This post provides you a complete solution of Chapter 10 Class 9th for Rajasthan Board of Secondary Education.
This post
contains all the main and important topics which have complete and detailed
description.
|
Board |
RBSE |
|
Subject |
Science |
|
Chapter |
Work and Energy |
|
No. of Intext questions |
12 |
|
No. of End exercise questions |
21 |
|
Prescribed Book |
NCERT Science |
Work and Energy Questions- Answers
These
solutions are based on NCERT Science book which includes Work, Energy, Rate of
Doing Work
Intext Questions
Answers of Questions on Page 115
Q.1 A force of 7 N acts on an object. The displacement is say
8 m , in the direction of the force. Let us take it that the force acts on the
object throughout the displacement. What is the work done in this case?
Ans.
Answers of Questions on Page 116
Q.1 When do we say that work is done?
Ans.
Work
is done, when following conditions are satisfied.
1. When force acts on an object.
2. The object is displaced from its initial
position.
Q.2 Write an expression for the work done when a force is
acting on an object in the direction of its displacement.
Ans. W
= Fs.
Q.3 Define 1 J of work.
Ans. IF
1 Newton force acting on a body displaces the body through 1 metre in its own
direction, then work is said to be 1 Joule.
Q.4 A pair of bullocks exerts a force of 140 N on a plough.
The field being ploughed is 15 m long. How much work is done in ploughing the
length of the field?
Ans.
Answers of Questions on Page 119
Q.1 What is the kinetic energy of an object?
Ans.
The
energy possessed by n object by virtue of its motion is called kinetic energy.
Q.2 Write an expression for the kinetic energy of an object.
Ans.
Kinetic
energy of an object `K_E= frac{1}{2}mv^2`
Where m = mass of the object,
v = velocity of
the object
Q.3 The kinetic energy of an object of mass m moving with a
velocity of 5 ms-1 is 25 J. What will be its kinetic energy when its
velocity is doubled? What will be its kinetic energy when its velocity is
increased three times?
Ans.
Answers of Questions on Page 123
Q.1 What is power?
Ans.
The
rate of doing work is known as power.
Q.2 Define 1 watt of power.
Ans.
When 1 Joule work is done in 1 second
then the power is said to be 1 Watt.
Q.3 A lamp consumes 1000 J of electrical energy in 10 s. What
is its power?
Ans.
Q.4 Define average power.
Ans.
When an agent do work at different rates at different intervals of time then the ration of the total work and total time taken is called average power.
Answers of End Exercise Questions on Page 124-126
Q.1 Look at the activities listed below. Reason out whether
or not work is done in the light of your understanding of the term ‘work’.
(a) Suma is swimming in
a pond.
(b) A donkey is carrying a load on its back.
(c) A wind mill is lifting water from a well.
(d) A green plant is
carrying out photosynthesis.
(e) An engine is pulling
a train.
(f) Food grains are
getting dried in the sun.
(g) A sail boat is
moving due to wind energy.
Ans.
Work is done when the following conditions are satisfied-
a force acts on a body.
There is displacement of the body occurs.
(a) Works is done [Reaction (force) of water on Suma during swimming
displaces Suma in the forward direction].
(b) No
work is done (force of gravity acting on the load is perpendicular to the
displacement of the load).
(c) Work
is done (force displaces water in the upward direction).
(d) No
work is done (There is no force and no displacement during photosynthesis).
(e) Work
is done (Force displaces the train).
(f) No
work is done (There is no external force and no displacement of the grains).
(g) Work
is done (force acting on the sail boat displaces the boat).
Q.2 An object thrown at a certain angle to the ground moves
in the curved path and falls back to the ground. The initial and final points
of the path of the object lie on the same horizontal line. What is the work
done by the force of gravity on the object?
Ans.
Work
done by the force of gravity on the object = mgh
Where h = difference in height of the
initial and find position of the object.
Since initial and final position of the
object are at the same level, so h = 0.
Work done = mgh = mg × 0 = 0 [ No
work is done by the force of gravity]
Q.3 A battery lights a bulb. Describe the energy changes
involved in the process.
Ans.
The
chemical energy of the battery changes into electrical energy which is then
converted into light and heat energy.
Q.4 Certain force acting on a 20 kg mass changes its velocity
from 5 ms-1 to 2 ms-1.
Calculate the work done by the force.
Ans.
Q.5 A mass of 10 kg is at a point A on a table. It is moved
to a point B. If the line joining A and B is horizontal, what is the work done
on the object by the gravitational force? Explain your answer.
