Category: CBSE Class 9

NCERT Solutions for Class 9 Science Chapter 12 Sound

These Solutions are part of NCERT Solutions for Class 9 Science. Here we have given NCERT Solutions for Class 9 Science Chapter 12 Sound. LearnInsta.com provides you the Free PDF download of NCERT Solutions for Class 9 Science (Physics) Chapter 12 – Sound solved by Expert Teachers as per NCERT (CBSE) Book guidelines. All Chapter 12 – Sound Exercise Questions with Solutions to help you to revise complete Syllabus and Score More marks.

More Resources

• NCERT Solutions for Class 9 Science
• HOTS Questions for Class 9 Science
• Value Based Questions in Science for Class 9
• NCERT Exemplar Solutions for Class 9 Science
• Previous Year Question Papers for CBSE Class 9 Science

NCERT TEXT BOOK QUESTIONS

IN TEXT QUESTIONS

Question 1.
Explain how sound is produced by your school bell.

                                                            Or

Sound is produced when your school bell is struck with a hammer. Why ? (CBSE 2011)
Answer:
When school bell is struck by a hammer, it starts vibrating. Since the vibrating bodies produce sound, so the vibrating school bell produces the sound.

Question 2.
Why are sound waves called mechanical waves ? (CBSE 2012)
Answer:
Sound waves are characterised by the motion of particles of a medium. Hence sound waves are called mechanical waves.

Question 3.
Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend ?
Answer:
Sound waves need material medium like air to move from one place to another place. Since there is no air on the moon, so sound cannot travel from one place to another place. Hence, we cannot hear sound on the moon.

Question 4.
Which wave property determines
(a) loudness,
(b) pitch ? (CBSE 2011, 2012)
Answer:
(a) Amplitude of the wave determines loudness.
(b) Frequency of the wave determines pitch.

Question 5.
Guess which sound has a higher pitch : guitar or car horn ?
Answer:
Guitar, because frequency of sound produced by guitar is higher than the sound produced by car horn.

Question 6.
What are wavelength, frequency, time period and amplitude of a sound wave ? (CBSE 2012)
Answer:
Wavelength (or length of a wave): The distance between two successive regions of high pressure or high density {or compressions) or the distance between two successive regions of low pressure or low density (or rarefactions) is known as wavelength of a sound wave. It is denoted by λ (read as lambda).
In S.I., unit of wavelength is metre (m).
Frequency: The number of compressions or rarefactions crossing a point per unit time is known as the frequency of a sound wave. It is denoted by μ (read as Neu). In S.I., unit of frequency is hertz (Hz).
1 hertz = one oscillation completed by a vibrating body or a vibrating particle in one second.
Time period: Time taken by two consecutive compressions or rarefactions to cross a fixed point. Amplitude. The maximum displacement of a vibrating body from its rest position or mean position.

Question 7.
How are the wavelength and frequency of a sound wave related to its speed ? (CBSE 2011, 2012)
Answer:
V = vλ.

Question 8.
Calculate the wavelength of a sound wave whose frequency is 200 Hz and speed is 440 m/s in a given medium.
Answer:

Question 9.
A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of sound. What is the time interval between successive compressions from the source ?
Answer:

Question 10.
Distinguish between loudness and intensity of sound.
Answer:

Loudness	Intensity of a sound
1.Loudness is a subjective quantity. It depends upon the sensitivity of the human ear. A sound may be loud for a person but the same sound may be feeble for another person who is hard of hearing even when both are sitting at the same distance from the source of sound.	Intensity of a sound is an objective physical quantity. It does not depend on the sensitivity of a human ear.
2. Loudness cannot be measured as a physical quantity because it is just sensation which can be felt only.	Intensity of a sound can be measured as a physical quantity.

Question 11.
In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature ?
Answer:
Sound travels the fastest in iron.

Question 12.
An echo returned in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m s^-1 ? (CBSE 2011)
Answer:
Time taken by sound to travel from the source to the reflecting surface, t = 3/2 = 1.5 s
Speed, v = 342 m s
Distance of reflecting surface from the source, S = vt = 342 x 1.5 = 513 m.

Question 13.
Why are the ceilings of concert halls curved ? (CBSE 2011, 2012)
Answer:
So that the sound after reflection from the ceiling reaches all the corners of the hall.

Question 14.
What is the audible range of the average human ear ? (CBSE, 2011, 2012, 2015)
Answer:
20 Hz to 20,000 Hz.

Question 15.
What is the range of frequencies associated with :
(a) infra sound ?
(b) ultra sound ? (CBSE 2011, 2013)
Answer:
(a) Frequencies less than 20 Hz and greater than zero.
(b) Frequencies greater than 20,000 Hz and equal to 107 Hz.

