Work and Energy class 9 physics MCQ & SAQ

Work and Energy class 9 physics NCERT SOLUTION

Q1. 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?

 Ans.

F = 7 N

s = 8 m

W = F × s = 7

N × 8 m

W = 56 J

∴ Work done is 56 J.

 

Q2. When do we say that work is done?

 Ans. Work is said to be done when force applied on an object shows the displacement in that object.

W = F × s.

 

Q3. Write an expression for the work done when a force is acting on an object in the direction of its displacement.

 Ans. Work done = W

Force = F

Displacement = s

∴ Work done is equal to the product of the force and displacement.

∴ W = F × s

 

Q4. Define 1 J of work.

 Ans. 1 Joule of work is said to be done when a force of 1 Newton acts on a body and shows displacement of 1 meter.

 

Q5. A pair of bullocks exerts a force of 140 N on a plow. The field being plowed is 15 m long. How much work is done in plowing the length of the field?

 Ans. Force = 140 N s = 15 m W = ?

∴ W = F × s = 140 N × 15 m = 2100 Nm

W = 2100 J

∴ Work done in plowing the length of the field is 2100 J.

 

Q6. What is the kinetic energy of an object?

 Ans. The energy possessed by a body by the virtue of its motion is called kinetic energy. Every moving object possesses kinetic energy. A body uses kinetic energy to do work. Kinetic energy of a hammer is used in driving a nail into a log of wood, kinetic energy of air is used to run wind mills, etc.

 

Q7. Write an expression for the kinetic energy of an object.

 Ans. If a body of mass m is moving with a velocity v, then its kinetic energy Ek is given by the expression,

Ek= 1/2 mv2

Its SI unit is Joule (J).

 

Q8. The kinetic energy of an object of mass, 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. K.E. = 25 J, v = 5 m/s

K.E. = (1/2) mv2 

25 = (1/2) m (5)2

∴ m = 2 kg

(i) If velocity is doubled i.e., v = 10m/s, m = 2kg

K.E. =(1/2) mv2 =(1/2) (2) × (10)2 = 100 J,

K.E will become four times.

(ii) If velocity is 3 times, v = 15 m/s,m = 2 kg

K.E. =(1/2) mv2

= (1/2) × 2 × (15)2

= 225 J, K.E will become nine times.

Q1. 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?

 Ans.

F = 7 N

s = 8 m

W = F × s = 7

N × 8 m

W = 56 J

∴ Work done is 56 J.

 

Q2. When do we say that work is done?

 Ans. Work is said to be done when force applied on an object shows the displacement in that object.

W = F × s.

 

Q3. Write an expression for the work done when a force is acting on an object in the direction of its displacement.

 Ans. Work done = W

Force = F

Displacement = s

∴ Work done is equal to the product of the force and displacement.

∴ W = F × s

 

Q4. Define 1 J of work.

 Ans. 1 Joule of work is said to be done when a force of 1 Newton acts on a body and shows displacement of 1 meter.

 

Q5. A pair of bullocks exerts a force of 140 N on a plow. The field being plowed is 15 m long. How much work is done in plowing the length of the field?

 Ans. Force = 140 N s = 15 m W = ?

∴ W = F × s = 140 N × 15 m = 2100 Nm

W = 2100 J

∴ Work done in plowing the length of the field is 2100 J.

 

Q6. What is the kinetic energy of an object?

 Ans. The energy possessed by a body by the virtue of its motion is called kinetic energy. Every moving object possesses kinetic energy. A body uses kinetic energy to do work. Kinetic energy of a hammer is used in driving a nail into a log of wood, kinetic energy of air is used to run wind mills, etc.

 

Q7. Write an expression for the kinetic energy of an object.

 Ans. If a body of mass m is moving with a velocity v, then its kinetic energy Ek is given by the expression,

Ek= 1/2 mv2

Its SI unit is Joule (J).

 

Q8. The kinetic energy of an object of mass, 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. K.E. = 25 J, v = 5 m/s

K.E. = (1/2) mv2 

25 = (1/2) m (5)2

∴ m = 2 kg

(i) If velocity is doubled i.e., v = 10m/s, m = 2kg

K.E. =(1/2) mv2 =(1/2) (2) × (10)2 = 100 J,

K.E will become four times.

(ii) If velocity is 3 times, v = 15 m/s,m = 2 kg

K.E. =(1/2) mv2

= (1/2) × 2 × (15)2

= 225 J, K.E will become nine times.

(c) Force Displacement of water W = +ve

(d) No displacement, hence no work is done

(e) Work is done

(f ) No displacement, hence no work is done.

(g) Work is done

 

Q2. 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?

