# Homework Help: Speed of object after collision related to spring

1. Oct 12, 2016

### songoku

1. The problem statement, all variables and given/known data
As shown in the figure below, object A (mass: m) is at rest on a smooth, horizontal surface, and a lightweight spring that compress / stretches in the horizontal direction is attached to it. Object B, which has the same mass m, approaches A from the left side with speed v and collides with the spring. The spring compresses and A begins to move. A, B, and the spring travel in a straight line. What is the speed of A when the spring is at maximum compression?

2. Relevant equations
conservation of energy
momentum?

3. The attempt at a solution
I tried using conservation of energy:

Kinetic energy of B before hitting the spring = elastic potential energy of spring at maximum compression + kinetic energy of A

But then I stuck because I don't know how to change the spring constant and maximum compression to other variables

2. Oct 13, 2016

### haruspex

so consider
Do the conditions for momentum conservation exist here?

3. Oct 17, 2016

### songoku

Yes, taking both boxes and spring as a closed system.

At maximum compression, object A and B will move with same speed?

Using conservation of momentum:
mAuA + mBuB = (mA+mB) V
V = 1/2 v

Is this correct?

4. Oct 17, 2016

### haruspex

That's it.

5. Oct 17, 2016

### songoku

Thanks a lot for the help