# Elastic Collisions?

1. Nov 19, 2013

### kimlu

1. The problem statement, all variables and given/known data
A 25g object moving to the right at 20cm/s overtakes and collides elastically with a 10g object moving in the same direction at 15 cm/s. Find the velocity of each object after the collision

2. Relevant equations
Ʃp=Ʃp'
v1+v1'=v2+v2'

3. The attempt at a solution
First I tried to find one one of the v primes by using the momentum equation

25(20)+10(15)=25v1'+10v2' and then simplifying it to
500+150=250(v1'+v2')
650-250v1'=v2'

Afterwards I plugged this in for v2' in v1+v1'=v2+v2' and solved through to get 2.57 cm/s for v1' which I know is incorrect. I know I did something wrong in the step above but I don't know how to approach this.

2. Nov 19, 2013

### ehild

The red part is wrong.

ehild

3. Nov 19, 2013

### kimlu

I'm not quite sure how to simplify that part it would help if you would tell me how :c

4. Nov 19, 2013

### ehild

25v1'+10v2' is not 250(v1'+v2'). Isolate v2' from the equation 25v1'+10v2'=650, and plug in for v2' in v1+v1'=v2+v2'.

ehild