 Ans. Work done by the force of gravity on the object is zero. Force of gravity acts in the vertical downward direction and the distance covered by the object is in the horizontal direction. As there is no displacement in the direction of force, hence the work done is zero.

 

Q3. A battery lights a bulb. Describe the energy changes involved in the process.

 Ans. Chemical energy of the battery → transformed into heat energy of the bulb filament, which is further transformed into light energy.

 

Q4. A force acting on a 20 kg mass changes its velocity from 5 m/s to 2 m/s.

 Calculate the work done by the force.

 Ans. mass = 20 kg u = 5 m/s v = 2 m/s W = ?

Q5. A mass of 10 kg is at point A on a table. It is moved to 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. Force acts on a body of mass 10 kg to move it horizontally 

from A to B as shown above. Work is said to be done on the object by force.

But the work done on the object by the gravitational force is zero.

Since work done by gravitational force depends upon change in vertical height.

Q6. The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Ans. A body at a particular height has maximum potential energy, when it falls the potential energy is transformed into kinetic energy, when the object is about to touch the ground, the body has maximum velocity and hence the kinetic energy is maximum.

Energy is not lost but it gets transformed into different forms. It abides by the Law of Conservation of Energy.

 

Q7. What are the various energy transformations that occur when you are riding a bicycle?

 Ans. Muscular energy gets transformed into mechanical energy.

 

Q8. Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

 Ans. When rock is pushed with all might, the energy does not allow the rock to move. No work is said to be done. The energy gets transformed in the form of heat energy.

 

Q9. A certain household has consumed 250 units of energy during a month. How much energy is this in joules?

 Ans. Commercial unit of energy = 250 units

1 unit = 1 kWh

∴ 250 units = 250 kWh

1 kWh = 3.6 × 106 J

∴ 250 units = 250 × 3.6 × 106 J

Energy in Joules = 9 × 108 J

 

Q10. 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. Mass = 40 kg

h = 5 m

P.E. = ?

P.E. = mgh = 40 × 10 × 5 Q

g = 10 m/s2

P.E. = 2000 J

When the object falls the potential energy gets transformed into kinetic energy.

When the object is half way down

∴P.E. will become half i.e., 20200 = 1000 J and P.E. = K.E.  

Kinetic Energy = 1000J

 

Q11. What is the work done by the force of gravity on a satellite moving around the earth? Justify your answer.

 Ans. As the force acting on the body is perpendicular to the direction of displacement,

∴ Work done is zero.

 

Q12. Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher.

 Ans. If an object is in a state of motion, then as per Newton’s first law of motion in the absence of any external force acting on it, the object will maintain its motion along a given straight line. So the object will have displacement. However, if the object is initially at rest then there can be no displacement, the work done by a person is zero.

 

Q13. 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. Work is said to be done when force is applied on a body and the body shows the displacement.

In this case force is applied on the hay bundle but there is no displacement of it.

Hence no work is said to be done on the hay.

 

Q14. An electric heater is rated 1500 W. How much energy does it use in 10 hours?

 Ans. Power = 1500 W → 1.5 kW

time = 10 hour

Energy = ?

Energy = Power × Time taken = 1.5 × 10

= 15 kWh

∴ Energy required = 15 kWh

 

Q15. 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?

 Ans.

Let ‘A’ be the mean position of the pendulum. When it is raised to point ‘B’ and released, 

it starts oscillating from B to A, A to C and back to A. It repeats this motion again and again.

At point B and C, the height of the pendulum is maximum, hence the potential energy at these points is maximum.

During oscillation, when the pendulum moves from B − A − C, its maximum speed is at A, hence the maximum kinetic energy is at A, and potential energy at this point is minimum.

This shows the potential energy at B is transformed to kinetic energy at A and again this energy is transformed into potential energy at C.

This transformation of energy goes on and the pendulum oscillates, it does not violate the law of conservation of energy.

Bob comes to rest because there are three forces acting on it,

(i) Gravitational force exerted by earth ‘g’ which tries to bring the bomb to rest.

(ii) Air resistance force that acts opposite the motion of the pendulum.

(iii) Restoring force, the pendulum is tied at a point, the upward tension pull also tries to get the pendulum to its original position.

 

Q16. 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. The mass of the object = m

Velocity = v

K.E. = (1/2) mv2 – This is the energy of the object with which the body is moving, hence the same amount of energy is required to stop the body.

∴ Work done on the object in order to bring the object to rest = K.E. =(1/2) mv2

 

Q17. Calculate the work done required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h.

 Ans. mass = 1500 kg

v = 60 km/h =

∴

 

Q18. In each of the following forces, 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.

Ans. (a) Incase ( a ) , the force F is perpendicular to the displacement , hence work done = 0. (b) In case

(b), the force F is in the same direction as that of displacement, hence work done is positive. (c) In case

(c), the force F is opposite to the direction of displacement, hence work done is negative.

