question_answer 49)

                  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.                

Answer:

                  (b, d)                 (b) Work down on man by internal forces (muscular effort)                 \[=+mgL\]                 Work down on man by gravity \[=-mgL\]                 Work down on man by friction = 0                 Thus, work done by all forces on man \[=+mgL-mgL=0\]                 (d) The option is self-explanatory.