Work, Energy and Power - Part I

We have seen how force and acceleration of a body is inter-linked. A force produces motion. We have also seen Newtonís laws of motion that give us the definition of force, momentum, inertia etc. In this chapter we will see how force can be converted into energy. We will also see what are the different types of energy available in our daily lives and how one type of energy can be converted into another.

What we will study in this chapter : 
Ý 
1. Definition of work and its units 
2. Types of energy 
3. Transformation of energy 
4. Definition of power and its units 

1. Definition of work and its units  
In Physics, the word ìworkî has a totally different meaning than what we imply in our everyday usage. ìWorkî does not mean any physical or mental activity. In fact the word ìworkî has a precise definition.

Work is defined as force into displacement. 
             
W  =  F   .   s 
                                                                         
where  W is the work done, F is the force applied and  s  is the displacement that occurs due to the applied force.

Work is a scalar quantity. If the displacement is in the direction of the applied force, then W is positive. On the other hand if the displacement produced is opposite to the direction of the applied force, then W is a negative quantity.

The following example will illustrate the definition of work more clearly.

Suppose you have to lift a box that weighs 10 kg from the floor to the table which is at a height of 2 m.

You know that your hands are feeling the force that you have to apply to do this task. The box can be lifted to a higher position because your hands have done some work on it : put a force to keep it at a height of 2m.

Units of force in the M.K.S. system is Joules (J). Unit of force and displacement in the M.K.S system is Newton (N) and meter (m) respectively. That is when a force of 1 Newton displaces a body through one meter, work done is said to be 1 joule (or 1 Newton-meter)

W = F.s

1 Joule = 1 Newton x 1 meter

1 Joule = 1 Nm

Unit of work in the C.G.S. System is an erg.

1erg = 1 dyne x 1 centimeter

1 J = 10^ñ7 erg

In the above example, the mass of the box = 10 kg. 
The gravitational acceleration on the box = 9.8 m/s².

The force therefore is given by

F = m.g  = 98 N

The work done for lifting the box is

W = F. s  = 98 N x 2 m

    = 196 N-m  = 196 Joules

Since work is a dot product, no work is done if the displacement is perpendicular to the applied force. Also if force applied produces no displacement, then we say that no work is done.

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