Discussion
Consider the two statements S1 and S2 given below and choose the correct option.
S1 - Non-zero work has to be done on a moving particle to change its momentum.
S2 - To change momentum of a particle, a non-zero net force should act on it.
Answer: D
S1 is false but S2 is true.
S2 is Newton's second law: the rate of change of momentum equals the net force, so momentum changes only if a non-zero net force acts.
S1 is false because momentum can change with no work done. In uniform circular motion, the centripetal force is always perpendicular to the velocity, so it does no work and the speed stays constant, yet the direction of motion, and hence the momentum, keeps changing. Work changes kinetic energy, not necessarily momentum.
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