 

Q19. Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

 Ans. Yes, when several forces are acting on a body, if the forces are opposite to each other, the acceleration produced can be zero.

 

Q20. Find the energy in kwh consumed in 10 hours by four devices of power 500 W each.

 Ans. Power = 500 W = 0.5 kW

time = 10 hours

Devices = 4

∴ Energy consumed in kWh = 0.5 kW × 10 hours × 4 = 20 kWh

 

Q21. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?

 Ans. When a body is freely falling the energy remains conserved. Potential energy is transformed into kinetic energy. But when the body reaches the ground, its velocity, (v = 0) becomes zero. Hence the kinetic energy is also zero.

Work and Energy class 9 physics MCQ 

Question: Water stored in a dam possesses:

• a) potential energy

• b) No energy

• c) kinetic energy

• d) electrical energy

Answer: potential energy

Question: When a body falls freely towards the earth then the total energy

• a) remains constant

• b) increase

• c) decrease

• d) first increases and then decreases

Answer: remains constant

Question: Power of a woman is equal to:

• a) work /time

• b) work /time x work

• c) work x time

• d) time/work

Answer: work /time

Question: A spring is compressed The P.E of the compressed spring

• a) decrease

• b) becomes zero

• c) remains unchanged

• d) increase

Answer: decrease

Question: The potential energy of a girl is maximum

• a) standing

• b) sleeping on the ground

• c) sitting on the ground

• d) sitting on the ground

Answer: standing

Question: A child on a skateboard is moving at a speed of 2 m/s. After a force acts on the child, her speed is 3 m/s. What can you say about the work done by the external force on the child?

• a) positive work was done

• b) negative work was done

• c) Zero work was done

• d) Infinite work was done

Answer: positive work was done

Question: Mike applied 10 N of force over 3 m in 10 seconds. Joe applied the same force over the same distance in 1 minute. Who did more work?

• a) both did the same work

• b) both did zero work

• c) Mike

• d) Joe

Answer: both did the same work

Question: Work done by tension in the string when a ball tied to a string is being whirled around in a circle is

• a) tension does not work at all

• b) tension does negative work

• c) tension does positive work

• d) tension does zero work

Answer: tension does not work at all

Question: The value of 1 KWH is

• a) 3.6 X 106 J

• b) 3.6 X 105J

• c) 3.6 X 107 J

• d) 3.6 X 109 J

Answer: 3.6 X 106 J

Question: When force acts in opposite direction the work done is

• a) negative

• b) Positive

• c) infinity

• d) Zero

Answer: negative

Question: When a spring is compressed work is done on it. Its elastic potential energy

• a) Decreases

• b) increases

• c) Disappears

• d) Does not change

Answer: Decreases

Question: When a body rolls down an inclined plane, then it has

• a) Both kinetic and potential energy

• b) Only Kinetic energy

• c) It has potential energy

• d) It has no energy

Answer: Both kinetic and potential energy

Question: On a rough surface a mass is (a) pulled,(b) pushed by a force acting at an angle with the surface.

• a) Pulling is easier

• b) Pushing is easier

• c) Pushing and pulling both are easier

• d) Pushing and pulling g are not easier

Answer: Pulling is easier

Question: A stone rubbed on a rough surface and placed on the skin will show heating sensation, because

• a) Friction causes heat

• b) Heat cannot flow from the skin to

• c) Heat flows from the stone to skin

• d) Heat cannot flow

Answer: Friction causes heat

Question: Work done by a body from Force-distance curve is

• a) Area under the curve

• b) Slope of the curve

• c) Line parallel to the distance axis

• d) Line parallel to the Force axis

Answer: Area under the curve

Question. 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

Question. In the SI system, the unit of potential energy is

(a) newton

(b) joule

(c) watt

(d) meter per second

Answer :    B

Question. Work done by centripetal force is

(a) >0

(b) ≥0

(c) <0

(d) 0

Answer :   D

Question. What is the commercial unit of energy?

(a) electron volt

(b) W/s

(c) kW h

(d) joule

Answer :   C

Question. When a body like earth is moving in a circular path the work done in that case is zero because:

(a) Centripetal force acts in the direction of motion of the body

(b) Centripetal force acts along the radius of circular path

(c) Gravitational force acts along the radius of circular path

(d) Centrifugal force acts perpendicular to the radius of circular path

Answer :   B

Question. 9800 joule of energy was spent to raise a mass of 50kg, the mass was raised to a height of

(a) 20m

(b) 980m

(c) 10m

(d) none of these

Answer :   A

Question. SI unit of power is

(a) watt

(b) joule

(c) newton

(d) meter

Answer :   A

Question. A girl is carrying a school bag of 3 kg on her back and moves 200 m on a leveled road. The work done against the gravitational force will be (g = 10 ms²)

