Question

A molecule in a gas container hits a horizontal wall with speed 200 $m\,s^{-1}$ and angle 30° with the normal, and rebounds with the same speed. Is momentum conserved in the collision ? Is the collision elastic or inelastic ?

Answer 1

Yes; Collision is elastic.
The momentum of the gas molecule remains conserved whether the collision inelastic. The gas molecule moves with a velocity of $200 \,m/s$ and strikes the stationary wall of the container, rebounding with the same speed. It shows that the rebound velocity of the wall remains zero.

Hence, the total kinetic energy of the molecule remains conserved during the collision. The given collision is an example of an elastic collision.