Ans.
The
work done is zero, because the gravitational force and displacement are
perpendicular to each other.
Work done by gravitational force = mgh
h=
0 (because A and B are at the same height)
Work done = 0.
Q.6 The potential energy of a freely falling body decrease
progressively. Does this violate the law of conservation of energy?
Ans.
No,
as the potential energy of a freely falling body decreases progressively, the
object acquires more and more speed. So, the kinetic energy of the object increases.
However, total energy of the object remains the same at every point.
Q.7 What are the various energy transformations that occur
when you are riding a bicycle?
Ans.
The
chemical energy of food taken changes into heat and then to muscular energy.
After that on paddling, our muscular energy is converted into the kinetic
energy of the bicycle.
Q.8 Does the transfer of energy takes place when you push a
huge rock with all your might fail to move it? Where is the energy you spend
going?
Ans.
When
we push a huge rock, the rock also exerts a huge force on us. The muscular
energy spent by us is used to oppose the huge force acting on us due to the
potential energy of rock.
Q.9 A certain household has consumed 250 units of energy
during a month. How much energy is this in joules?
Ans.
Q.10 An object of mass 40 kg is raised to a height of 5 m
above the ground. What is its potential energy? If the object is allowed to
fall, find its kinetic energy when it is half-way down.
Ans.
Q.11 What is the work done by the force of gravity on a
satellite moving round the earth? Justify your answer.
Ans.
The
force of gravity acts at right angle to the displacement of the satellite so
work done.
.
Q.12 Can there be displacement of an object in the absence of any force acting on it?
Ans.
Yes,
when an object moves with a constant velocity, then no force acts on the
object. However, the object is displaced from one position to another position.eg-
In deep space an object moves from one point to another point in a straight
line, displacement takes place without any force acting on it.
Q.13 A person holds a bundle of hay over his head for 30
minutes and gets tired. Has he done some work or not? Justify your answer.
Ans.
Since the person is standing at one place and
there is no displacement so the work done is zero. But his energy is exhausted
W= Force × displacement
Since the person is at rest, so
displacement = 0.
W= F ×0 = 0
Hence, no work is done by the person.
Q.14 An electric heater is rated 1500 W. How much energy does
it use in 10 h.?
Ans.
Q.15 Illustrate the law of conservation of energy by
discussing the energy changes which occur when we draw a pendulum bob to one
side and allow it to oscillate. Why does the bob eventually come to rest? What
happens to its energy eventually? Is it a violation of law of conservation of
energy?
Ans.
(i) When a pendulum bob is
stretched towards left to the position A, it has Potential energy (PE) stored
in it. When it is released form A, it begins to move towards O and this time it
has maximum Kinetic energy (KE). When it reaches at point B, its kinetic energy
changes into Potential energy again.so energy changes continuously which
illustrates the law of conservation of energy.
(ii) When
the bob of a simple pendulum oscillates in air, the air friction opposes its
motion. The kinetic energy of oscillating bob is used to overcome the air
friction which tries to oppose the motion of the bob of the pendulum.
As the oscillating bob strikes the air
molecules, the kinetic energy of the bob of the pendulum is converted into heat
energy. This heat energy is transferred to the atmosphere. Ultimately, the
entire kinetic energy of the bob of the pendulum is converted into heat energy and
hence, the oscillating bob comes to rest.
In this case, the law of conservation of
energy is not violated as one form of energy is converted into another form of
energy.
Q.16 An object of mass m is moving with a constant velocity,
v. How much work should be done on the object in order to bring the object to
rest?
Ans.
Q.17 Calculate the work required to be done stop a car of 1500
kg moving at a velocity of 60 km/h.
Ans.
Q.18 In each of the following, a force F is acting on an object of mass, m. The direction of displacement is from west to east shown by the longer arrow. Observe the diagram carefully and state whether the work done by the force is negative, positive or zero.
Ans.
Q.19 Soni says that the acceleration in an object could be
zero even when several forces are acting on it. Do you agree with him? Why?
Ans.
Yes
this is possible when all the forces cancel each other, then net force acting
on the object is zero.
Q.20 Find the energy in kWh consumed 10 hours by four devices
of power in 500 W each.
Ans.
Q.21 A freely falling object eventually stops on reaching the
ground. What happens to its kinetic energy?
Ans.
Kinetic
energy is converted into sound energy and heat energy.
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