Question

Which of the following quantities is zero on an average for the molecules of an ideal gas in equilibrium?
(a) Kinetic energy
(b) Momentum
(c) Density
(d) Speed.

Answer 1

The molecules move in all possible directions in an ideal gas at equilibrium. Since momentum is a vector quantity for every direction of motion of the molecules, there exists an opposite direction of motion of the other. Hence, the average momentum is zero for an ideal gas at equilibrium.

Thus, (b) is the correct answer.