Question 16.
A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff ?
Answer:
Time taken by the pulse to go from submarine to the cliff, t = 1.02/2 =0.51 s
Speed of sound, v = 1531 m/s
Distance of cliff from the submarine, S = vt = 1531 x 0.51 = 780.81 m.

NCERT CHAPTER END EXERCISE

Question 1.
What is sound and how is it produced ? (CBSE2012)
Answer:
Sound is a form of energy which produces the sensation of hearing in our ears. Sound is produced by forcing an object to vibrate. In other words, sound is produced by a vibrating object.

Question 2.
Why is sound wave called a longitudinal wave ? (CBSE 2012)
Answer:
When sound waves travel in medium, the particles of the medium vibrate about their equilibrium positions along the direction of the propagation of the waves.

Question 3.
Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room ? (CBSE 2011)
Answer:
Timber or quality of sound.

Question 4.
Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen. Why ?

          Or

There is some time interval between observing a flash and hearing a thunder. Explain. (CBSE 2013)
Answer:
Thunder is heared after some time interval the flash is seen because speed of sound is less than the speed of light.

Question 5.
A person has a hearing range of 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies ?
Answer:
Take the speed of sound.in air as 344 m s^-1.

Question 6.
Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound waves in air and in the aluminium to reach the second child. (Speed of sound in aluminium is 6420 m s^-1 and in air is 346 m s^-1). (CBSE 2013)
Answer:
Let l = length of the rod
Time taken by sound to travel distance / in aluminium rod,

Question 7.
The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute ?
(CBSE 2011, 2012)
Answer:
Frequency of source = 100 Hz.
∴ Number of times the source of sound vibrates in 1 s = 100
∴ Number of times the source vibrates in a minute or 60 s = 100 x 60 = 6000.

Question 8.
Does sound follow the same laws of reflection as light does ? Explain.
Answer:
Yes. Sound waves are reflected just like light waves.

Question 9.
When a sound is reflected from a distant object, an echo is produced. Let the distance of the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day ?
Answer:
Let d = distance between the reflecting surface and the source of sound
v = speed of sound in air.

On a hotter day, speed of sound increases with increase in temperature. Hence, the time after which echo is heard decreases. If the time taken by the reflected sound is less than 0-1 s after the production of original sound, then echo is not heard. However, if this time is greater than 0-1 s, then echo will be heard.

Question 10.
Give two practical applications of multiple reflection of sound waves. (CBSE 2011, 2012)
Answer:

1. Megaphone
2. Hearing aid.

Question 11.
A stone is dropped from the top of a tower 500 m high into a pond,of water at the base of the tower. When is the splash heard at the top ? Given g = 10 m s^-2 and speed of sound = 340 m s^-1 .
(CBSE 2011, 2012)
Answer:
Time, after which splash is heard = time taken by the stone to reach the surface of water in a pond + time taken by the sound of splash to reach the top of tower.

Question 12.
A sound wave travels at a speed of 399 m s^-1. If its wavelength is 1.5 m, what is the frequency of the wave ? Will it be audible ?
Answer:

Since audible range of frequencies is 20 Hz to 20,000 Hz. Hence, the given frequency will not be audible.

Question 13.
What is reverberation ? How can it be reduced ? (CBSE 2012, 2014)
Answer:
The phenomenon of prolongation of original sound due to the multiple reflection of sound waves even after the source of sound stops producing sound is called reverberation.
Reverberation can be reduced by covering the roof and walls of a hall by sound absorbing materials.

Question 14.
What is loudness of sound ? What factors does it depend on ? (CBSE 2011)
Answer:
Loudness of a sound is a subjective quantity which causes unpleasant effect in our ear.
Loudness depends upon the amplitude of the vibrating body and the sensitivity of human ear.

Question 15.
Explain how bats use ultrasound to catch a prey. (CBSE 2011, 2012)

                                                  Or

Bats have no eyes, yet they can ascertain distances. (CBSE 2013)
Answer:
Bats can produce ultrasonic waves by flapping their wings. They can also detect these waves. The ultrasonic waves produced by the bats after reflection from the obstacles like buildings guide them to remain away from the obstacles during their flights. Hence, they can fly during night without hitting the obstacles. Bats also catch their prey during night with the help of ultrasonic waves. The ultrasonic waves produced by a bat spread out. These waves after reflecting from a prey say an insect reach the bat. Hence, the bat can easily locate its prey (Figure 24).

Question 16.
A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m. (CBSE 2011)
Answer:

NCERT Solutions for Class 9 Science Chapter 12 Sound

Hope given NCERT Solutions for Class 9 Science Chapter 12 are helpful to complete your science homework.

