Work Power and Energy: X Physics Chapter 8

Q1. Define Work done.
Ans: WORK DONE:
When a force acts on a body, it moves the body in the direction of its displacement in
Science, it is said that work is done.
OR
The product of force and displacement is called WORK.
Work = Force x Displacement
W = F.d
Work is scalar quantity and it depends upon force and displacement.
Q2. Give unit of work and energy in different systems.
Ans:
SYSTEMS UNITS OF WORK
M.K.S.System
S.I. System
C.G.S System
F.P.S. or B.E.System Joule
Joule
Erg
Lbs - Foot




Q3. Derive the equation of work done. Also explain maximum and minimum work
done.
Ans: DERIVATION FOR WORKDONE:
Consider the following diagram.
F F

F

In the above figure. Force “F” divided into two components vertical Fy and horizontal Fx.0 is the angle between them. So the dot product of Fx and displacement “S” is given by:
W = Fx. S _____________ (i)
We know that
Cos 0 = base / hyp
Cos 0 = Fx / F

Put this Fx = F Cos 0 in equation (i)
W = F Cos 0 S
W = FS Cos 0

The above equation is called work done equation. Where Cos 0 is an angle between F and S.
MAXIMUM WORKDONE:
When the angle between force and displacement is “0”, then the work done is
maximum.
W = FS Cos 0 O = 0
W = FS. Cos 0 Cos 0 = 1
W = F.S(1)
W = FS
MINIMUM WORKDONE:
When the angle between Force and displacement is 90.
W = FS Cos 0
W = FS Cos90
W = FS (0)
W = 0
Q4. Define energy. Write its types.
Ans: ENERGY:
The ability of a body to do work or over come resistance is called Energy.It is a scalar quantity. Its unit is same as work.
Examples:
i. We use the chemical energy of coal, oil and gas released in the form of heat to
drive steam turbines and internal combustion engines.
ii. .Electrical energy is used in electric heater which appears in the form of heat
energy.
iii. Wind mills transfer the energy of wind into mechanical energy which is used for
pumping water from the wall as well as for mill in grains or sawing timber.
iv. In Pakistan nuclear energy is used for the production of electricity.
TYPES OF ENERGY:
i. Mechanical energy.
ii. Chemical energy.
Iii .Electric energy.
iv. Heat energy.
v. Light energy.
vi. Nuclear energy.
vii. Solar energy.
Viii .Magnetic energy.
ix. Wave energy.
x. Geothermal energy.
Q5. What is meant by Kinetic energy? Prove that K.E = ½ mv2.
Ans: DEFINITION:
The energy possessed by a body by virtue of its motion is called Kinetic energy it is denoted by K..E.. Kinetic energy of body depend upon the velocity of body, as the velocity is greater kinetic energy is also greater.
Equation for Kinetic Energy:
K.E = ½ mv 2
i. Let body having mass “m” at rest.
W = Fs
F V
s
ii. Now an acceleration “a” is produced by the application of force “F”. So,
according to Newton 2nd Law.
F = ma
a = F/m ____________ 1
iii. When body starts from rest. So its initial velocity Vi = 0. So, according to
equation of motion.
Vf 2 - Vi 2 = 2aS __________2
Put the value of “a” and Vi in equation (ii)
Vf 2 - (0)2 = 2 (F/m) S
Vf 2 = 2F S
M
½ Vf 2 = F S
F.S = ½ Vf 2
Where F.S is amount of work done which is changed into Kinetic Energy. So F.S=K.Ej
And Vf 2 = Vi 2
K. E = ½ mV2
This called equation of Kinetic energy

Q6. Define Potential Energy. Drive its equation.
Ans: POTENTIAL ENERGY:
The energy possessed by a body by virtue of its position or mechanical condition is
called its Potential Energy. The work done against already present force on body is
called potential energy.

EXAMPLES:
i. If we move a body against gravitational force then the work done is called
Potential Energy.
ii. In winding of the spring of a clock, we do certain amount of work on it l.e., we
spend certain amount of energy. This energy is stored in the spring in the form
of Potential Energy.

DERIVATION:
Consider a body whose mass “m” and which is at vertical height “h” from the ground.
so the weight of a body.
W = mg
The work done is raising the body through the height “h” is
W = F x h ____________ 1
But here weight is just equal to force. So,
F = mg

Put the value of “F” in equation 1.
W = F h
W = mgh

Where “W” is according potential energy of the body. So, W = P.E
P.E = mgh
This is called equation of potential energy
Q7. Differentiate between Kinetic Energy and Potential Energy.
Ans: KINETIC ENERGY POTENTIAL ENERGY
1. The energy possessed by body by virtue
of its motion is called Kinetic Energy.
2. Kinetic energy is depend upon the
velocity of body.
3.It can be calculated by the following formula.
K.E – 1 mv2
4. It is directly proportional to the mass and
square of velocity. 1.The energy possessed by a body by virtue
of its position is called Potential Energy.
2. It is increased with increase in height.

3 It can be calculated by the following formula
P. E = mgh
4. It is directly proportional to mass,
Gravitation and height.
Q8. What is meant by Law of Conversation of Energy? Give examples.
Ans: LAW OF CONSERVATION OF ENERGY:
Statement:
“Energy may change its form but it can neither be created nor be destroyed.”
Examples:
i. When an electric current passes, through a ceiling fan, the fan starts rotating.
Electrical energy is converted into mechanical energy.
ii. In an automobile, burning of diesel and petrol releases chemical energy which
makes the automobile to move and showing that chemical energy is changed
into heat Kinetic Energy.
Iii .The chemical energy stored in foods is converted into heat energy as a result of
digestion in the body. This energy keeps our bodies warm and allows us to do
work.
Q9. What is meant by Inter Conversation of Energy.
Ans: INTER CONVERSATION OF ENERGY:
Suppose a body having mass “m” placed at a height of “h” in the position of rest
So, its kinetic energy is zero. So its potential energy is. Mgh
Total energy = PE + K. E
E = mgh + 0
E = mgh
Suppose the body is released from the height “h” . Now the height of body.
BC = h – x
In this case we use equation of motion to calculate velocity.
AT POSITION “A”
Vi = 0 S = x
Vf = Vi a = g
2aS = Vf 2 - Vi 2
2gx = V2 = 0
V2 = 2g x
AT POSITION “B”
K.E = ½ mv 2 __________ 1
Put the value of V2 in equation 1.
K.E = ½ m 2gx
K.E = mgx
Potential energy at Point “B”
P.E. = mg (h-x)
So total energy at point “B” is,
T.E = K.E + P.E.
T.E = mgx + mg (h-x)
T.E = mgx + mgh – mgx
T.E = mgh
Q10. Explanation Law of Conversation of Energy with the help of simple pendulum.
Ans: EXPLANATION:
Consider the motion of simple pendulum. When it reaches at extreme position the
velocity is zero and as well as K.E = 0 but P.E is maximum. Similarly, when bob passing
through the mean position then its K.E. is maximum while P.E. is zero. During vibratory
motion of simple pendulum, energy changes continuously from one form to another
form. But the total energy remains constant.