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 no work because the point of the application of the force does not move while the force exists.

 
Choose the correct option:
1. (a), (d)
2. (a), (c)
3. (b), (d)
4. (a), (b), (c)

(b, d) Hint: Apply the work-energy theorem and the concept of work.

Step 1: Find the work done by all the forces.

When a man of mass m climbs up the staircase of height L work done by the gravitational force on the man is-mgl work done by internal muscular forces will be mgL as the change in kinetic energy is almost zero.

Hence, total work done = -mgL + mgL = 0

Step 2: Find the work done by contact force.

As the point of application of the contact, forces do not move hence work done by reaction forces will be zero.