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NCERT Exemplar Solutions for Class 9 Science Chapter 11 Work, Power and Energy

These Solutions are part of NCERT Exemplar Solutions for Class 9 Science . Here we have given NCERT Exemplar Solutions for Class 9 Science Chapter 11 Work, Power and Energy

MULTIPLE CHOICE QUESTIONS

Question 1.
When a body falls freely towards the earth, then its total energy
(a) increases
(b) decreases
(c) remains constant
(d) first increases and then decreases,
Answer:
(c) Total energy = K.E. + P.E.
When a body falls freely, its K.E. increases and P.E. decreases but the sum of K.E and P.E. remains the same.

More Resources

• NCERT Exemplar Solutions for Class 9 Science
• NCERT Solutions for Class 9 Science
• Value Based Questions in Science for Class 9
• HOTS Questions for Class 9 Science
• Previous Year Question Papers for CBSE Class 9 Science

Question 2.
A car is accelerated on a levelled road and attains a velocity 4 times of its initial velocity. In this process, the potential energy of the car
(a) does not change
(b) becomes twice to that of initial
(c) becomes 4 times that of initial
(d) becomes 16 times that of initial.
Answer:
(a). Potential energy does not depend on the velocity of a body.

Question 3.
In case of negative work, the angle between the force and displacement is
(a) 0°
(b) 45°
(c) 90°
(d) 180°.
Answer:
(d) Explanation : W = FS cos θ. When θ = 180°, cos 180° = – 1 and W = – FS

Question 4.
An iron sphere of mass 10 kg has the same diameter as an aluminium sphere of mass is 3.5 kg. Both spheres are dropped simultaneously from a tower.
When they are 10 m above the ground, they have the same
(a) acceleration
(b) momenta
(c) potential energy
(d) kinetic energy.
Answer:
(a). Freely falling bodies moves with constant acceleration.

Question 5.
A girl is carrying a school bag of 3 kg mass on her back and moves 200 m on a levelled road. The work done against the gravitational force will be (g = 10 m s^-2)
(a) 6 x 10³ J
(b) 6 J
(c) 0.6 J
(d) zero.
Answer:
(d) Explanation : W = FS cos 90° = 0.

Question 6.
Which one of the following is not the unit of energy ?
(a) joule
(b) newton metre
(c) kiiowatt
(d) kolwatt hour.
Answer:
(c) It is the unit of power.

Question 7.
The work done on an object does not depend upon the
(a) displacement
(b) force applied
(c) angle between force and displacement
(d) initial velocity of the object.
Answer:
(d) W = FS cos θ.

Question 8.
Water stored in a dam possesses
(a) no energy
(b) electrical energy
(c) kinetic energy
(d) potential energy
Answer:
(d). The energy possessed by a body by virtue of its position is called potential energy.

Question 9.
A body is falling from a height h. After it has fallen a height h/2, it will possess
(a) only potential energy
(b) only kinetic energy
(c) half potential and half kinetic energy
(d) more kinetic and less potential energy.
Answer:
(c).

SHORT ANSWER QUESTIONS

Question 10.
A rocket is moving up with a velocity u. If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies ?
Answer:

Question 11.
Avinash can run with a speed of 8 m s^-1 against the frictional force of 10 N, and Kapil can move with a speed of 3 m s^-1 against the frictional force of 25 N. Who is more powerful and why ?
Answer:
P = Fu
Power of Avinash = 10 x 8 = 80 W
Power of Kapil = 25 x 3 = 75 W
So, Avinash is more powerful.

Question 12.
A boy is moving on a straight road against a frictional force of 5 N. After travelling a distance of 1.5 km he forgot the correct path at a round about (Fig. 1) of radius 100 m.

Answer:
However, he moves on the circular path for one and half cycle and then he moves forward upto 2.0 km. Calculate the work done by him.
Here, F = 5N, Distance travelled S = 1500 + 3 π r + 2000 = 4442.86 m
∴ W = F x S = 5 x 4442.86 = 22214.3 J.

Question 13.
Can any object have mechanical energy even if its momentum is zero ? Explain.
Answer:

Question 14.
Can any object have momentum even if its mechanical energy is zero ? Explain.
Answer:

Question 15.
The power of a motor pump is 2 kW. How much water per minute the pump can raise to a height of 10 m ? (Given g = 10 m s^-2).
Answer:

Question 16.
The weight of a person on a planet A is about half that on the earth. He can jump upto 0.4 m height on the surface of the earth. How high he can jump on the planet A ?
Answer:

Question 17.
The velocity of a body moving in a straight line is increased by applying a constant force F, for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body.
Answer:
Consider a body or an object of mass m moving with velocity u. Let a force F be applied on the body so that the velocity attained by the body after travelling a distance S is v (Figure 5).

