Class 9 Science Solutions English Medium

11. Work and Energy

Introduction:

• All living beings need food. Living beings have to perform several basic activities to survive. We call such activities ‘life processes’.
• The energy for these processes comes from food. We need energy for other activities like playing, singing, reading, writing, thinking, jumping, cycling and running. Activities that are strenuous require more energy.
• Machines also require energy to work, for which diesel and petrol are used.

Work: The work done on a body is defined by the result of the force applied on it and the product of the distance fixed by it in the direction of the force.

Work = Force × Displacement

W = F × S 

S.I Unit of Work: 

Force is measured in newtons (N) and displacement is measured in meters (m). 

By the above formula we gets 

S.I unit of works => N (Newtons) × m (meter) = Nm 

This Nm is known as Joule (J). 

Therefore work is a type of energy so S.I Unit of work is Joule (J).

Work is a scalar quantity: by the definition of work, Work is the product of force (a vector quantity) and displacement (a vector quantity). Whereas work is a scalar quantity because work has magnitude but does not have direction. It is a scalar quantity similar to energy.

Note: All works done by you are not scientifically work.

Our Which actions are work? 

Activities like preparing for examination, reading books, drawing a picture, discussing on problems etc use a large amount of energy but according to scientific definition you can see there happens a very few work. 

Examples of some our activities: 

(i) Suppose you are pushing a very large rock with force, if in spite of your millions of efforts, the rock does not move, then this work will not be considered because the force applied did not cause the displacement of the object while the energy was spent too much.

(ii) We do many physical and mental tasks in daily life such as - playing in the field, interacting with friends, humming a tune, watching cinema, deep discussion on a subject etc. But all these tasks will not be understood as work.

(iii) Push a pebble lying on a surface. The pebble moves through a distance. You exerted a force on the pebble and the pebble got displaced. In this situation work is done.
(iv) A girl pulls a trolley and the trolley moves through a distance. The girl has exerted a force on the trolley and it is displaced. Therefore, work is done.

(v) Lift a book through a height. To do this you must apply a force. The book rises up. There is a force applied on the book and the book has moved. Hence, work is done.

SCIENTIFIC CONCEPTION OF WORK:

When we displace an object to apply a force then work will be done. 

Therefore, two conditions need to be satisfied for work to be done:

Conditions for work:

(i) a force should act on an object, and

(ii) the object must be displaced.

If any one of the above conditions does not exist, work is not done. This is the way we view work in science.

Define 1 Joule of Work:

When an object is displaced 1 m applying 1 N of force along the line of action of
the force it is called 1 Joule of work has done.

1 J = 1 N x 1 m

1 J = 1 Nm