Question

In which of the following situations is no work being done by the object exerting the force?

• A. An elevator floor is exerting a force perpendicular to the bottom of a man's shoes while the man is moving up.
• B. A person is applying a normal force to a box to push it along a horizontal surface at a constant speed.
• C. A person is holding a string that is exerting a force on a ball attached to the other end. The ball is moving in a circular path.
• D. More than one of the preceding answers is correct.
• E. A person is slowly lowering a brick to the floor.

Answer 1

Answer: (C)

We know work done by a force is given by ,

$W=Fd\cos \theta$ ,

(A) The angle between force exerted by elevator on man's shoes and displacement of shoes is 0 , therefore $W=Fd\ne 0$ .

(B) The angle between force exerted by man on box and displacement of box is 0 , therefore $W=Fd\ne 0$ ,(though net work done is zero) constant velocity means force exerted by man equals the frictional force .

(C) The angle between force exerted by man and displacement of ball is 90 degree, therefore $W=Fd=0$ .

(D) The angle between force exerted by person on brick and displacement of brick is 180 , therefore $W=-Fd\ne 0$