Nothing
can move or work without energy. We need energy for our survival. The
amount
of work we do everyday depends on our energy level. We get energy from the
food
we take. Plants need energy for growth. Engine also needs energy for its
functioning.
Some engines use electricity and some need fuel for energy. Energy is
stored
in fuel.
What
do we mean by energy? The energy of a body means its ability to work. So the
body
which is able to work has energy in it and the body that does not have energy
in it
cannot
work.
displacement
(
time )2
(displacement)2
(
time )2
ML2
T2
Given,
Mass
of man, m = 70kg
Force,
F = weight of the man =
mg
=
70kg × 9.8 ms-2
=
686N
Displacement,
S = 200m
Work,
W =?
Physics
71
When
we say a body has energy in it we mean the body can apply force on other and
can
work.
Again the amount of work done on a body is equal to the energy we use.
The
energy of a body refers to its ability to work. Here work means the
transformation of
energy
from one form to another. It means that the total amount of work that a body
can
do
is its energy. The amount of work a body can do is the measure of its energy.
So, the
amount
of work done is the amount of energy used.
Therefore,
Work done = Energy used
Energy
has no direction. So it is a scalar quantity.
The
unit of energy and work is the same and it is joule (J).
Different forms of energy
We
need different types energy for doing different kinds of work. For example we
need
heat
to boil water. We get light energy from an electric bulb. There is sound energy
in
the
music we hear. We need muscular energy to shift or to lift any object. Electrical
energy
is necessary to operate an electrical device. We get chemical energy by
chemical
reaction
in the electric cell. A piece of paper flies due to energy of air. Nuclear
energy is
released
when the atoms are accumulated or broken.
The
universe is in motion as there is energy. If no energy existed the universe
would be
motionless.
As there is light energy we can see and hear because of sound energy. We
can
move for mechanical energy. Fan rotates and factory runs with the help of
electric
energy.
Energy exists in the universe in different forms.
Generally
we observe the following forms of energy. Such as, mechanical energy, heat
energy,
sound energy, light energy, magnetic energy, electrical energy, chemical
energy,
nuclear
energy and solar energy.
The
most common form of energy is mechanical energy. The energy that is stored in a
body
due to its position or motion is called mechanical energy. In this lesson we
will
discuss
two forms of mechanical energy-kinetic energy is produced due to motion and
potential
energy is produced due to the position of object.
Kinetic Energy: We sometimes notice that cricket ball hits the stamp and
strikes it
down.
If anything hard hits the glass of window the glass breaks down. If we throw
stones
at mango or jujube it may fall down.
From
the above example we can see energy exists in the body in motion. The capacity
of
doing
work acquired by a moving body due to its motion is called kinetic energy.
Do it yourself: Keep a pen on a table or a desk in front of you. Put a light
object before the
pen.
Hit the pen by your finger towards the object.
Why
does the object displace from its initial position? This is because the hit
makes the
pen
move and the pen obtains the ability to work that is kinetic energy is produced
in it.
So
it could displace the object.
Creating
velocity in a body in rest or increasing the velocity of a moving object means
to
produce
acceleration in it. For this force has to be applied. As a result work will be
done
on
the body. For this the body will obtain the ability to work and this work will
be stored
72
Physics
in
the object as kinetic energy. This is why all moving objects are in possession
of
kinetic
energy. The body will be able to perform this amount of work before it rests.
Let
a force F be applied on a body of mass m at rest. The body attains a velocity v.
Suppose
the body moves a distance s
in the direction of the force. The work
done to
produce
this velocity of the body is its kinetic energy.
Therefore,
Kinetic
energy = Work done
=
Force × displacement
=
F × s
or,
Ek = mas; [ as, F = ma]
Fig:
4.2
But,
v2 = u2 + 2as
or, as = v2
2 ; [since, initial velocity, u =
0]
∴ Ek
=
1
2 mv2 … … … (4.2)
∴ Kinetic energy = 1
2 × mass × (velocity)2
Kinetic
energy depends on the mass of object. The more is the mass the higher is the
kinetic
energy. With the same velocity a light tennis ball and a heavy cricket ball are
thrown
at you. The hit will be more by the cricket ball than that of tennis ball.
