Question

A 10-g bullet of unknown speed is shot horizontally into a 2-kg block of wood suspended from the ceiling by a cord. The bullet hits the block and becomes lodged in it. After the collision, the block and the bullet swing to a height 30cm above the original position. What was the speed of the bullet? (This device is called the ballistic pendulum). Take

Answer 1

the height of the swing is a change in potential energy of the combined bullet\block system after the collision

The change in potential energy must be EQUAL TO the change in kinetic energy that the system had before the collision.

(since the block wasn't moving before the collision, all the kinetic energy must have been in the bullet)