Work done by the force on the body is given by
W =      FS                                                                               …(i)
Since velocity of the body changes so the body is accelerated. Let a be the acceleration of the body. Therefore, according to Newton’s second law of motion,
F =     ma                                                                             …(ii)
Using eqn. (ii) in eqn. (i), we get

Thus, work done by a force on a body is equal to the change in kinetic energy of the body This is known as work-energy theorem.

Question 18.
Is it possible that an object is in the state of accelerated motion due to external force acting on it, but no work is being done by the force. Explain it with an example.
Answer:
Yes. When body moves in a circular path.

Question 19.
A ball is dropped from a height of 10 m. If the energy of the ball reduces by 40% after striking the ground, how much high can the ball bounce back ? (g = 10 m s^-2). (CBSE 2013)
Answer:

Question 20.
If an electric iron of 1200 W is used for 30 minutes everyday, find electric energy consumed in the month of April.
Answer:
Energy consumed in one day = P x t= 1200 W x ½ h = 600 Wh
Energy consumed in 30 days = 600 Wh x 30 = 1800 Wh =18 kWh.

LONG ANSWER QUESTIONS

Question 21.
A light and a heavy object have the same momentum. Find out the ratio of their kinetic energies. Which one has a larger kinetic energy ?
Answer:

Question 22.
An automobile engine propels a 1000 kg car (A) along a levelled road at a speed of 36 km h^-1. Find power if the opposing frictional force is 100 N. Now, suppose after travelling a distance of 200 m, this car collides with another stationary car (B) of same mass and comes to rest. Let its engine also stops at the same time. Now car (B) starts moving on the same level road without getting its engine started. Find the speed of car (B) just after the collision.
Answer:
Force = 100 N, v = 36 km h^-1 = 36 x (5/18) = 10 m s^-1
∴ Power, P = Force x velocity = 100 x 10 = 1000 W
According to law of conservation of linear momentum
Momentum of car A + Mementum of car B before collision = momentum of car A + momentum of car B after collision
i.e. 1000 x 10 + 1000 x 0 = 1000 x 0 + 1000 x v
∴ v = 10 m s^-1
Thus, speed of car B just after collision =10 ms^-1

Question 23.
A girl having mass of 35 kg sits on a trolley of mass 5 kg. The trolley is given an initial velocity of 4 ms^-1 by applying a force. The trolley comes to rest after travelling a distance of 16m.
(a) How much work is done on the trolley ?
(b) How much work is done by the girl ?
Answer:

Question 24.
Four men lift a 250 kg box to a height of 1 m and hold it without raising or lowering it.
(a) How much work is done by the men in lifting the box ?
(b) How much work do they do in just holding it ?
(c) Why do they get tired while holding it ? (g = 10m s^-2)
Answer:

Question 25.
(a) W = F x S = mgS= 250 x 10 x 1 =25.0 J.
(b) Zero. This is because displacement of box is zero.
(c) They get tired because muscular force applied by them is needed to balance the weight of the box.
Answer:
The Jog Falls in Karnataka State are nearly 200m high. 2000 tonnes of water falls from it in a minute. Calculate the equivalent power if all this energy can be utilized ? (g = 10 ms^-2)

Question 26.
How is the power related to the speed at which a body can be lifted ? How many kilograms will a man working at the power of 100 W, be able to lift at constant speed of 1 ms^-1 vertically ? (g = 10 ms^-2)
Answer:

Question 27.
Define watt. Express kilowatt in terms of joule per second. A 150 kg car engine develops 500 W for each kg. What force does it exert in moving the car at a speed of 20 m s^-1 ?
Answer:

Question 28.
Compare the power at which each of the following is moving upwards against the force of gravity ? (given g = 10 ms^-2)

1. a butterfly of mass 1.0 g that flies upward at a rate of 0.5 m s^-1 .
2. a 250 g squirrel climbing up on a tree at a rate of 0.5 m s^-1.

Answer:

1. P = Fv = mgv = 1 x 10^-3 x 10 x 0.5 = 5 x 10^-3 W
2. P = Fv = mg= 250 x 10^-3 x 10 x 0.5 = 1.25 W.

Hope given NCERT Exemplar Solutions for Class 9 Science Chapter 11 Work, Power and Energy are helpful to complete your science homework.

If you have any doubts, please comment below. Learn Insta try to provide online science tutoring for you.

Value Based Questions in Science for Class 9 Chapter 11 Work, Power and Energy

These Solutions are part of Value Based Questions in Science for Class 9. Here we have given Value Based Questions in Science for Class 9 Chapter 11 Work, Power and Energy

VALUE BASED QUESTIONS

Question 1.
Akshit is a student of class IX. He was waiting for a bus on a bus stand. He saw an old man trying to keep his box on the roof of a bus but was unable to do so. Akshit picked up his box and placed the box on the roof of the bus. The old man thanked Akshit.
Answer the following questions based on the above paragraph.

