Question

Two blocks are initially at rest with a compressed spring between them. When the spring is released, the 2m block moves to the left and the m block moves to the right. This represents an inelastic collision in a closed system.

A diagram of two square masses sitting on a flat horizontal surface connected by a spring. The mass on the left is labeled 2 m and the mass on the right is labeled m.
What best describes the blocks after the spring is released?

The total energy is zero.
The total momentum is zero.
The momentum of the m block is zero.
The momentum of the 2m block is zero.

Answer 1

The total momentum is zero.

Reason: No external horizontal force acts, so momentum is conserved. Initially the system is at rest so total momentum is 0; after release the two blocks have equal and opposite momenta (so they cancel). The individual momenta are not zero, and the total energy is not zero (spring potential is converted into kinetic energy).