Work And Energy

In our daily life work means "to do anything". But in science, work has specific meaning, i.e. when force is applied and some distance is covered. For example, a man carrying a physics book is doing work but he is not doing work if he is not moving while keeping the physics book on his head. Scientifically, work is done only when an effort or force moves an object. When work is done, energy is used. Thus, work and energy are related to each other. The concept of energy is an important concept in Physics. It helps us to explain the changes that occur when work is done This unit deals with the concepts of work, energy and power.

6.1 WORK

Work is said to be done when a force displaces a body in its own direction. When an object moves distance S in the direction of applied force F (figure 6.1), then work done W is given mathematically as

Work - done = Force x Displacement W=Fxd

6.1

Work is a scalar quantity and in the International System of Units (SI), work is measured in joules (J). One joule is equal to the work done by a one-newton (1 N) force acting over a one-metre (1 m) distance.

1J = 1N x 1m

SCIENCE TIDBITS

X

VECTOR REPRESENTATION

Force is not always applied perfectly in the direction of motion. For example consider a toy car which is moving in horizontal direction and force 'F' is applied making certain angle '0' with the horizontal. In such situations the force is resolved into its rectangular components as shown in figure 6.2.

CAN YOU TELL?

A man is pushing the truck but truck is at rest, is he doing work? Explain.

EXAMPLE 6.1: WORK DONE

Muhmmad Tuaha was pushing a box with a force of 100 N on a flat frictionless surface. How much work he does on the box if he pushes it through a displacement of 200 m?

GIVEN

Force 'F' = 100 N

Distance 'd' = 200 m

REQUIRED

Work 'W' =?

SOLUTION

By the definition of work: W=Fxd Putting values:

W = 100N x 200m

Therefore,

W = 20,000 J = 20 kJ

Answer

Muhammad Tuaha does 20 kJ of work on the box.