Question

State to examples of zero work.

Answer 1

Hint: Zero work is done when the displacement of the body is perpendicular to the force acting on it.

Complete step by step answer:
According to Newton’s first law of motion everybody in the universe is at rest or in uniform motion (constant velocity) until an unbalanced force acts on it. In other words, for a body to change its state of rest or uniform motion, an unbalanced force is to be applied on it. We can also say that when an unbalanced force is exerted on a body, the body accelerates.
When a body accelerates, there is a change in its energy i.e. kinetic energy. Change in kinetic energy occurs when some work is done on the body. Work done(W) on a body is defined as the product of the force applied(F) on the body and the displacement(s) of the body parallel to the direction of that force and is given by $W=Fs\cos \theta $, where $\theta $ is the angle between the force and the displacement of the body.
Look at the expression for work. Even for the same magnitudes of F and s, the value of work will be different depending on the angle between them. If $\cos \theta =1$, maximum amount work can be done on the body i.e. if $\theta =0$ or the directions of F and s are parallel. If $\cos \theta =0$ then the work done on the body will be zero i.e. when the angle between the force and the displacement is $90^o$. When the work done is zero, it is called zero work.

Examples of zero work:
1) A simple example of zero work is when you stand holding a bag in your hands and do not move it. Your hands apply a force on the bag to balance the force of gravity exerted on it but since there is no displacement of the bag, the work done on it by you (your force) and also the gravity is zero.
2) When a body is in uniform motion, the work done on it is zero. As told earlier, work done on a body is a change in its kinetic energy. Therefore, if the body is in uniform motion i.e. constant velocity, the kinetic energy of the body is constant. This implies that no work is being done on the body.
3) When a body is moving on a horizontal surface, work done by the gravity is zero since the direction of the gravitational force exerted on the body is downwards and perpendicular to the direction of the displacement of the body.

Note: In example (3), it is explained that the work done by the gravitational force on the body is zero. This does not imply that the total work done on the body is zero. It is said that the body is moving on a horizontal surface. If it's accelerating that means a force is being applied on it and there will be work done on the body by this force. Therefore, the total work done on the body is not equal to zero.