Question

Justify by giving proper reasoning whether the work done in the following cases is positive or negative:

(i) Work done by friction on a body sliding down an inclined plane. 

(ii) Work done by a man in lifting a bucket from a well.

Answer 1

(i) When the body will slide down on an inclined plane then the frictional force will acts upwards to the inclined plane, so the angle between the motion of the body and the frictional force will be. 

Since, Work= FS cos θ 

F = force 

S = displacement 

θ = angle between direction of force and direction of motion = 180^o 

So, work done = F×s cos 180^o

Cos 180^o = -1 

So work done = - F×s 

Work done by friction on a body sliding down an inclined plane will be negative. 

(ii) In this case the man is doing work against the gravitational force, by applying force in an upward direction to lift the bucket. Since the direction of force applied and the direction of motion of bucket is the same, work done will be positive. 

Work= FS cos θ 

θ = angle between direction of force and direction of motion = 0^o 

Cos 0^o = 1 

So, work done = F × s