Question

A 0.0135-kg bullet is fired straight up at a falling wooden block that has a mass of 4.02 kg. The bullet has a speed of 809 m/s when it strikes the block. The block originally was dropped from rest from the top of a building and had been falling for a time t when the collision with the bullet occurs. As a result of the collision, the block (with the bullet in it) reverses direction, rises, and comes to a momentary halt at the top of the building. Find the time t.