Question

On a horizontal frictionless table, masses A and B (3 kg, 6 kg) slide to the right and left, respectively. They have speeds of 3 m/s and 1 m/s, respectively. The two masses collide, and bounce off each other. After the collision, they travel in opposite directions at speeds of 1.5 m/s and 1.250 m/s, respectively.

Calculate the kinetic energy of the A+B system:
before collision:
after collision

Answer 1

They tell you the masses of both and their speeds before and after the collision. All you have to do is calculate and add up the kinetic energies (1/2) M V^2 of each mass, before and after.

They have already provided velocities that satisfy the momentum conservation requirement, so you don't have to apply conservation laws to do this problem.