PHYSICS-WORK ENERGY AND POWER

CLASS IX(2020-21)

RESOURCE PERSON-DEVIKA GANDHI

Q 1 Define work.Give its SI unit.

Ans 1 Work is said to be done by a force on a body if the force applied causes a displacement in the
body.SI unit of work is joules(J)

Eg work is done when we hit a football

When a box is lifted from ground to some height

Q 2 What are the two factors on which work done by a body depends?

Ans 2 1.Force applied

2.displacement travelled by a body on application of force

Q 3 Derive the formula of work done under following conditions.

WORK DONE BY A CONSTANT FORCE

(i) When a constant force is applied in the horizontal direction

Let a constant force F is applied on a wooden block placed at position A.Suppose that the block moves
to new position B so that its displacement is s

A s B

Work done is given by W=F*s

Let a force F is applied on a wooden block at an angle ʘ with the horizontal direction.Let block moves
horizontally and occupies new position B so that it travels a distance s horizontally.

Fsinʘ

Fcosʘ

A B

Work done is given by

W=Fcosʘ*s

W=Fs cosʘ

Q 4 Define positive, negative and zero work done.

Ans Positive work done

Work done by a force on a body is said to be positive if force and displacement acts in same direction.

i.e angle between force and displacement is zero

ʘ=0

W=FScos 0 (cos0=1)

W=FS

Examples

1. When a boy is throwing ball in upward direction then he is applying force in the upward direction and
the displacement of the ball is also in upward direction then work done by boy in throwing ball is
positive.

2. When we are push a box in forward direction and box is also moving in forward direction.Hence work
done is positive.

3. In game of tug of war, work done by winning team is positive because direction of force and
displacement is same.

Negative work done

Ans Work done by a force on a body is said to be negative if force and displacement acts in opposite
direction.

i.e angle between force and displacement is 180o

W=FS cos ʘ

ʘ=180o

W=FScos 180 (cos180=-1)

Examples

1. When a boy is throwing ball in upward direction, the direction of gravitational force acting on ball is in
downward direction and displacement of ball is in upward direction so work done by force of gravity on
ball thrown up is negative.

2. When we are pushing box in forward direction then force of friction is acting in backward direction
and displacement of box is taking place in forward direction.So work done by force of friction on box is
negative.

3. In game of tug of war, work done by losing team is positive because direction of force applied is
opposite to the direction of displacement .

Zero work done

Work done by a force on a body is zero when force acts at right angle to displacement or when the
force applied does not produce displacement in the body.

i.e angle between force and displacement is 90

W=FS cos ʘ

ʘ=90

W=FScos90 (cos90=0)

W=0

Or

S=0

W=F*0=0

Examples
1.When a coolie is carrying a luggage on his head and moving in forward direction then work done by
the coolie against the weight of the luggage is zero as weight of the luggage is acting vertically
downward and motion is along horizontal direction. So angle between force and displacement is 90o.So
work done is zero.

F=W=mg

3. When a person is carrying packet in his hand he is applying force on it in upward direction to hold it
and displacement is taking place in forward direction so work done by person on packet will be zero
4. When moon moves revolves around earth, then the centripetal force is provided by the gravitational
force of the earth which acts towards the centre and the motion is along the tangent to the circular
path. As force and displacement are at right angles to each other , so work done by gravitational force of
earth on the moon is zero.

5. Work done by force of gravity on a luggage lying on roof of moving bus is zero because force and
displacement are at right angles to each other.

6. When we apply force on heavy rock but it does not move then also work done is zero as displacement
is zero.

Q 5 Define I J of work.
Ans

W=FS

IJ=IN*1m

Work is said to be 1 J when a force of 1N displaces a body by 1m.

ENERGY
Q 6 Define energy and give its SI unit.

Ans The capacity of doing work is known as energy

→ Working body losses energy and body on which work is done gains energy.

→ Energy is a scalar quantity.

Unit: The SI unit of energy is Joule (J) and its bigger unit is kilo joule (kJ). 1 kJ = 1000 J
Heat energy
Chemical energy
Electrical energy
Light energy
Sound energy
Nuclear energy

Q 7 Define kinetic energy and give some examples

And The energy possessed by body due to its motion is known as kinetic energy.

• Examples of kineticenergy
→ A moving cricket ball
→ Running water
→ A moving bullet
→ Flowing wind
→ A moving car
→ A running athelete
→ A rolling stone
Q 8 Derive expression for kinetic energy of an object.

Ans

If an object of mass ‘m’ moving with uniform velocity ‘u’, it is displaced through a distance ‘s’. Constant
force ‘f’ acts on it in the direction of displacement. Its velocity changes from ‘u’ to ‘v’.