(a) 6 × 10³ J

(b) 6 J

(c) 0.6 J

(d) zero

Answer :   D

Question. Work done by a body is

(a) negative, positive or zero

(b) always positive

(c) always zero

(d) always negative

Answer :   A

Question. A rocket rises up in the air due to the force generated by the fuel. The work done by the:

(a) fuel is negative work and that of force of gravity is positive work

(b) fuel is positive work and that of force of gravity is negative work

(c) both fuel and force of gravity do positive work

(d) both fuel and force of gravity do negative work

Answer :   B

Question. A body is falling from a height of 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

Question. If speed of a car becomes 2 times, its kinetic energy becomes

(a) 4 times

(b) 8 times

(c) 16 times

(d) 12 times

Answer :   A

Question. The form of energy possessed by a flying bird is:

(a) Kinetic energy

(b) Potential energy

(c) Both kinetic and potential energy

(d) Can’t say

Answer :   C

Question. A ball is dropped from a height of 10 m.

(a) Its potential energy increases and kinetic energy decreases during the falls

(b) Its potential energy is equal to the kinetic energy during the fall.

(c) The potential energy decreases and the kinetic energy increases during the fall.

(d) The potential energy is ‘O’ and kinetic energy is maximum while it is falling.

Answer :   C

Question. The work done is '0' if

(a) The body shows displacement in the opposite direction of the force applied.

(b) The body shows displacement in the same direction as that of the force applied.

(c) The body shows a displacement in the perpendicular direction to the force applied.

(d) The body masses obliquely to the direction of the force applied.

Answer :   B

Question. Work done by the gravitational force of earth on a satellite along a circular path is

(a) O

(b) minimum

(c) max

(d) all of these

Answer :   A

Question. A body is falling from a height of h. After it has fallen to a height of h/2, it will possess:

(a) Only kinetic energy

(b) Half kinetic and half potential energy

(c) Only potential energy

(d) More kinetic and less potential energy

Answer :   D

Question. When speed of the moving object is doubled its

(a) Acceleration is doubled

(b) Momentum becomes four times more

(c) K E. is increased to four times

(d) Potential energy is increased

Answer :   C

1. Work done by centripetal force is

(a) >0

(b) ≥0

(c) <0

(d) 0

► (d) 0

2. When a body falls freely towards the earth, then its total energy

(a) Decreases

(b) Increases

(c) First increases and then decreases

(d) Remains constant

► (d) Remains constant

3. If work is done at a faster rate, then

(a) power is more

(b) No power is required to do work

(c) power is less

(d) Infinite power is required

► (a) power is more

4. Moon revolves around the earth due to the gravitational force (F) of earth on the moon. The work done by the gravitational force is (r = radius of circular orbit of moon)

(a) F.2πr

(b) F.πr

(c) zero

(d) negative work

► (c) zero

5. Work done by a body is

(a) negative, positive or zero

(b) always positive

(c) always zero

(d) always negative

► (a) negative, positive or zero

6. 1 W. S = ________?

(a) 10 joule

(b) 1 joule

(c) 3.6 × 106 joule

(d) 2 joule

► (b) 1 joule

7. Scientifically, work is said to be done in which of the following cases

(a) Manish pulls the trolley for a certain distance

(b) Mira studies for her exam

(c) Manoj pushes the wall with no change in the position of wall

(d) Mira pulls the wall with no change in the position of wall

► (a) Manish pulls the trolley for a certain distance

8. P.E of a spring increases when it is in the state of

(a) compression

(b) neither a nor b

(c) expansion

(d) both a and b

► (d) both a and b

9. When speed of the moving object is doubled its

(a) Acceleration is doubled

(b) Momentum becomes four times more

(c) K E. is increased to four times

(d) Potential energy is increased

► (c) K E. is increased to four times

10. A ball is thrown vertically upwards. Its velocity keeps on decreasing. What happens to its kinetic energy when it reaches the maximum height?

(a) Kinetic energy is double

(b) Kinetic energy is zero

(c) Kinetic energy is maximum at maximum height

(d) Kinetic energy is half of initial value

► (b) Kinetic energy is zero

11. A stone weighing 1 kg is dropped from rest from a height of 4 meters above the ground. When it has free-fallin 1 meter its total energy with respect to the ground is

