Q. In terms of Physics, which among the following situations result in work done?
1) Brakes are applied to a moving vehicle and vehicle stops
2) A ball is projected vertically upward and it comes back due to force of gravity
3) A body moves in a circular path
Select the correct option from the codes given below:

Explanation by: Jitendra Singh

When a body is acted upon by a force, there are two conditions when work done is zero. Firstly, the force must not cause displacement in its own direction; secondly, the force must act at right angle to direction of displacement. However, the work can be positive or negative. Positive work is done, when the angle between the direction of force applied and the displacement of body is zero. For example, when a horse pulls a cart or an engine pulls a train, the positive work is done. Negative work is done, when the angle between the direction of the force applied and the displacement is 180° i.e., the displacement is in a direction opposite to the force. For example, when brakes are applied to a moving vehicle, the work done by the applied force is negative. Similarly, when a ball is projected vertically upward, the work done by the force of gravity is negative. When a body moves in a circular path, no work is done. It is because, the centripetal force acting on the body is always at right angles to the displacement of the body along the circular path.
Thus, cos 90° = 0
So, in the expression
W = F cos 90° × S = 0 × S = Zero

At the same time, we note that when satellite revolves around the earth in a circular orbit. Work done by the force due to gravity is zero, because it acts at right angles to the direction of displacement of the satellite