1. Is the work done by Akshit while placing the box on the roof of the bus positive or negative ?
2. Is the work done by gravity on the box is positive or negative ?
3. What values are shown by Akshit ?

Answer:

1. Positive,
2. Negative,
3. Akshit is helpful. He believes in helping old people. He has dignity of labour.

More Resources

• Value Based Questions in Science for Class 9
• HOTS Questions for Class 9 Science
• NCERT Solutions for Class 9 Science
• NCERT Exemplar Solutions for Class 9 Science
• Previous Year Question Papers for CBSE Class 9 Science

Question 2.
Rajiv is a student of class IX. He bought a toy gun from the market. The bullets used in gun were made of rubber. His mother asked Rajiv not to fire bullets on the window panes. Rajiv went to a park and fired bullets on a displaying screen made of plastic. Rajiv’s friend Rohan objected the action of Rajiv. Rajiv started quarelling with Rohan.
Answer the following questions based on above paragraph.

1. Name the type of energy stored in the spring of gun.
2. Name the type of energy of a moving bullet.
3. What values are shown by Rohan ?
4. Comment on the attitude of Rajiv.

Answer:

1. Elastic potential energy,
2. Kinetic energy.
3. According to Rohan, one must not destroy or disfigure the public or private property. It is sin and crime.
4. Rajiv’s attitude is undesirable. He has no right to destroy the public property. He must be ashamed of his action.

Question 3.
Ramesh was walking on a road in the morning. He found that an old mail was lying on a road side. Ramesh touched the body of the old man. His body was very cold. Ramesh rubbed the hands and feet of the old man. The old man opened his eye. Ramesh helped the old man to reach his home.
Answer the following questions based on above paragraph.

1. Why hands become warm, when rubbed ?
2. What values are shown by Ramesh ?

Answer:

1. When hands are rubbed against each other, mechanical energy is converted into heat energy. Therefore, hands become warm.
2. Ramesh is helpful, feels concerned for others. He has high degree of general awareness.

Question 4.
An old man was standing on a bus stand carrying a very heavy luggage. Saurabh was looking at the old man and finding him uncomfortable, requested him to put down the luggage and helped him in doing so.
(a) Did the old man do any work while holding the luggage ?
(b) What is the equation for work done against gravity ?
(c) Why did Saurabh ask the old man to put down the luggage ? (CBSE 2015)
Answer:
(a) No work is done by the old man.
(b) W = mgh
(c) Saurabh is a good boy. He always helps needy and old people. He wanted to help the old man and hence asked him to put down the luggage.

Hope given Value Based Questions in Science for Class 9 Chapter 11 Work, Power and Energy are helpful to complete your science homework.

If you have any doubts, please comment below. Learn Insta try to provide online science tutoring for you.

NCERT Solutions for Class 9 Science Chapter 11 Work, Power and Energy

These Solutions are part of NCERT Solutions for Class 9 Science. Here we have given NCERT Solutions for Class 9 Science Chapter 11 Work, Power and Energy. LearnInsta.com provides you the Free PDF download of NCERT Solutions for Class 9 Science (Physics) Chapter 11 – Work and Energy solved by Expert Teachers as per NCERT (CBSE) Book guidelines. All Chapter 11 – Work and Energy Exercise Questions with Solutions to help you to revise complete Syllabus and Score More marks.

More Resources

• NCERT Solutions for Class 9 Science
• HOTS Questions for Class 9 Science
• Value Based Questions in Science for Class 9
• NCERT Exemplar Solutions for Class 9 Science
• Previous Year Question Papers for CBSE Class 9 Science

NCERT TEXT BOOK QUESTIONS

IN TEXT QUESTIONS

Question 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 through the displacement. What is the work done in this case ?
(CBSE 2011)
Answer:
Here, Force, F = 7 N
Displacement, S = 8 m
Work done = F x S = 7 x 8 = 56 J.

Question 2.
When do we say that work is done ? (CBSE 2012)
Answer:
Work is done, when force acts on an object and the object is displaced from its initial position.

Question 3.
Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Answer:
W = FS

Question 4.
Define 1 J of work. (CBSE 2012)

                                           Or

Define SI unit of work. (CBSE Sample Paper 2010 ; CBSE 2011, 2012)
Answer:
Work is said to be 1 J if 1N force displaces an object through 1 metre in its own direction.

Question 5. 
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 ? (CBSE 2010)
Answer:
Here, Force, F = 140 N
Displacement, S = 15 m
Work done = F x S = 140 x 15 = 2100 J.

