Question

Work done by the force of friction
A) Can be positive
B) Can be negative
C) Can be zero
D) All of the above

Answer 1

Hint: This problem can be solved by recalling the definition of positive work done by a force on an object and that it occurs when the displacement of the body is in the direction of the force. Negative work is when the displacement of the body is opposite to the direction of the force.

Complete answer:
The work done is the product of the force exerted on the body and the displacement of the body in the direction of that force. Positive work is done by a force when the displacement of the body is in the direction of the force applied whereas negative work is done by the force when the direction of the displacement of the body is opposite to the direction of the force. Zero work is done when the displacement of the body in the direction of the force is zero.

Now, let us analyze the options one by one.

The work done by frictional force will be positive when the displacement of the body is in the direction of the force. This can be understood by an example. Consider there are two blocks stacked one over the other. If the lower block starts to move slowly in one direction, then there is a frictional force between the two blocks that prevents the sliding of the two blocks. This force acts on the upper block in the direction of motion of the lower block. The upper block also moves along with the lower block, that is, in the direction of the frictional force. Hence, the work done by friction is positive in this case.

The work done by friction can be negative. For example, when a block is sliding over a rough ground, the frictional force acts in a direction opposite to the direction of motion of the block and tries to bring it to a stop. Since, the direction of motion of the block and the direction of frictional force is opposite to each other, the work done by friction is negative.

The work done by friction can also be zero. For example, if we try to push a very heavy block on a rough floor, the block does not move. This is because the force of static friction opposes our force. However, since the block does not move, the displacement is zero. Therefore, zero work is done by friction.

Therefore, friction can do positive, negative as well as zero work.

So, the correct answer is “Option D”.

Note:
Students must note that generally friction is considered to be a dissipative force in nature that generally acts opposite to the direction of motion of an object and tries to decrease its kinetic energy. Hence, in general friction does negative work. However, in some special cases as discussed in the answer above, friction can also do positive and zero work which might not seem so intuitive at the first thought.