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  • A man, of mass m, standing at the bottom of the staircase, of height L climbs it and stands at its top. (a) Work done by all forces on man is equal to the rise in potential energy mgL.

A man, of mass m, standing at the bottom of the staircase, of height L climbs it and stands at its top.
(a) Work done by all forces on man is equal to the rise in potential energy mgL.
(b) Work done by all forces on man is zero.
(c) Work done by the gravitational force on man is mgL.
(d) The reaction force from a step does not do work because the point of application of the force does not move while the force exists.

Answers (1)

The answer is the option (b) and (d)

The gravitational force acts in a downward direction. The displacement is labelled L, which is in the upward direction. So, the work done on man due to gravitational force amounts to -mgL. Also, the work is done to lift the man also amounts to the force in the direction of displacement. Hence the net force amounts to -mgL+ mgL = 0. Hence, option (b) is correct.

The displacement at the point where the force acts are zero. Hence, the amount of work done by the force is also zero. So, KE =0 and hence option (d) is correct.

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