An Alternate Solution Using Conservation of Momentum

Example 3: Bullet and Block

A bullet with a mass of 0.020 kg collides inelastically with a wooden block of mass 2.5 kg, initially at rest. After the collision, the bullet + block has a speed of 1.2 m/s. What was the initial speed of the bullet?


problem diagram

This problem is essentially the same as Example 1. Since no external forces act on the bullet + block system, the system's total momentum is conserved. The system's momentum before the collision is:

pbefore = m1v1 + m2v2 = m1v1

and the system's momentum after the collision is:

pafter = (m1 + m2)v

If momentum is conserved, then:

pafter = pbefore

(m1 + m2)v = m1v1

We want to know v1, so:

v1 = 150 m/s

The velocity of the bullet immediately before the collision was 150 m/s.

last update April 4, 2006 by JL Stanbrough