(a) 15 J

(b) 40 J

(c) 20 J

(d) 30 J

► (b) 40 J

12. The energy possessed by an oscillating pendulum of a clock is        

(a) kinetic energy

(b) potential energy

(c) restoring energy

(d) mechanical energy

►(d) mechanical energy

13. What is the commercial unit of energy?

(a) electron volt

(b) W/s

(c) kW h

(d) joule

► (c) kW h

14. Calculate the work done in moving a body of mass 50 Kg through a height of 5 m.(g = 10m/s2)​

(a) 250 J

(b) 2500 J

(c) 25 x 103 J

(d) 2.5 x 105 J

► (b) 2500 J

15. Find the correct statement

(a) 1 Gigawatt is equal to 109W

(b) power is the rate of doing work

(c) both a and b

(d) none of these

► (c) both a and b

16. If a force of F newton moves a body with constant speed v, the power delivered by it is

(a) F/v

(b) Fv

(c) F2v

(d) v/F

► (b) Fv

17. According to the law of conservation of energy

(a) Energy can’t be destroyed

(b) Energy can’t be created

(c) Energy can be transformed from one form to another

(d) All of the above

► (d) All of the above

18. In SI system, the unit of P.E. is

(a) erg

(b) dyne-cm

(c) J

(d) None of these

► (c) J

19. In the SI system, the unit of potential energy is

(a) newton

(b) joule

(c) watt

(d) meter per second

► (b) joule

20. Two bodies of equal weight are kept at heights of h and 1.5 A, respectively. The ratio of their PE is

(a) 3 : 2

(b) 2 : 3 

(c) 1 : 1

(d) 4 : 3

► (b) 2 : 3 

21.  On tripling the speed of motion of a body, the change in K.E. is

(a) 9 times

(b) 8 times

(c) 4 times

(d) 2 times

► (a) 9 times

22. A stone is tied to a string and whirled in a circular path. The work done by the stone is :

(a) negative

(b) zero

(c) positive

(d) none of the above

► (b) zero

23. 9800 joule of energy was spent to raise a mass of 50kg, the mass was raised to a height of

(a) 20m

(b) 980m

(c) 10m

(d) none of these

► (a) 20m

24. The linear momentum of a body of mass 2kg is 16kgms−1. What is its K. E?

(a) 64J

(b) 70J

(c) 60J

(d) none of these

► (a) 64J

25. Potential energy and kinetic energy are types of

(a) Electrical energy

(b) Magnetic energy

(c) Thermal energy

(d) Mechanical energy

► (d) Mechanical energy

26. A body of mass 5 kg is moving with an acceleration of 5 ms-2 gets a displacement of 2m, the work done by the force is:​

(a) 75 J

(b) 25 J

(c) 100 J

(d) 50 J

► (d) 50 J

27. Work done is said to be positive when a force causes displacement:

(a) In its own direction

(b) In the direction opposite to the applied force

(c) In the direction at right angles to the direction of applied force

(d) None of the above

► (a) In its own direction

28. Two masses m and 2m are dropped from a certain height ‘h’. Then on reaching the ground,

(a) K.E. of them will be equal

(b) K.E. of the heavier is 4 times the K.E. of the lighter

(c) K.E. of the lighter is 4 times the K.E. of the heavier

(d) K.E. of the heavier is more than that of the lighter

► (d) K.E. of the heavier is more than that of the lighter

29. A rocket rises up in the air due to the force generated by the fuel. The work done by the:

(a) fuel is negative work and that of force of gravity is positive work

(b) fuel is positive work and that of force of gravity is negative work

(c) both fuel and force of gravity do positive work

(d) both fuel and force of gravity do negative work

► (b) fuel is positive work and that of force of gravity is negative work

30. A body is moved through a distance of 3 m in the following different ways. In which case is the maximum work done?

(a) when pushed over an inclined plane

(b) when lifted vertically upward

(c) when pushed over smooth rollers

(d) when pushed on a plain horizontal surface

► (b) when lifted vertically upward

31. In winters, rubbing of hands together for some time, causes a sensation of warmth mainly because of 

(a) heat caused by the force of friction 

(b) heat caused by the momentum 

(c) heat caused by the motion 

(d) heat flows from the blood to skin

► (a) heat caused by the force of friction 

32. An electric lamp of 100w is used for 5 hours per day to calculate the units of energy consumed by the lamp in one day

(a) 1 unit

(b) 0.5 unit

(c) 1.5 units

(d) none of these

► (b) 0.5 unit

33. One unit of electricity is consumed by

A : P = 40 W bulb used for t = 25 hours

B : P = 20 W bulb used for t = 50 hours

Both A and B are true as, energy E in kWh is best related as

(a) E = Pt

(b) E = Pt/1000

(c) E = P/1000

(d) E = t/1000

► (b) E = Pt/1000

34. Work done by the gravitational force of earth on a satellite along a circular path is

(a) O

(b) minimum

(c) max

(d) all of these

► (a) O

35. In which case work is not done:

(a) A girl swimming in a pond

(b) A windmill lifting water from a well

(c) A standing man holding a suitcase in his hand

(d) A sail boat moving in the direction of wind

► (c) A standing man holding a suitcase in his hand

36. A boy holds a mass on his stretched hand. Then

(a) Work done against gravity is zero.

