Question

A rubber bullet of mass m is fired from a rifle into a stationary block of 25 m. The bullet remains in the block and both the bullet and the block move off with a common velocity. Assume this is an inelastic collision in a closed system.

Which best describes the final momentum of the bullet/block?

The final momentum of the bullet/block is zero.
The final momentum of the bullet/block is equal to the initial momentum of the bullet.
The final momentum of the bullet/block is less than the initial momentum of the bullet.
The final momentum of the bullet/block is more than the initial momentum of the bullet.

Answer 1

The final momentum of the bullet+block is equal to the initial momentum of the bullet.

Reason: momentum is conserved in this closed system. If the bullet (mass m, speed v) hits a stationary block (mass 25m) and sticks, the combined mass is 26m and final speed v_f = (m v)/(26m) = v/26. The final momentum is (26m) v_f = m v, which equals the bullet's initial momentum.