NCERT Book Class 9 Science Chapter 10 Work and energy

C hapter 10
WORK AND ENERGY
In the previous few chapters we have talked draws diagrams, organises her thoughts,
about ways of describing the motion of collects question papers, attends classes,
objects, the cause of motion and gravitation. discusses problems with her friends, and
Another concept that helps us understand and performs experiments. She expends a lot of
interpret many natural phenomena is ‘work’. energy on these activities. In common
Closely related to work are energy and power. parlance, she is ‘working hard’. All this ‘hard
In this chapter we shall study these concepts. work’ may involve very little ‘work’ if we go by
All living beings need food. Living beings the scientific definition of work.
have to perform several basic activities to You are working hard to push a huge rock.
survive. We call such activities ‘life processes’. Let us say the rock does not move despite all
The energy for these processes comes from the effort. You get completely exhausted.
food. We need energy for other activities like However, you have not done any work on the
playing, singing, reading, writing, thinking, rock as there is no displacement of the rock.
jumping, cycling and running. Activities that You stand still for a few minutes with a
are strenuous require more energy. heavy load on your head. You get tired. You
Animals too get engaged in activities. For have exerted yourself and have spent quite a
example, they may jump and run. They have bit of your energy. Are you doing work on the
to fight, move away from enemies, find food load? The way we understand the term ‘work’
or find a safe place to live. Also, we engage in science, work is not done.
some animals to lift weights, carry loads, pull You climb up the steps of a staircase and
carts or plough fields. All such activities reach the second floor of a building just to
require energy. see the landscape from there. You may even
Think of machines. List the machines that climb up a tall tree. If we apply the scientific
you have come across. What do they need for definition, these activities involve a lot of work.
their working? Why do some engines require In day-to-day life, we consider any useful
fuel like petrol and diesel? Why do living physical or mental labour as work. Activities
beings and machines need energy? like playing in a field, talking with friends,
humming a tune, watching a movie, attending
10.1 Work a function are sometimes not considered to
be work. What constitutes ‘work’ depends
What is work? There is a difference in the on the way we define it. We use and define
way we use the term ‘work’ in day-to-day life the term work differently in science. To
and the way we use it in science. To make understand this let us do the following
this point clear let us consider a few examples. activities:

10.1.1 NOT MUCH ‘WORK’ IN SPITE OF Activity _____________ 10.1
WORKING HARD!
• We have discussed in the above
Kamali is preparing for examinations. She paragraphs a number of activities
spends lot of time in studies. She reads books, which we normally consider to be work