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Work done, W = F × s = mg × s raised hammer falls on a nail placed on a piece
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= 15 kg × 10 m s × 1.5 m of wood, it drives the nail into the wood. We
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= 225 kg m s m have also observed children winding a toy
= 225 N m = 225 J (such as a toy car) and when the toy is placed
Work done is 225 J. on the floor, it starts moving. When a balloon
is filled with air and we press it we notice a
Q 1. When do we say that work is when the force is withdrawn. However, if we
change in its shape. As long as we press it
uestions
gently, it can come back to its original shape
done?
press the balloon hard, it can even explode
2. Write an expression for the work
producing a blasting sound. In all these
done when a force is acting on
an object in the direction of its
different means, the capability of doing work.
displacement. examples, the objects acquire, through
An object having a capability to do work is
3. Define 1 J of work. said to possess energy. The object which does
4. A pair of bullocks exerts a force the work loses energy and the object on which
of 140 N on a plough. The field the work is done gains energy.
being ploughed is 15 m long. How does an object with energy do work?
How much work is done in An object that possesses energy can exert a
ploughing the length of the field?
force on another object. When this happens,
energy is transferred from the former to the
10.2 Energy latter. The second object may move as it
receives energy and therefore do some work.
Life is impossible without energy. The demand Thus, the first object had a capacity to do
for energy is ever increasing. Where do we work. This implies that any object that
get energy from? The Sun is the biggest possesses energy can do work.
natural source of energy to us. Many of our The energy possessed by an object is thus
energy sources are derived from the Sun. We measured in terms of its capacity of doing
can also get energy from the nuclei of atoms, work. The unit of energy is, therefore, the same
the interior of the earth, and the tides. Can
as that of work, that is, joule (J). 1 J is the
you think of other sources of energy?
energy required to do 1 joule of work.
Activity _____________10.5 Sometimes a larger unit of energy called kilo
joule (kJ) is used. 1 kJ equals 1000 J.
• A few sources of energy are listed
above. There are many other sources
10.2.1 FORMS OF ENERGY
of energy. List them.
• Discuss in small groups how certain Luckily the world we live in provides energy in
sources of energy are due to the Sun. many different forms. The various forms
• Are there sources of energy which are include mechanical energy (potential energy
not due to the Sun?
+ kinetic energy), heat energy, chemical
The word energy is very often used in our energy, electrical energy and light energy.
daily life, but in science we give it a definite
and precise meaning. Let us consider the
following examples: when a fast moving Think it over !
cricket ball hits a stationary wicket, the wicket How do you know that some entity is a
is thrown away. Similarly, an object when form of energy? Discuss with your friends
raised to a certain height gets the capability and teachers.
to do work. You must have seen that when a
116 SCIENCE
Rationalised 2023-24
