balls. Once theories and formulas for that are worked out, then they can be modified to account for the somewhat different behavior of subatomic and/or relativistic (close to the speed of light) objects. So far as pool goes, if you study how the movement of pool balls is driven by the momentum and collisions of the balls and bumpers, then you can apply the formulas in a predictive fashion. So, then if you know how the balls are hit, you can say (roughly (due to the additive effects of small uncertainties)) where they will end up. So, if you can do that, contemplate how that could be useful if you know where you would like the balls to end up and need to know what momentum in what direction (how hard and where to hit it) to give the cue ball.

Answer 2:

The momentum of a ball is its mass times its velocity.In a collision with another ball, they exert a force on each other called an impulse. Newton's third law of Physics says that the forces things exert on each other are always equal in strength and opposite in direction. This means that the impulse is also equal and opposite. The connection between impulse and momentum is that impulse also equals the change in momenutm of each ball. If the change in each is exactly the opposite of the other, when you add them together, the change in the total momentum must be zero. If the change in total momentum is zero, then the total momentum must be the same before and after the collision. This is called conservation of momentum.

Let's say you're trying to put the 8 ball in the corner pocket. After you hit the cue ball, its momentum is mass times velocity. The momentum of the 8 ball is zero because it is not moving yet. If you hit it straight on and the cue ball stops at the collision, then all of momentum of the cue ball must go into the 8 ball because momentum is conserved. If you hit it at an angle, both balls keep moving after the collision. If you add the momentum of the balls together, it must equal the momentum of the cue ball before the collision. The 8 ball travels at an opposite angle from the direction you hit it, so you can always direct where you want it to go, by hitting it at the correct angle.

Answer 3:

Understanding how conservation of momentum and collisions work probably would be helpful to your pool game to some extent but there are some complications. When playing pool you have to take into account that the balls might be sliding, rolling, and/or spinning. In addition, you have to worry about friction and irregularities in the pool table surface. So in the end, if you were to sit down and try to calculate what should happen when you hit the cue ball it could be a very difficult problem.

We all have an intuition about how things work from living in a world that is governed by the laws of physics. If you practice playing pool a lot you will get better because you will have done a lot of "experiments" to learn the physics of pool (including the physics of you hitting the ball). I believe that if you know more about the physics of momentum conservation (both linear and angular), collisions, and friction that you will have a greater appreciation of the complexities of the game and hopefully enjoy playing more.

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