Class 9 Science Chapter 10 question answer Work and Energy

Ncert Solutions for Class 9 Science Chapter 10: work and energy class 9 questions and answers

Textbook	Ncert
Class	Class 9
Subject	Science
Chapter	Chapter 10
Chapter Name	Work and Energy class 9 ncert solutions
Category	Ncert Solutions
Medium	English

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In Text Questions Page No: 115

Question 1: A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force (Fig. 10.3). Let us take it that the force acts on the object through the displacement. What is the work done in this case?

Answer 1: When a force F acts on an object to move it in its direction through a distance S,  work is done.

The work on the body is done by force.

Work done = Force × Displacement

W = F × S

• Where,
• F = 7 N
• S = 8 m
• So, work done,
• W = 7 × 8
• W = 56 Nm
• W = 56 J

In Text Questions Page No: 116

Question 1: When do we say that work is done?

Answer 1: Work is said to be done when a force causes displacement of an object in the direction of applied force.

Question 2: Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Answer 2: When a force F displaces a body through a distance S in the direction of the applied force, then the work done W on the body is given by the expression:
Work done = Force × Displacement
W = F × s

Question 3: Define 1 J of work.

Answer 3: 1 J is the amount of work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force.

Question 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?

Answer 4: Work done by the bullocks is given by the expression: W= F × d Where, Applied force, F = 140 N Displacement, d = 15 m W = 140 x 15 = 2100 J Therefore, 2100 J of work is done in ploughing the length of the field.

In Text Questions Page No: 119

Question 1: What is the kinetic energy of an object?

Answer 1: The energy possessed by an object due to its motion is called kinetic energy.

Question 2: Write an expression for the kinetic energy of an object.

Answer 2: The expression for the kinetic energy of an object is:

\(KE = \frac{1}{2}mv^2\)

Where:

• KE is the kinetic energy (in joules, J)
• m is the mass of the object (in kilograms, kg)
• v is the speed of the object (in meters per second, m/s)

Question 3: 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 three times?

Answer 3: Given

• K.E. of the object = 25J
• Velocity of the object (v) = 5 m/s
• K.E. = (½) mv²
• 25 = (½) m (5)²
• 50 = 25 x m
• m = 50/25
• m = 2 kg

• Now, when velocity is doubled
• v = 10 m/s
• m = 2 kg
• K.E. = (½) x 2 x (10)²
• K.E. = 10²
• K.E. = 100 J

• When velocity is increased three times, then
• v = 15 m/s
• m = 2 kg
• K.E. = (½) x 2 x (15)²
• K.E. = (15)²
• K.E. = 225 J

In Text Questions Page No: 123

Question 1: What is power?

Answer 1: Power is the rate of doing work or the rate of transfer of energy. If W is the amount of work done in time t, then power is given by the expression,

The rate of doing work or energy conversion rate is called power.

Power = Work/time

or P = W/t

Where,

• P = Power
• w = Work
• t = Time

Question 2: Define 1 watt of power.

Answer 2: Power of an agent is 1watt if work is done at the rate of 1 joule per second.
Watt = joule/second 1 W = 1 Js^-1 = 10⁷ ergs^-1.

Question 3: A lamp consumes 1000 J of electrical energy in 10 s. What is its power?

Answer 3: Power = Work Done/Time
Work done = Energy consumed by the lamp = 1000 J
Time = 10 s
Power= 1000 / 10 = 100 Js^-1=100 W

Question 4: Define average power.

Answer 4: Average power is defined as the ratio of total work done by the body to the total time taken by the body.

Average Power = Total work done/Total time taken

Exercises

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’.
• Suma is swimming in a pond.
• A donkey is carrying a load on its back.
• A wind mill is lifting water from a well.
• A green plant is carrying out photosynthesis.
• An engine is pulling a train.
• Food grains are getting dried in the sun.
• A sailboat is moving due to wind energy.

Answer 1: Work is said to be done when a force is applied on an object and the object moves in the direction of the force.

Suma is swimming in a pond.

