# Homework Help: Elastic ball collision.

1. Oct 24, 2012

### DatAshe

1. The problem statement, all variables and given/known data
A ball of mass 0.206 kg with a velocity of 1.54 m/s meets a ball of mass 0.291 kg with a velocity of -0.396 m/s in a head-on, elastic collision.
(a) Find their velocities after the collision.
1f = m/s
2f = m/s
(b) Find the velocity of their center of mass before and after the collision.
cm, before = m/s
cm, after = m/s

2. Relevant equations
conservation of momentum? p=mv, pf=pi+I, KEf=KEi (because its elastic), Vcm=Ʃmivi/Ʃmi

3. The attempt at a solution
I have spent hours fiddling around with equations and I can't get anything to result in a correct answer. I am stumped and frustrated. This is past homework and I am just looking for an explanation on how I get from this information to the final answers. Step by step would be much appreciated, especially if you use calculus because I am lost and I am only to derivatives in calc I so far. Thank you for any and all help!

2. Oct 24, 2012

### haruspex

Yes, conservation of energy and of momentum.
What is the total momentum before? Total energy before?
In terms of the unknown final velocities, what are the energy and momentum after collision?
What equations do you get?

3. Oct 25, 2012

### DatAshe

wouldn't the before and after be the same do to conservation of momentum and energy?
so mivi^2=mfvf^2 and mivi=mfvm?

4. Oct 25, 2012

### AJ Bentley

You have to calculate the total momentum and energy.

That is the momentum of both balls added together. The sum total is the same before and after the collision. m1*v1i + m2*v2i = m1*v1f + m2*v2f

Same with energy.

That gives you a couple of equations with two unknowns (v1f and v2f) to solve.

Last edited: Oct 25, 2012