# Elastic collistions

1. Jul 16, 2008

### spidey12

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

Kevin has a mass of 89.5 kg and is skating with in-line skates. He sees his 25.50 kg younger brother up ahead standing on the sidewalk, with his back turned. Coming up from behind, he grabs his brother and rolls off at a speed of 2.13 m/s. Ignoring friction, find Kevin's speed just before he grabbed his brother.
2. Relevant equations

1/2m1v012+1/2m2v022=1/2m1vf12+1/2m2vf22

3. The attempt at a solution

I have tried many ways of doing this problem, and I keep arriving at the same answer: 2.41 m/s which is incorrect. I then decided to pretend that the kid and his brother are one unit so i added their masses together multiplied times the 2.132 divided by 2 and set that equal to the left side of the above equation. However i again got 2.41. It seems that every way i try this problem i get 2.41

What am i doing wrong?

2. Jul 16, 2008

### LowlyPion

Aren't you supposed to be conserving momentum? Isn't that simply m x v ?

Last edited: Jul 16, 2008
3. Jul 16, 2008

### spidey12

I am an idiot. I read waaaay to much into this problem. I got it. Thank you.