UNIT 5: WORK,
ENERGY & POWER
Work
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We
do so many activities in our daily life, but according to scientific and
technical definition of work all activities are not work
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A
watch man does his duty in a gate.
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Here
neither force is applied nor is distance covered.
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An
officer does his duty sitting on a chair whole day in his office.
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In
this case also, neither force is applied nor is distance covered.
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Porter
caries a heavy load on his back and carries to bus stop.
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Here
force is applied to the load but the distance is not covered by load.
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Force
is applied here in upward direction (against gravity) but distance is covered
in horizontal direction
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When
you push a wall, wall remains in its position.
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Here
force is applied but the distance is not covered.
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So,
all above mentioned activities are not work as per scientific definition.
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When
you push a car and it moves for a distance, then it is work done.
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Force
is applied by pushing car and the car covers distance in the direction of force
applied.
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When
you pick up your bag from the floor of your room and keep it on the table, then
it is also work done.
-
You
apply force in upward direction in bag and bag also covers the distance in the
upward direction.
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So,
for work, two conditions must be satisfied:
-
i.
Force must be applied.
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ii.
Distance must be covered in the
direction of force applied.
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So,
Work done is defined as the distance
covered by a body in the direction of force applied.
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Mathematically,
Work done (W) = Force (F) x Distance (d)
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The
SI unit of work done is Joule (J).
1J Work
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W
= F x d
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or,
W = 1N x 1m\
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or,
W = 1Nm
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or,
W = 1J
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When a body covers 1m distance by
applying 1N force in the direction of force applied, then the work done is
known as 1J work.
Types of Work done
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Work
done can be categorized into two types:
-
i.
Work against friction
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ii.
Work against gravity
Work done against friction
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When
you push a table, does it pushed easily? There is some resistance you feel,
which known as friction.
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In
above mentioned example of work, when you push a car, then there is work done,
which against friction.
-
So,
When a body covers a distance by pushing
or pulling, then the work done is known as work done against friction.
Work done against Gravity
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The
earth pulls any objects downward due to gravity.
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When
you lift a body upward, then the work done is against gravity.
-
In
above mentioned example of work done, when you pick up the bag from the floor
of your room and keep it on the table, then the work done is against gravity.
-
So,
When a body covers a distance in upward
direction by lifting, then the work done is known as work done against gravity.
-
Mathematically,
Work done against gravity (W) = m x g x
h.
-
Where,
m = mass of the body, g = acceleration due to gravity and h = height the body
covers.
Energy
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Can
an electric water pump pull water by itself? No, it requires electricity.
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Can
a vehicle move by itself? No, it requires petrol or diesel.
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Can
we do any work without taking food for a long time? No, we need food.
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In
all above examples, electricity, diesel or food are sources of energy.
-
If
there is no energy in a body, no any work can be performed.
-
That’s
why, The capacity to do a work by a body
is known as energy.
-
The
SI unit of energy is Joule (J).
Forms of Energy
-
There
are so many forms of energy, which are given below:
-
Mechanical
Energy (2 types): Energy possessed by moving or rest body.
o
Kinetic
Energy
o
Potential
Energy
-
Electrical
Energy
-
Light
Energy
-
Heat
Energy
-
Magnetic
Energy
-
Nuclear
Energy
-
Sound
Energy
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Chemical
Energy
Mechanical Energy
-
The
energy possessed by a body due its position of rest of motion is known as
mechanical energy.
Kinetic Energy
-
The
energy possessed by a body due to its motion is called as kinetic energy. For
example: moving water can rotate turbine to produce electricity.
-
Mathematically,
K.E. = (1/2)mv2 , Where, m = mass of the body and v = velocity of
the body.
Potential Energy
-
When
you stretch the rubber of catapult, what happens to the rubber?
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What
about the water stored in a dam?
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When
you lift a stone up to some height and store it, what happens to it?
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In
all above cases, rubber of catapult, water of dam and lifted stone have stored
energy, which is known as potential energy.
-
That’s
why, The energy possessed by a body due
its position is known as potential energy.
-
Mathematically,
P.E. = m x g x h, Where, m = mass, g
= acceleration due to gravity, h = height.
Electrical Energy
-
The
energy possessed by a body due to continuous flow of electrons in known as
electrical energy.
Light Energy
-
Light
is the form of energy which gives the sensation of vision.
-
Sun
is the main source of Light energy
Heat Energy
-
The energy possessed by a body
due motion of molecules is known as heat energy.
-
Rub
your hand sharply for some minutes and feel it.
-
You
will feel some heat, because molecules of your hand got in motion due to
rubbing and heat energy is produced.
Magnetic Energy
-
The
energy possessed by a magnet is called magnetic energy.
Nuclear Energy
-
The
energy possessed by a body due the nuclear fission or nuclear fusion is known a
nuclear energy.
Sound Energy
-
The
energy possessed by a body due to vibration of its molecules is known as sound
energy.
Chemical Energy
-
The
energy released due chemical reaction is known as chemical energy.
Conservation of Energy
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Principle
of conservation of energy states that, “ Energy neither can be created nor can
be destroyed, it can only be changed from one form to another form.”
Transformation of Energy
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When
you switch on, fan rotates and bears kinetic energy.
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When
you switch on, bulb glows and bears light energy.
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When
you switch on, heater glows and bears heat energy.
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When
you speak on microphone, sound is converted in electrical energy.
-
So,
the process of conversion of one form of
energy into another form of energy is known as transformation of energy.
Power
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If
person A can carry 50 bricks to the top of 5 steroid building in 5 minutes and
B can carry 12 minutes, then obviously A has more power.
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Power
depends upon time taken to perform a work.
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So,
the rate of doing work is known a power.
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Mathematically,
Power(p) = Work done(w)/Time taken(t)
o
or, p = Fxd/t
o
or, p = mgh/t
-
SI
unit of Power is Watt (W).
-
Power
can also be measured in terms of horse power (HP).
-
1
HP = 746 watt.
1 Watt Power
We know that, P = w/t
or, P = 1J/1s
-
or, P = 1 Watt
-
So,
If 1J work is done in 1s, then the power
is defined as 1 watt power.
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