Question

A student lifts an object in the upward direction. In doing so, he applies the force on the object in

the upward direction and displaces it in that direction: (However, the force of gravity is also

acting on the object.)

(a) State the direction in which force of gravity is acting on it.

(b) Which one of these forces is doing positive work? Give reason.

(c) Which one of these forces is doing negative work? Give reason.

Answer 1

a) The direction in which the force of gravity works is downward direction.
b) The force of gravity is doing positive work, because, the force pulls down and the value of theta is 0 . So, cos theta=cos 0 degree=1. Thus, work done by force of gravity is positive.
c) The student does a negative work, because, the angle between lifting the body and force of gravity is 180 degree. Thus, cos theta=cos 180 degree= -1.
Hence, the work done by the boy is negative.

Answer 2

Answer:

a. The force of gravity is in the downward direction.

b. The work done by the student is positive work because the force applied by him is along the direction of displacement. (cos 0° = 1 ; i.e, positive)

c. The work done by gravity is negative work because the force applied by gravity is opposite to the direction of displacement. (cos 180° = -1 ; i.e, negative)