Important Questions for Work and Energy for Class 9 Science
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
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
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}
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
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
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