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?

The linear momentum is always conserved irrespective of the nature of collision. Since the gas molecule rebounds with the same speed, the rebound velocity of the wall is zero. Hence, the kinetic energy is conserved as there is no change in velocities. Thus, this is an elastic collision.

NOTE: Initial and final velocities of the molecule and wall remain same before and after the collision. Hence, the kinetic energies also remain same.