
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: 20000324   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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