Then acceleration is ‘a’. Work done, W = F × s (i)

F = ma (ii)

According to third equation of motion, relationship between u, v, s and a is as follows: v2 - u2 = 2as

⇒ s = (v2- u2)/2a (iii)

Now putting the value of f and s from (ii) and (iii) in equation (i),

If u = 0 (when body starts moving from rest) W =½mv2

KE=½mv2

NOTE:If initial velocity of body is not zero then work done on the body is given by

W=FS (i)

F = ma (ii) (from newton’s second law of motion)

According to third equation of motion,

⇒ s = (v2- u2)/2a (iii)

Now putting the value of F and s from (ii) and (iii) in equation (i),

WD=1/2mv2 -1/2mu2

Work done=final kinetic energy-initial kinetic energy

This is known as work energy theorem.

Q 9 Derive relation between kinetic energy and momentum

KE=1/2mv2

Multiplying and dividing eqn 1 by m

KE=1/2m2v2/m=1/2m(mv)2=1/2mp2=p2/2m

KE=p2/2m

Q 10 Define potential energy and give its SI unit.

Ans The energy possessed by any object due to its position(height),shape or size is known as potential
energy.Its SI unit is Joule(J).
Q 11 Define mechanical Energy

Ans It is the sum of kinetic and potential energy of an object. Therefore, it is the energy obtained by an
object due to motion or by the virtue of its location. Example, a bicycle climbing a hill possesses kinetic
energy as well as potential energy.

Q 12 Derive expression for gravitational potential energy of an object.

Ans When an object is raised through a height, work is said to be done on it against gravity. The
energy possessed by such an object is called the gravitational potential energy.

Consider a body with mass m, raised through a height h, from the ground,
Force required to raise the object
= weight of object mg.
GPE = work is done in raising a body from the ground to a point against
gravity

NOTE:Gravitational potential energy does not get affected due to the path taken by the object to
reach a certain height.
 h2

h1

work done in (i) case work done in (ii) case

w=mgh w=mg(h1+h2)=mgh

Work done in both the cases (i) and (ii) is same as a body is raised from position A to B, even if the
path taken is different but the height attained is the same.

Q 13 What do you mean by transformation of energy?

Ans The change of one form of energy to another form of energy is known as transformation of energy

Example:

1. A stone on a certain height has entire potential energy. But when it starts moving downward,
potential energy of stone goes on decreasing as height goes on decreasing but its kinetic energy goes on
increasing as velocity of stone goes on increasing.
At the time stone reaches the ground, potential energy becomes zero and kinetic energy is maximum.
Thus, its entire potential energy is transformed into kinetic energy.

2.At hydroelectric power house, the potential energy of water is transformed into kinetic energy which
is then converted into electrical energy.

3. Plants use solar energy to make chemical energy in food by the process of photosynthesis.

4.When an arrow is released from bow then potential energy of stretched bow is converted into kinetic
energy of moving arrow

Q 14 State law of conservation of energy.

Ans Energy can neither be created nor destroyed, it can only be transformed from one form to
another.
Q 15 Explain law of conservation of energy taking an example of pendulum

Position of bob KE PE
At position B(extreme position) min max
From B to A Increases decreases
At position A(mean position) max min
From A to C decreases increases
At position C(extreme position) min max

Q 16 Law of conservation of energy for a particle falling from certain height.

.
At position A
Height is maximum so PE is also maximum
Velocity is zero as object is at rest so Kinetic energy is minimum
PE=MAX
KE=MIN

As the object falls down height keeps on decreasing so PE will keep on decreasing and velocity increases as the
object falls down due to earth’s attraction so KE will keep on increasing
PE=DECREASING
KE=INCREASING

At position C
Height is zero so PE will be zero or minimum
And velocity will be maximum so KE will be maximum there
PE=MIN
KE=MAX

So we have concluded that if one form of energy increases then other form of energy decreases so that total
energy remains constant.

POWER
Q Define power and give its SI unit.

Power is the rate of doing work.

P=Work done/time=Energy/time

SI unit-Watt

Q Define 1 watt

Ans P=WD/t

1watt=1joule/sec

Power is said to be 1 watt when 1 joule of work is done in 1second.

NOTE-

1.

Commercial unit of power is kilowatt(kw)

Commercial unit of energy is kilowatt-hour(kwh)

P=WD/t=F*s/t

P=F*v

Q Give relation between SI unit of energy and commercial unit of energy.

Ans

1kwh=?

1kw=1000W

1h=3600s

1kwh=1000W*3600s

=36*105 Ws

1kwh=3.6*106 J (since P=WD/t

W=J/s ,therefore Ws=J)