Work, Energy and Power - Part II

2. Types of energy  
Energy is defined as the capacity of a body to do work. In the example of lifting a weight of 10 kg, your arms were expending energy to apply force and do work. The unit of energy is the same as that of work.

There are number of ways in which a body can do work. One of the most common ones is mechanical energy. Mechanical energy comes about due to a bodyís motion or its position. The energy that a body possesses because of its motion or velocity is known as kinetic energy. The energy that is possessed by virtue of its height is known as potential energy. The concept of potential energy is a bit difficult to understand.

To understand the concept of kinetic energy, consider a ball falling from a height s.

The work done by the gravitational pull on the ball = force of gravity on the ball x s

W = mg . s

Since the ball is falling, the work done is due to its motion.

Hence W = kinetic energy of the ball = mg . s

Using the third equation of motion v² = u² + 2as, putting u = 0 and a = g

We get

v² =  2 gs

gs  = ‡ v²

substituting we get kinetic energy of the ball = ‡ m v²

Kinetic energy is shortened and is written as K.E.

Thus K.E. = ‡ m v²

To understand the concept of potential energy, consider again the example of a box of 10 kg lifted from the ground to the table. We have seen that some work is done in lifting the box against the force of gravity. Your muscles have expended energy. This energy is stored in the box in the form that is called the potential energy. If you now drop the box from the table to the floor, the box might fall with a thud. Thus part of the energy stored in the form of potential energy is converted into another form of energy like the sound energy.

It is a convention that the potential energy of a body is said to be zero when it is on the surface of the earth. We cannot calculate the total potential energy of a body, but a change in its potential energy relative to its previous position can be calculated.  

The formula for potential energy is given by Work done to lift a body from the ground to a height h = Force of gravity x h Potential energy  = m g h The potential energy is shortened and written as P.E.

Another example of potential energy is a spring loaded wrist watch. In your grandfatherís time, a wrist watch used to run on a wound spring. Every day the spring had to be wound for the watch to function. The energy stored in the winding of the spring that is potential energy stored was able to run the watch. These days batteries have replaced the spring.

Other types of energy are solar energy, nuclear energy, wind energy, chemical energy, electrical energy etc. Heat, light, and sound are also types of energy but they can be categorized under mechanical energy itself. We will study about these types of energies in later chapters.

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