How Do You Calculate Initial Speed in an Inelastic Collision?

[pstfleur]

1. Kevin has a mass of 87kg and is skating with in-line skates. He sees his 22-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 3.4 m/s. Ignoring friction, find Kevin's speed just before he grabbed his brother.



2. VF(ma+mb)=MTVF



3. From the problem I know it is an inelastic collision..
I did v= 87*v+22*2.4/109
v=2.4 which is wrong. the answer is suppose to be 3.0

Please help.. I really don't think I've grasped the idea of collisions

 

[Kurdt]

You have the correct equation in the second part. I will rewrite it for you.

$$Mv=(M+m)v_f$$

All you have to do is rearrange and find v.

The answer should also be greater than 3. How can he gain speed if he picks up his little brother as well.