(b) Muscular energy is used

(c) Both (a) and (b)

(d) Neither (a) nor (b)

► (c) Both (a) and (b)

Work and Energy class 9 physics SAQ

Does work done depend upon the velocity of the body. [SAII-2014]

Answer. No.

State the law of conservation of energy. [SAII-2014]

Answer. It states that energy can neither be created nor destroyed. It can only change its form.

In a tug-of-war one team gives way to the other. What work is being done and by whom ? [SAII-2014]

Answer.

The winning team does work. The work is equal to the product of the resultant force and the displacement undergone by the losing team.

List two essential conditions for work to be done. [SAII-2010]

Answer. (i) A force must act and (ii) There should be displacement in the body.

What is power? What is its SI unit ?

Answer. It is defined as the rate of doing work. Its unit is watt.

 Write an expression for the work done in lifting a body of mass ‘m’ through a vertical height ‘h’. [SAll-2012]

Answer. Work done W = mgh, where g is acceleration due to gravity.

 When a book is lifted from a table, against which force work is done ?

Answer. Work is done against the force of gravity.

 What is the work done when the force acting on the body and the displacement produced in the body are at right angles to each other ?

Answer. Zero.

A light and a heavy body have equal kinetic energy. Which one is moving fast ? [SAII-2011]

Answer. The lighter body is moving fast.

Is it possible that some force is acting on a body but still the work done is zero ?

Answer. Yes, when force acts at an angle of 90° with the displacement.

 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 ?

Answer.

Given, displacement = 8 m,

Force = 7N

Now, Work done = Force x Displacement

= 7 x 8 = 56 J

When do we say that work is done ? ~

Answer.

Work is said to be done when a force causes displacement of an object in the direction of applied force.

A pair of bullocks exert a force of 140 N on a plow. The field being plowed is 15 m long. How much work is done in plowing the length of the field ?

Answer.

Work done = Force x Displacement = 140 x 15 = 2100 J

What is the kinetic energy of an object?

Answer.

The energy possessed by a body by virtue of its motion is called kinetic energy.

Define average power.

Answer.

When a machine or person does different amounts of work or uses energy in different intervals of time, the ratio between the total work or energy consumed to the total time is average power.

Define energy.

Answer.

Energy is the ability of a body to do work. It is also defined as the capacity to do work

A body performs no work. Does it imply that the body possesses no energy ?

Answer.

When a body does not perform any work, it never implies that the body has no energy. The body may have energy but still does not perform any work, e.g., a book placed on a table has potential energy but is not performing any work.

A cell converts one form of energy into another. Name the two forms.

Answer.

It converts chemical energy into electrical energy.

 When an arrow is shot from its bow, it has kinetic energy. From where does it get the kinetic energy ? [SAll-2010]

Answer.

A stretched bow possesses potential energy on account of a change in its shape. To shoot an arrow; the bow is released. The potential energy of the bow is converted into the kinetic energy of the arrow.

Name at least three commonly used units of energy.

Answer. (i) Joule (ii) Erg (iii) Kilowatt hour.

Name at least six forms of energy.

Answer.

(i) Chemical energy

(ii) Heat energy

(iii) Light energy

(iv) Electrical energy

(v) Sound energy

(vi)Solar energy

How many watt are there in 1 horse – power ?

Answer. 746 watt.

What is horsepower ?

Answer. It is a unit of power.

 State the relation between kWh and joule. Define 1 watt. [SAII-2014]

Answer.

1 kW h – 1000 W h = 1000  Js-1  x 60 x 60 s = 3.6 x  106 J

1 watt is the power of an agent which can do one joule of work in one second.

Is it possible for a body to be in accelerated motion under a force acting on the body, yet no work is being done by the force? Explain your answer giving a suitable example. [SAII-2012]

Answer. Yes, it is possible, when the force is perpendicular to the direction of motion. The moon revolving around the earth under the centripetal force of attraction of the earth but earth does not do any work on the motion of The moon.

Define work. How is work measured ? When is work done by a force negative? [SAII-2013]

Answer. Work is said to be done if force acting on an object displaces it through a certain distance.

It is measured as the product of force and displacement.

Work done is negative if force and displacement are in the opposite direction

What is the work done by the force of gravity in the following cases ?

(a) Satellite moving around the earth in a circular orbit of radius 35000 km.

(b) A stone of mass 250 g is thrown up through a height of 2.5 m. [SAII-2013]

Answer.

(a) Zero, as the displacement in one complete revolution is zero.

(b) Given m = 250 g = 0.25 kg, h = 2.5 m, g = 10 ms-2, W = ?