Kinetic
energy also depends on the velocity of objects. The more is the velocity the
higher
will be the kinetic energy. The damage will be less if a truck hits a wall with
less
velocity
but the damage will be more in case of higher velocity.
Mathematical Example 4.2: The kinetic energy of a runner of mass 70 kg is 1715 J.
What
is his velocity?
We
know,
Ek = mv2
or,
v2 =
∴ v = ( )
=
( )
=
7 ms-1
Ans:
7 ms-1
1
2 2Ek
m
2Ek
m
2 × 1715 J
70 kg
Here,
Mass,
m = 70 kg
Kinetic
energy, Ek =
1715 J
Velocity,
v =?
Physics
73
Potential Energy:
If
a piece of stone or brick falls on a body from the roof of a building it may
flatten or
break
the body. When the stone or brick was in rest on the roof potential energy was
stored
in it but the potential energy works when it falls down. The energy was stored
in
the
stone because it was above the ground.
What
will happen when the two ends of a spring is stretched and tied to two objects
and
then
released? The objects will move fast and collide with each other. The stretched
spring
was at rest but potential energy was there in it. If it is released it may
work. The
energy
was stored in the stretched spring because it was strained.
The
ability of a body to do work when its normal position or configuration is
changed to
some
other position or configuration is called potential energy.
Expanded Activities: Take a pulley and place a rope on it. Tie
a
heavy object A with its one end and a light object B with the
other
end. Such that, A remains above the ground but B
remains
on the ground [Fig: 4.3]. Remove your hand.
What
did you see? The object A goes down and object B goes up.
The
potential energy was stored in the object A as it was above the
ground
from its normal position and gained ability to work. It can
work till
it reaches the ground that is it can raise the object B.
Experiment: Take a spring and tie its one end with a strong support and
a block to its other
end.
Place them on a smooth surface. Now apply force on the block and contract the
spring and
keep
another object in front of the block [Fig: 4.4] then remove your hand.
Why
did the object move fast? Spring could work while regaining its original
configuration
and was able to displace the other body. This ability of
spring
to work for the change of its normal configuration is its
potential
energy. If some work is done against the force at the time of
changing
from normal position or configuration to some other
position
or configuration then the body obtains the capacity of doing
work
that is same amount of energy is stored in it. This principle is
applicable
within the sphere of influence of conservative force such
as
electric force, magnetic force, spring force etc. This sphere of
influence
is called the field of that force such as gravitational field,
electric
field etc. We work against the force of gravity when we lift
anything
higher from the ground. As a result the object obtains some
amount
of potential energy. It can perform the same amount of work
when
it falls on the ground.
If
a body of mass m is raised to a height h (fig:4.5) above the surface of the
earth, the
work
done in such a process is a measure of potential energy stored in the body. In
this
Fig:
4.3
Fig:
4.4
Fig:
4.5
74
Physics
case,
the work done is the product of the applied gravitational force i.e. the weight
of the
body
and the vertical height.
∴ Potential energy = weight of the body ×
vertical height
=
mgh
∴ Ep = mgh (4.3)
i.e.
potential energy = mass of the body × acceleration due to gravity × vertical
height
Potential
energy depends on the vertical height of the object from the surface of the
earth.
The more is the height, the higher will be the potential energy. Potential
energy
also
depends on the mass of the object. The more the mass the more will be the
potential
energy
of the object.
To
use the potential energy stored in a body it is necessary to transform it into
other form
of
energy. For example, a piece of stone is not dangerous as long as it is on the
roof and
its
potential energy transforms into kinetic energy i.e. it starts to fall.