Question 6.
What is the kinetic energy of an object ? (CBSE 2011, 2012, 2014, 2015)
Answer:
The energy possessed by an object by virtue of its motion is known as the kinetic energy of the object.

Question 7.
Write an expression for the kinetic energy of an object.
Answer:
K.E. of an object = 1/2 mv² , where m is the mass of the object and v is the speed of the object.

Question 8.
The kinetic energy of an object of mass m moving with a velocity of 5 m s^-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
(CBSE 2010, 2012)
Answer:

Question 9.
What is power ?
Answer:
Power is defined as the rate of doing work.

Question 10.
Define 1 Watt of power.
Define SI unit of power.
Answer:
Power of an agent is said to be 1 watt if it does 1 joule work in 1 second.

Question 11.
A lamp consumes 1000 J of electrical energy in 10 s. What is its power ?
Answer:

Question 12.
Define average power.
Answer:
Average power is defined as the ratio of total work done by an agent to the total time taken.

NCERT CHAPTER END EXERCISE

Question 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 lilting water from a well.
(d) A great plant is carrying out photosynthesis.
(e) An engine is pulling a train.
(f) Food grains are getting dried in the sun.
(g) A sailboat is moving due to wind energy.
Answer:
(a) Work 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).

Question 2.
An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground.
The initial and the 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 ? (CBSE 2011, 2012)
Answer:
Work done by the force of gravity on the object = mgh
Here, h = difference in height of the initial and final positions of the object.
Since, initial and final positions of the object are at the same level, so h = 0.
∴ Work done = mg x 0 = 0.

Question 3.
A battery lights a bulb. Describe the energy change involved in the process. (CBSE 2012)
Answer:
The chemical energy of the battery changes into electrical energy which is then converted into light energy and heat energy.

Question 4.
Certain force acting on a 20 kg mass changes its velocity from 5 m s^-1 to 2 m s^-1. Calculate the work done by the force. (CBSE 2011, 2012)
Answer:

Question 5.
A mass of 10 kg is at a point A on a table. It is moved to a point B which is at a distance of 2 m from A.
If the line joining A and B is horizontal, what is the work done on the object by the gravitational force ? Explain your answer. (CBSE 2012)
Answer:
Work done by the gravitational force = mgh
Since h = 0 (because both points A and B are at the same height.)
∴ Work done = 0.

Question 6.
The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy ? Why ? (CBSE 2011)
Answer:
No, As the potential energy of a freely falling object decreases, the object acquires more and more speed. So the kinetic energy of the object increases at the cost of its potential energy. However, total energy of the object remains the same at every point.

Question 7.
What are the various energy transformations that occur when you are riding a bicycle ?
Answer:
Our muscular energy is converted into the kinetic energy of the bicycle.

Question 8.
Does the transfer of energy takes place when you push a huge rock with all your might and fail to move it ? Where is the energy you spend going ?
Answer:
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 rock.

Question 9.
A certain household has consumed 250 units of energy during a month. How much energy is this in joules ?
Answer:

Question 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. (CBSE 2011)
Answer:

1. Potential energy = mgh
   = 40 kg x 10 m s^-2 x 5 m = 2000 J.
2. According to the law of conservation of energy :
   K.E. when it is half-way down = P.E. when it is half-way down
   

Question 11.
What is the work done by the force of gravity on a satellite moving round the earth ? Justify your answer. (CBSE 2012)

Answer:
The force of gravity acting on a satellite moves the satellite in a circular path. Since, the force of gravity acts at right angle to the displacement of the satellite (Figure), so work done,
W = FS cos 90° = 0

Question 12.
Can there be displacement of an object in the absence of any force acting on it ?
Answer:
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.

Question 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. (CBSE 2011, 2012)
Answer:
Work = Force x Displacement.
Since the person is at rest, so displacement = 0.
Hence no work is done by the person.

Question 14.
An electric heater is rated 1500 W. How much energy does it use in 10 hours ? (CBSE 2011, 2012)
Answer:

Question 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 the law of conservation of energy ?
Answer:
For part

1. Consider a simple pendulum suspended from a rigid support at S. Let OS be the undisturbed position of the pendulum. Now, let the pendulum be displaced to position A where it is at rest (Figure 9).
   
   At position A, the pendulum has potential energy (mgh). When the pendulum is released from position A, it begins to move towards position O. The speed of the bob of the pendulum increases and its height decreases. So potential energy of the pendulum is changed into its kinetic energy. At position O, whole of the potential energy of the pendulum is converted to its kinetic energy.
2. When the bob of a simple pendulum oscillates in air, the air friction opposes its motion. The kinetic energy of the 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. In other words, the kinetic energy of the bob of the pendulum is converted into heat energy with the passage of time. Ultimately, the entire kinetic energy of the bob 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.

