Introduction
The concept of work is closely related to the concept of energy. In fact, the concept of work provides a link between force and energy. The general idea of work and energy can be applied to a wide range of phenomena in different fields of physics. Further from the practical point of view, it is important to know not only the work done to an object but also the rate at which it is being done. This involves the concept of power.
Definition of Work
The traditional meaning of work is quite different from its scientific meaning. In our everyday activity, the term 'work' is used equally for mental and physical work. As we say the watchman is continuously standing at the gate and has become completely tired. So, he has done work.
Similarly, a student is preparing for his exam by continuously reading and practicing questions. So, we can say that she is working hard.
But scientifically speaking, both of them have not done any work because they have not changed their position in spite of the expenses of their force.
In physics, "work is said to be done if any object changes its position by the application of force on it."
Thus, for the work to be done,
- A force should be applied to the object and,
- The object should displace form its original position.
Some examples of work are lifting an object from the floor and putting it on the table, moving of cart due to the pulling of a bullock, pushing of a van, etc. mathematically, work is the product of the force applied on an object and displacement made by an object in the direction of the force.
i.e., Work (W) = Force (F) × Displacement (S)
∴ W = F × S
Hence, when the body covers the displacement of 1 meter by the application of 1 Newton force, then the work done is said to be 1-joule work.
Work done by Constant Force
The direction of the displacement of an object and the direction of the force can have different relations with each other. The directions of the force and the displacement may be:
- Same
- Opposite
- Perpendicular to each other
1. Force acting in the same direction of displacement
If the displacement of a body is the same as teh direction of the force, then the work done by the body is positive, e.g. a body falling under the effect of gravity.
2. Force acting in the opposite direction of the displacement
If the displacement of the body is in the opposite direction of the force then the work done by the body is negative, e.g. a ball thrown upwards against gravity, work done by the frictional force, etc.
3. Force acting in the perpendicular direction of the displacement
If the displacement of the body is perpendicular to the direction of the force then no work is said to be done, as there is no displacement in the direction to the force, e.g. when a block moving around a circular path, the work done by the centripetal force is zero.
Types of Work
There are mainly two types of work done. They are:
- Work done against gravity
- Work done against friction
1. Work done against Gravity
If the displacement of a body is opposite to the direction of the gravitational force of the earth then the work is called the work against gravity, e.g. lifting a load form the ground by applying some force.
2. Work done against Friction
If the displacement of the body is oppostite to the direction of the frictional force then the work is called work against friction. E.g. pushing a car to a certain distance, pulling a wooden plank to a certain distance.