Given below are the Class 9 Science Important Questions for work and energy
a. Concepts questions
b. Calculation problems
c. Multiple choice questions
d. Long answer questions
e. Fill in the blank's
Short Answer Based questions
 What is unit of energy?
 The total energy when a body falls freely towards earth remains constant. True or false?
 What do you mean by kinetic energy of the body?
 If the mass of the body is doubled and velocity is made halved, the kinetic energy will become?
 Potential energy is a vector quantity True or false?
 When the force acts at right angle to the direction of motion, what is the work done by the force
 What is Power?
 What is Law of conservation of energy?
Answer
a. Joule
b. True
c. Energy associated with the motion of the object is called Kinetic energy. A body of mass m and moving with velocity v has a kinetic energy
$K=\frac {1}{2}mv^2$
d. Now,
$K=\frac {1}{2}mv^2$
If mass is double and velocity is made halved
$K_1=\frac {1}{2} 2m (v/2)^2 =\frac {1}{2} K$
So Kinetic energy get halved
e. False
f. Workdone is zero
g. Power is defined as the rate of doing work. The SI unit of power is watt. 1 W = 1 J/s
h. Energy can not be created or destroyed.It can be transformed from one form to another
Match the column
Column A
(Energy Conversion)

Column B
(Converters)

Chemical energy into Mechanical energy

Electric Generator

Heat energy into Mechanical engine

Electric Motor

Mechanical energy into Electrical energy

Car engine

Electrical energy to Mechanical energy

Steam engine

Light energy into electrical energy

Electric bulb

Electrical energy into light energy

Solar Cell

Answer
Table type question
A force 10 N act on the body and body is displaced by 10 m. The angle between force and displacement are given below. Calculate the work done
Angle

30

45

60

90

Work done

?

?

?

?

Answer
Workdone is given by
$W= Fd cos \theta$
Where $\theta$ is the angle between force and displacement
$cos 30 = \frac {\sqrt {3}}{2}$
$cos 45 = \frac {1}{\sqrt {2}}$
$cos 60 = \frac {1}{2}$
$cos 90 = 0$
So,answer is
Table type question
A body of mass 10 kg falls from the height 10 m towards the earth. Given g=10m/s
^{2}
Height from earth(m)

10m

?

?

2m

Potential energy(J)

?

?

500J

?

Kinetic energy(J)

?

1000J

?

?

Mechanical energy (J)

?

?

?

?

Answer
This can be easily solved using below formula
$PE =mgH$ , Mechanical Energy= PE + KE
Mechanical Energy remains constant during the Free Fall
Fill in the blanks
 1 Kilo Watt hour of energy is equal to ________ Joule
 Power is the rate of doing ______. And its unit is ___________
 Work done by the force can be _______ and ______.
 The water stored in the reservoir of the tank possessed _______ energy
 The total energy of the swinging pendulum remains ______ at all the points
 When the body falls freely towards earth ,potential energy of the body _______while kinetic energy of the body______. The total remains ______at all the point during the motion
Answer
 3.6X10^{6}
 Work, watt
 Negative,zero,positive
 Potential
 Constant
 Decrease, increase, constant
Crossword Puzzle
Across
2. The scientist with whom unit of power is named
5. The kinetic energy of the body depends on mass and this quantity
7. The energy possessed by the car moving on the road
8. The Bigger unit of power
9. Work and power are these quantities
Down
1. The work done by the earth on the satellite moving in circular path
3. Solar water heater converts this energy into heat energy
4. The gravitation potential energy is present because of this
6. In hydroelectric power house, the potential energy of the water is finally transformed to this energy
Answer
Crossword Solution
Across
James,velocity,kinetic,kilowatt,scalar
Down
Zero,light,Position,electrical
Short Answer type question
Question 1.An electric bulb of 100 W is used for 8 h per day. Calculate the 'units' of energy consumed in one day by the bulb?
Answer
Power of electric bulb = 60 W
= 0.1 kW.
Time used, t = 8 h
$Energy = power \times \text{time taken}$
$= 0.1 \times 8 $
$= 0.8 kWh$
= 0.8 'units'.
The energy consumed by the bulb is 0.8 'units'.
Question 2. Find the energy possessed by an object of mass 10 kg when it is at a height of 2 m above the ground. Given, g = 10 m/s
^{2}.
Answer
Energy possessed = PE= $mgH= 10 \times 10 \times 2 = 200 J$
Question 3. An engineer is asked to design a playground slide such that the speed a child reaches at the bottom does not exceed 4.0 m/s. Determine the maximum height that the slide can be.?Given, g = 9.8 m/s
^{2}.
Answer
Entire PE is converted to KE when the child reaches bottom
So
KE = PE
$\frac {1}{2} mv^2 = mgH$
or $H= \frac {v^2}{2g} = \frac {16}{2 \times 9.8} = .816 m$
Question 4. Which one of the following is not the unit of energy?
(a) joule
(b) newton metre
(c) kilowatt
(d) kilowatt hour
Answer
(c)
Question 5. A rocket is moving up with a velocity v. If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies?
Answer
$KE_1 = \frac {1}{2} mv^2 $
$KE_2 = \frac {1}{2} m(3v)^2 =9 \times \frac {1}{2} mv^2$
$KE_1 : KE_2 = 1 :9 $
Question 6. A force of 10 N displaces a body by a distance of 3 m at an angle 60° to its own direction. Find the amount of work done.
Answer
Workdone is given by
$W= Fd cos \theta$
Where $\theta$ is the angle between force and displacement
$cos 60 = \frac {1}{2}$
Hence $W= 10 \times 3 \times \frac {1}{2} = 15 J$
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