Question 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 ? (CBSE 2012)
Answer:

Question 17.
Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h. (CBSE 2011, 2016)
Answer:

Question 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 diagrams carefully and state whether the work done by the force is negative, positive or zero. (CBSE 2011)

Answer:
First Case: Angle between force and displacement, 0 = 90°
∴ work done = FS cos 6 = FS cos 90° = 0
Second Case: Angle between force and displacement, 0 = 0°
∴ work done = FS cos 9 = FS cos 0° = FS ( positive work) (∴ cos 0° = 1)
Third Case: Angie between force and displacement, 9 = 180°
∴ work done = FS cos 9 = FS cos 180° = -FS ( negative work) (∴ cos 180° = -1)

Question 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 ?
Answer:
Yes, If the sum of several forces acting on an object is zero, then net force acting on it is zero. Hence,

Question 20.
Find the energy in kWh consumed in 10 hours by four devices of power 500 W each.
Answer:
Total power, P = 500 W x 4 = 2000 W
Time, t = 10 h
∴ Energy consumed = Power x Time
= 2000 W x 10 h = 20,000 Wh = 20 kWh.

Question 21.
A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy ?
(CBSE 2011)
Answer:
Kinetic energy is converted into sound energy and heat energy.

NCERT Solutions for Class 9 Science Chapter 11 – Work and Energy

Hope given NCERT Solutions for Class 9 Science Chapter 11 are helpful to complete your science homework.

If you have any doubts, please comment below. Learn Insta try to provide online science tutoring for you.

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NCERT Exemplar Solutions for Class 9 Science Chapter 10 Gravitation

These Solutions are part of NCERT Exemplar Solutions for Class 9 Science . Here we have given NCERT Exemplar Solutions for Class 9 Science Chapter 10 Gravitation

MULTIPLE CHOICE QUESTIONS

Question 1.
Two objects of different masses falling freely near the surface of moon would
(a) have same velocities at any instant
(b) have different accelerations
(c) experience forces of same magnitude
(d) undergo a change in their inertia.
Answer:
(a) Explanation : During free fall, acceleration remains the same irrespective of the mass of the object. Force is directly proportional to the mass of a freely falling object.

More Resources

• NCERT Exemplar Solutions for Class 9 Science
• NCERT Solutions for Class 9 Science
• Value Based Questions in Science for Class 9
• HOTS Questions for Class 9 Science
• Previous Year Question Papers for CBSE Class 9 Science

Question 2.
The value of acceleration due to gravity
(a) is same on equator and poles
(b) is least on poles
(c) is least on equator
(d) increases from pole to equator.
Answer:
(c) Explanation :

Equatorial radius (R) is more than the polar radius.

Question 3.
The gravitational force between two objects is F. If masses of both objects are halved without changing distance between them, then the gravitation force would become
(a) F/4
(b) F/2
(c) F
(d) 2F
Answer:

Question 4.
A boy is whirling a stone tied with a string in an horizontal circular path the string breaks, the stone
(a) will continue to move in the circular path
(b) will move along a straight line towards the centre of the circular path
(c) will move along a straight line tangential to the circular path
(d) will move along a straight line perpendicular to the circular path away from the boy.
Answer:
(c) Explanation : Due to inertia of directions.

Question 5.
In the relation F = G M m/d² , the quantity G
(a) depends on the value of g at the place of observation
(b) is used only when the earth is one of the two masses
(c) is greatest at the surface of the earth
(d) is universal constant of nature.
Answer:
(d) Explanation : The value of ‘G’ is same throughout the universe.

Question 6.
Law of gravitation gives the gravitational force between
(a) the earth and a point mass only
(b) the earth and sun only
(c) any two bodies having some mass
(d) two charged bodies only.
Answer:

Question 7.
The value of quantity G in the law of gravitation
(a) depends on mass of earth only
(b) depends on radius of earth only
(c) depends on both mass and radius of earth
(d) is independent of mass and radius of the earth.
Answer:
(d) Explanation : G is universal constant.

Question 8.
Two particles are placed at some distance. If the mass of each of the two particles is doubled, keeping the distance between them unchanged, the value of gravitational force between them will be
(a) 1/4 times
(b) 4 times
(c) 1/2 times
(d) unchanged.
Answer:

Question 9.
The atmosphere is held to the earth by
(a) gravity
(b) wind
(c) clouds
(d) earths magnetic field.
Answer:
(a).

