# A ball with a mass of #4 kg# moving at #6 m/s# hits a still ball with a mass of #9 kg#. If the first ball stops moving, how fast is the second ball moving? How much kinetic energy was lost as heat in the collision?

##### 1 Answer

We can answer this question by recognizing that momentum is conserved, but kinetic energy is only conserved in a fully elastic collision.

#### Explanation:

The momentum before the collision is all in the 4 kg ball:

Since momentum is conserved, and the first ball comes to a stop, all the momentum goes to the 2nd ball:

So the second ball moves away at

The full equation, should it be needed, is:

Now, the second part of the equation asks about energy, so we need to find the difference between the two kinetic energies:

or the kinetic energy of the 9 kg ball after the collision, minus the kinetic energy of the 4 kg ball before the collision:

So 40 J of kinetic energy is lost to entropy in the collision.