An object is placed on a horizontal surface with friction. With a string we pull
ID: 1525947 • Letter: A
Question
An object is placed on a horizontal surface with friction. With a string we pull the object along the surface with constant speed
a) Net work done on the object is zero
b) Net work done on the object is larger than 0 (that it’s positive)
c) Net work done on the object is smaller than zero (that it’s negative)
d) Net work may be positive or negative depending of chosen positive direction
Since the speed is constant then the pulling force and the friction force must be equal. That is, no net force acting on object. I have asked similar question before and was told that the correct answer to a similar question should be b) net work done is positive since the force applied and the displacement is the same direction
The reason I am confused (I make it more difficult than it is probably) is that I am confused by the concept of net work. When I pull the string and it moves, I will do work on the object. But since the pulling force and friction is equal (since the speed is constant and there’s no acceleration) there’s no net force acting on the object. If there’s no net force acting, how can there be net work?
Explanation / Answer
From Work-energy theorem,
Net work done by all the forces = change in KE
as block is moving with constant velocity
hence change in KE = 0
So net work done = 0 .........Ans
Ans(A)