Now, W = FS = mg x h = 0.25 x 10 x 2.5 = 6.25 J

A mass of 10 kg is at point A on a table. It is moved to 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.

Answer. The work done is zero. This is because the gravitational force and displacement are perpendicular to each other.

The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why? [SAII-2010]

Answer. It does not violate the law of conservation of energy. Whatever, is the decrease in PE due to loss of height, same is the increase in the KE due to increase in velocity of the body.

 What are the various energy transformations that occur when you are riding a bicycle?

Answer. The chemical energy of the food changes into heat and then to muscular energy. On paddling, the muscular energy changes into mechanical energy.

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 ? [SAll-2011 ]

Answer. Since the body returns to a point which is on the same horizontal line through the point of projection, no displacement has taken place against the force of gravity; therefore, no work is done by the force due to gravity.

A battery lights a bulb. Describe the energy changes involved in the process.

Answer. Within the electric cell of the battery the chemical energy changes into electrical energy. The electric energy flowing through the filament of the bulb first changes into heat energy and then into light energy.

Can there be displacement of an object in the absence of any force acting on it? Think;

discuss this question with your friends and teacher. [SAII-2012]

Answer. The answer is. both Yes and No. Yes, because when an object moves in deep space from one

point to another point in a straight line, the displacement takes place, without the application of

force. No, because force cannot be zero for displacement on the surface of earth. Some force is essential.

A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy ? [SAI1-2011]

Answer. The KE on reaching the ground changes into heat energy, sound energy etc. and, therefore, dissipates in air.

Name the transformation of energy involved in the following cases :

(a) When a body is thrown upwards.

(b) When a body falls from the top of a hill.

(c) When coal burns.

(d) When a gas bums.

(e) When water falls from a height.

Answer.

(a) Kinetic energy into potential energy.

(fa) Potential energy into kinetic energy.

(c) Chemical energy into heat energy.

(cf) Chemical energy into heat energy.

(e) Potential energy into kinetic energy.

 What are the factors on which the work done depends ? [SAII-2010]

Answer.The work done by a force depends upon:

(i) The magnitude of the force.

(ii) The magnitude of the displacement and

(iii) The angle between force and displacement.

 What is the work done by a coolie walking on a horizontal platform with a load on his head ?

Answer. In order to balance the load on his head, the coolie applies a force on it in the upward direction, equal to its weight. Its displacement is along the horizontal direction. Thus, the angle between force F and displacement is 90°. Therefore, work done W = FS cos θ = FS cos 90° =0.

We wind our watch once a day, what happens to the energy ?

Answer. When we wind our watch, we wind the spring inside the watch. As a result, energy is stored in the spring in the form of elastic potential energy. This elastic potential energy is used to make the watch work the whole day. .

 What is the amount of work done by a force when a body moves in a circular path ? [SAll-2012]

Answer. Work done is given by the expression W = FS cos θ. When a body moves in a circular path, then the displacement (S) is zero. Therefore, work done is W = F x 0 = 0.

Work and Energy class 9 physics long QUESTION

 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. When the pendulum bob is pulled (say towards left), the energy supplied is stored in it is the form of PE on account of its higher position. When the pendulum is released so that it starts moving towards the right, then its PE changes into KE, such that in the mean position, it has maximum KE, and zero PE. As the pendulum moves towards the extreme right, its -KE changes into PE such that at the extreme position, it has maximum PE and zero KE. When it moves from this extreme position to the mean position, its PE again changes to KE. This illustrates the law of conservation of energy. Eventually, the bob comes to rest, because during each oscillation a part of the energy possessed by it transferred to air and- m overcoming friction at the point of suspension. Thus, the energy of the pendulum is dissipated in air.

The law of conservation of energy is not violated because the energy merely changes its form and is not destroyed.

Look at the activities listed below.

Reason out whether or not work is done in the light of your understanding of the term ‘work’

(i) Suma is swimming in a pbnd.

(ii) A donkey is carrying a load on its back.

(iii) A wind-mill is filling water from a well.

(iv) A green plant is carrying out photosynthesis.

(v) An engine is pulling a train.

(vi)Food grains are getting dried in the sun.

(vii) A sailboat is moving due to wind energy.

Answer.

(i) Work is done because the displacement of the swimmer takes place in the direction of applied force.

(ii) If the donkey is not moving, no work is done as the displacement of load does not take place in the direction of applied force.

(iii) Work is done, as the displacement takes place in the direction of force.

(iv) No work is done, because no displacement takes place.

(v) Work is done, because displacement takes place in the direction of applied force.

(vi)No work is done, because displacement does not take place.

(vii)Work is done because displacement takes place in the direction of the force.

 Distinguish between work, energy and power. State the SI units for each of these quantities.

