## Introduction

Category : 9th Class

### Introduction

In this chapter we shall study about the various concepts of work and energy. All the living organisms needs energy to perform several activities to survive. Work has different meaning in different aspect of life. Scientific notation of the work is that when a force is applied on an object and the object moves through a distance, we can say that work is done. The work done by a force is equivalent to the product of force and displacement of the body in the direction of force.

$\mathbf{W=F\times D}$

Where, F is the force and D is the displacement of the object.

When the displacement is in the direction of force, it is called positive work and if the displacement is in the opposite direction of the force, it is called negative work.

For example:

When we throw a ball in the upward direction, its displacement is in upward direction, whereas the force due to gravity acts in downward direction, hence we can say that work done is negative.

$\mathbf{W=}-\mathbf{F\times D}$

On the other hand, when the ball falls from a certain height both the displacement and the gravitational force acts in downward direction. Here the work done is positive.

$\mathbf{W=F\times D}$

Unit of Work

We know that work is the product of force and displacement. The SI unit of force is Newton and that of displacement is meter, so the unit of work is Newton - meter which is also equal to joule.

One joule of work is defined as the work done by a force of 1 newton, when it displaces a object by a distance of 1 meter in the direction of force. If the displacement of the object takes place in the direction perpendicular to the direction of force, then work done by the force on the object is zero.

For example:

The work done by the lift moving upward direction or by a potter carrying a load moving on a platform is zero.

When the displacement takes place at an angle to the force. Let us consider an object of mass 'm' sliding on an inclined surface making an angle ' Q' with the direction of force 'F' acting on the object. Then the work done by the force on the object is given by $\mathbf{W=F\times d}\times \mathbf{cos}\,\text{ }\!\!\theta\!\!\text{ }$.

A small child pushes a desk using a force of 10 N. Find the work done by this force if the desk is displaced to a distance of 6 meters.

(a) 40 joules

(b) 50 joules

(c) 60 joules

(d) 80 joules

(e) None of these

A boy throws a ball in upward direction that reaches a height of 10 m. If the mass of the ball is 980 gm, find the work done by the force of gravity.

(a) -96.04 joules

(b) 96 joules

(c) 98.08 joules

(d) -98 joules

(e) None of these

A child pull a block by applying a force of 15 N at an angle of 60°. Find the work done in pulling the block by a distance of 20 meters.

(a) 120 joules

(b) 130 joules

(c) 140 joules

(d) 150 joules

(e) None of these