Question 10.
The force of attraction between two unit point masses separated by a unit distance is called
(a) gravitational potential
(b) acceleration due to gravity
(c) gravitational field
(d) universal gravitational constant.
Answer:

Question 11.
The weight of an object at the centre of the earth of radius R is
(a) zero
(b) infinite
(c) R times the weight at the surface of the earth
(d) 1/R² times the weight at surface of the earth.
Answer:
(a) Explanation : W = mg. The value of ‘g’ at the centre of earth is zero.

Question 12.
An apple falls from a tree because of gravitational attraction between the earth and apple. If F₁ is the magnitude of force exerted by the earth on the apple and F₂ is the magnitude of force exerted by apple on earth, then
(a) F₁ is very much greater than F₂
(b) F₂ is very much greater than F₁
(c) F₁ is only a little greater than F₂
(d) F₁ and F₂ are equal.
Answer:

SHORT ANSWER Of QUESTIONS

Question 13.
What is the source of centripetal force that a planet requires to revolve around the sun ? On what factors does that force depend ?
Answer:
The source of centripetal force that a planet requires to revolve around the sun is the gravitational force between the sun and the planet. Thus,

where m is the mass of the sun, is the mass of the planet and r is the distance between the sun and the planet.
Thus, the force depends upon

1. the mass of the sun,
2. the mass of the planet and
3. the distance between the sun and the planet.

Question 14.
On the earth, a stone is thrown from a height in a direction parallel to the earths surface while another stone is simultaneously dropped from the same height. Which stone would reach the ground first and why ?
Answer:
Both stones will reach the ground simultaneously. Initial velocity of both the stones in the downward direction is zero and the acceleration of both the stones in the downward direction is same and equal to g.

so both stones take same time to reach the ground.

Question 15.
Suppose gravity of earth suddenly becomes zero, then in which direction will the moon begin to move if no other celestial body affects it ?
Answer:
Gravity of earth provides necessary centripetal force to the moon to move in a circular path around the earth. If gravity becomes zero, there is no centripetal force and hence, the moon will begin to move in a straight line along to the tangent at the point on the circular path due to inertia of direction.

Question 16.
Identical packets are dropped from two aeroplanes. One above the equator and the other above the north pole both at height h. Assuming all conditions are identical will those packets take same time to reach the surface of earth. Justify your answer.
(CBSE Sample Paper)
Answer:
Time taken by an object to fall through height h at a place is given by

Since, value of ‘g’ at poles is greater than at the equator, therefore, packet dropped above the north pole will take less time than the packet dropped above the equator to reach the surface of the earth.

Question 17.
The weight of any person on the moon is about 1/6 times that on the earth. He can lift a mass of 15 kg on the earth. Whatwill be the maximum mass, which can be lifted by the same force applied by the person on the moon ? (CBSE Sample Paper)
Answer:

Question 18.
Calculate the average density of earth in terms of g, G,m?
Answer:

Question 19.
The earth is acted upon by gravitation of sun, even though it does not fall into the sun. Why ? (CBSE 2012)
Answer:
The earth revolves around the sun. The centripetal force is needed by the earth to revolve around the sun. This centripetal force is provided by the gravitational force between the sun and the earth. The earth keeps on moving around the sun as long as gravitational force between the earth and the sun acts on it.

Question 20.
How does the weight of an object vary with respect to mass and radius of the earth. In a hypothetical case, if the diameter of the earth becomes half of its present value and its mass becomes four times of its present value, then how would the weight of any object on the surface of the earth be affected ? (CBSE 2012)
Answer:

Question 21.
How does the force of attraction between two bodies depend upon their masses and distance between them ? A student thought that two bricks tied together would fall faster than a single one under the action of gravity. Do you agree with this hypothesis or not ? Comment.
Answer:
The force of attraction between two bodies of masses m₁ and m₂ and separated by a distance r is given by

This force is known as gravitational force. The gravitational force is

1. directly proportional to the product of the masses of two bodies and
2. inversely proportional to the square of the distance between them.

The hypothesis is not correct. This is because, all bodies in the absence of any force of friction fall with the same acceleration (known as acceleration due to gravity) irrespective of their masses. Hence two bricks tied together will not fall faster than a single brick under the action of gravity.

Question 22.
Two objects of masses m₁ and m₂ having the same size are dropped simultaneously from heights h₁ and h₂– respectively. Find out the ratio of time they would take in reaching the ground. Will this ratio remain the same if

1. one of the object is hollow and the other one is solid and
2. both of them are hollow, size remaining the same in each case. Give reason.

Answer:

Ratio will be same as it does not depend on the mass and size of objects.

Hope given NCERT Exemplar Solutions for Class 9 Science Chapter 10 Gravitation are helpful to complete your science homework.

If you have any doubts, please comment below. Learn Insta try to provide online science tutoring for you.