# Homework Help: Two particle system involving conservation laws

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1. Sep 28, 2014

### ghostfolk

1. The problem statement, all variables and given/known data

A billiard ball of mass M is initially at rest on a horizontal frictionless table. Another ball
of mass m < M and velocity $\vec{v}$ in the positive x-direction hits the first ball in a pefectly elastic
collision. After the collision, the balls move with (unknown) velocities $\vec{U}$ and $\vec{u}$ respectively
(not necessarily in the x-direction).
Find the maximum amount of kinetic energy $∆T$ that the second ball can impart on
the first ball.

2. Relevant equations
$T=\frac{1}{2}mv^2$
$\rho_i=\rho_f$
$T_i=T_f$
3. The attempt at a solution
I'm entirely stuck and I'm not sure how to tackle this problem. Any help is appreciated.

2. Sep 29, 2014

### Orodruin

Staff Emeritus
You have written down some conservation laws, did you try applying them?

3. Sep 29, 2014

### ghostfolk

The thing is I'm not entirely sure how to. I can derive some basic things such as due to conservation of momentum $m\vec{v}=m\vec{u}+M\vec{U}$ and due to the system be isolated $\frac{1}{2}mv^2=\frac{1}{2}(mv^2+MU^2)$. After this I'm not sure where to go to get the maximum kinetic energy trasnferred.

4. Sep 29, 2014

### BvU

You mean $\frac{1}{2}mv^2=\frac{1}{2}(m{\bf u}^2+MU^2)$, right ?

So there are three equations, but four unknowns. That leaves one degree of freedom.
What is the kinetic energy you 'want' to maximize? Write it as a function of this one free variable left over.
Then try to find a maximum for that function.

5. Sep 29, 2014

### ghostfolk

Well since the object in question is the first ball, we have to find the a function of $v$ right? We can't use $u$ because that is the velocity after impact.

6. Sep 29, 2014

### BvU

No such freedom exists: $\vec v = (v_x, v_y)$ is a given ! $v_y=0$ and $v_x$ will appear in the answer, but as a parameter, not as a variable.

7. Sep 29, 2014

### ghostfolk

Could you explain that in more detail?

8. Sep 30, 2014

### ehild

The kinetic energy before impact is 1/2 mv2. After impact, it is 1/2 mu2+1/2 M U2, u is the speed of the ball with mass m and U is the speed of the billiard ball of mass M.

Write up the conservation laws also for the momentum components. Choose a coordinate system with x axis parallel with the initial velocity v of the the small ball and express the components with the angle they enclose with the x axis.

ehild

9. Sep 30, 2014

### ghostfolk

Okay, so when exactly can I find the maximum kinetic energy? I'm thinking if it's when the velocity of m is zero.

10. Oct 1, 2014

### ehild

The masses are given, and so is the velocity of the small ball. But the collision is not necessarily central. The direction of the velocities are the free parameters, but the conservation laws make them related, so only one angle is free.

Find the kinetic energy of the billiard ball after the collision, as function of its angle it encloses with the x axis. You can figure out at what angle is the KE maximum.

ehild

11. Oct 1, 2014

### ghostfolk

[QUOTE="ehild, post: 4868343, member: 481" You can figure out at what angle is the KE maximum.

ehild[/QUOTE]How exactly would I do that?

12. Oct 1, 2014

### BvU

Play with coins n a smooth table to get ideas.

13. Oct 1, 2014

### ghostfolk

Okay, but how exactly would one determine the angle where there is maximum kinetic energy transfer. That part doesn't make sense to me. How would a drawing of vectors show that?

14. Oct 1, 2014

### ehild

The kinetic energy will depend on some angle. To have maximum, its derivative with respect to the angle should be zero.
Drawing the velocity vectors help you to write up equations for the momentum components.

ehild