# Kinetic energy problem

1. Jun 16, 2008

### emeraldempres

On a frictionless, horizontal air table, puck A (with mass 0.245 kg) is moving toward puck B (with mass 0.360 kg), that is initially at rest. After the collision, puck A has a velocity of 0.115 m/s to the left, and puck B has velocity 0.660 m/s to the right.

Calculate the change in the total kinetic energy of the system that occurs during the collision.

I tried putting it all in to the change of kinetic energy equation but it does not work, this is my equation

.5(.245 kg)(-.155^2)+ .5(.36)(.66^2)= .08 J

But that answer is wrong, can smeone help?

2. Jun 16, 2008

### Staff: Mentor

What's the initial velocity of puck A? Use that to find the initial KE of the system.

3. Jun 16, 2008

### emeraldempres

The initial velocity i believe to be 0 m/s because the answer does not specify. using this, since the ke is equal to .5mv^2, the initial ke is 0 J. am i on the right track?

4. Jun 16, 2008

### calef

Here, you have to solve for the initial conditions of the system. Puck A had some velocity that you can solve for using conservation of momentum, so its kinetic energy is not zero.

Puck B's initial kinetic energy is zero, though. But that's only important insofar that all of the initial momentum is in puck A.

5. Jun 16, 2008

### Staff: Mentor

The initial speed cannot be zero, otherwise there would be no collision:
Make sure you post the complete problem exactly as it was given.