Answer.

Work: It is defined as the product of force applied and the distance moved by the body on the application of the force. In SI it is measured in joules.

Energy : It is defined as the capacity of a body to do work. In SI it is measured in joules.

Power: It is defined as the rate of doing work. It measures how fast or slow the work is done. In SI it is measured in watts.

Question.5 A girl sits and stands repeatedly for 5 minutes. Draw a graph to show the variation of potential energy with time.

Answer.

 Calculate the electricity bill amount for a month of 31 days, if the following devices are used as specified :

(a) 3 bulbs of 40 W for 6 hours.

(b) 4 tube lights of 50 W for 8 hours,

(c) A TV of 120 W for 6 hours.

The rate of electricity is Rs 2.50 per unit. [SAll-2014]

Answer.

(a) What is meant by mechanical energy ? State its two forms. State the law of conservation of energy. Give an example in which we observe a continuous change of one form of energy into another and vice-versa.

(b) Calculate the amount of work required to stop a car of 1000 kg moving with a speed of 72 km h”1. [SAII-2013]

Answer.

(a) It is the sum of KE and PE of an object. It states that energy can neither be created nor be destroyed. We observe a continuous change in energy in a simple pendulum and its : explanation. At the mean position, the energy is wholly kinetic while at the extreme position it is wholly potential. As the pendulum oscillates, its energy continuously changes between kinetic and potential.

(a)Derive an expression for kinetic energy of a body having mass m and moving with a velocity v.

(b)When the velocity of a body is increased 5 times, what is the change in its kinetic energy ?

(c)Two masses m and 2m are dropped from heights h and 2h. On reaching the ground, which will have greater kinetic energy and why ? [SAII-2013]

Answer.

(a)For derivation see above questions.

(b)Kinetic energy is given by the expression

KE = 1/2 mv2, therefore, if velocity is made 5 times KE will increase by 25 times.

(c)More potential energy will be the kinetic energy of the body when it falls. Hence, the body with mass 2m will have greater kinetic energy as it has more potential energy.

(a)State the law of conservation of energy.

(b)What is the work done to increase the velocity of a car from 36 km h-1 to 72 km h~1 if the mass of the car is 1500 kg ? Does the work done by the force have a negative or a positive magnitude ?

(c)Where does an oscillating pendulum have maximum PE and KE ? [SAII-2013]

Answer.

 

What is the work done by a coolie walking on a horizontal platform with a load on his head? [SAII-2010]

Answer. In order to balance the load on his head, the coolie applies a force on it in the upward direction, equal to its weight. Its displacement is along the horizontal direction. Thus, the angle between force F and displacement is 90°. Therefore, work done W = FS cos θ = FS cos 90° = 0.

 The work done in lifting a box onto a platform does not depend upon how fast it is lifted up.

Explain your answer giving proper reasoning. [SAll-2013]

Answer. The work done (W) in lifting a box through a distance (S) against the gravitational force (F) is given by W = FS. Hence, it is obvious that it is independent of the rate at which the box is lifted.

(a) How much work is done when a force of 1 N moves a body through a distance of 1 m in its direction?

(b) Is it possible that a force is acting on a body but still the work done is zero? Explain

giving one example. [SA II – 2011]

Answer.

(a) 1 J of work is done.

(b) Yes, it is possible when force acts at right angles to the direction of motion of the body. Example Gravitational force of earth acts on a satellite at right angles to its direction of motion.

(a) What is meant by potential energy? Is potential energy a vector or a scalar quantity?

(b) Give one example of a body having potential energy. [SA II – 2011]

Answer.

(a) The energy possessed by a body by virtue of its position or configuration. It is a scalar

quantity.

(b) Stretched string of a bow.

When is the work done by a force said to be negative? Give one situation in which one of the forces acting on the object is doing positive work and the other is doing negative work. [SA II – 2012]

Answer. We know that work done W = FS cos 0, where 0 is the angle between F and S. Clearly, W will be -ve, if 0 is between 90° and 180° because then cos 0 will be -ve. Consider the case of a body falling under gravity. The body experiences an upward frictional force and downward force due to gravity. Since the body is moving downwards, the work done by force to gravity will be +ve but that is against the upward thrust will be -ve.

Is it possible for a body to be in accelerated motion under the action of a force, yet no work is being done by the force? Explain with an example.

(b) Two bodies of masses m, and m2 have equal kinetic energies. What is the ratio of their

linear momenta? [SA II – 2012]

Answer.

(a) Yes, it is possible in the case of a body moving in a circular path with a speed v. The body has a centripetal acceleration directed along the radius of the circular path. The

displacement is, however, tangential to the radius i.e., 0 = 90°. Thus, work done, W = FS cos 90° = 0.