✅ Work is done – She is applying force to move herself through water.

• A donkey is carrying a load on its back.

❌ No work is done (scientifically) – The force (load) is vertical, but the motion is horizontal. No displacement in the direction of the force.

• A windmill is lifting water from a well.

✅ Work is done – The windmill applies force to move water upwards (displacement in the direction of force).

• A green plant is carrying out photosynthesis.

❌ No mechanical work is done – There is no physical displacement of an object.

• An engine is pulling a train.

✅ Work is done – The engine applies force, and the train moves in that direction.

• Food grains are getting dried in the sun.

❌ No mechanical work is done – Drying is a chemical/physical process, not due to applied force and displacement.

• A sailboat is moving due to wind energy.

✅ Work is done – Wind applies force, and the boat moves (displacement happens).

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?

Answer 2: 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.

Question 3: A battery lights a bulb. Describe the energy changes involved in the process.

Answer 3: When a bulb is connected to a battery, then the chemical energy of the battery is transferred into electrical energy. When the bulb receives this electrical energy, then it converts it into light and heat energy. Hence, the transformation of energy in the given situation can be shown as: Chemical Energy → Electrical Energy → Light Energy + 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.

Answer 4: Given

• Initial velocity u = 5 m/s
• Mass of the body = 20kg
• Final velocity v = 2 m/s
• The initial kinetic energy
• E_i = (1/2) mu² = (1/2) × 20 × (5)² 
• = 10 × 25
• = 250 J
• Final kinetic energy 
• E_f = (1/2) mv² = (1/2) × 20 × (2)² 
• = 10 × 4
•  = 40 J
• Therefore,
• Work done = Change in kinetic energy
• Work done = E_f – E_i
• Work done = 40 J – 250 J
• Work done = -210 J

Question 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.

Answer 5: Work done by gravity depends only on the vertical displacement of the body. It does not depend upon the path of the body. Hence, work done by gravity is given by:

W = mgh

Where,

Vertical displacement, h=0

⇒W = mg(0)=0J

Thus, the work done by gravity on the object is zero joules.

Question 6: The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Answer 6: The energy conservation law is not broken by this procedure. This is because the body gradually transforms its potential energy into kinetic energy when it descends from a height. The body’s kinetic energy increases in proportion to a decrease in potential energy. The body’s whole mechanical energy is conserved during the process. As a result, the law of energy conservation remains intact.

Question 7: What are the various energy transformations that occur when you are riding a bicycle?

Answer 7: The food which the cyclist has eaten has chemical energy. While cycling, the cyclist transfers this chemical energy into mechanical energy. In turn, mechanical energy of the cyclist does work against rolling friction between the cycle and the ground. In doing work against friction, he converts part of the energy into sound energy and heat energy.

Question 8: 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?

Answer 8: No, when we push a huge rock with all our strength but cannot move it, there is no transfer of energy. When we push the rock, our muscles tense and blood is displaced very rapidly to these muscles. These changes consume energy and we feel tired.

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

Answer 9: 1 unit of energy is equal to 1 kilowatt hour (kWh).
1 unit = 1 kWh
1 kWh = 3.6 × 10⁶ J
Therefore, 250 units of energy = 250 x 3.6 × 10⁶ = 9 × 10⁸ J.

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.

Answer 10: Given

• Mass (m) = 40 kg
• Acceleration due to gravity (g)= 10m/s²
• Height (h)= 5m

(1) Potential Energy at 5 m:

PE = mgh = 40×9.8×5 = 1960J

Potential energy = 1960 J

Kinetic Energy when it is halfway down (i.e., at 2.5 m)

At halfway, some potential energy has been converted into kinetic energy.

• Potential energy lost = mgh = 40×9.8×2.5 = 980 J
• This lost potential energy becomes kinetic energy.

Kinetic energy halfway down = 980 J

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

Answer 11: When a satellite moves round the Earth, then at each point of its path, the direction of force of gravity on the satellite (along the radius) is perpendicular to the direction of its displacement (along the tangent). Hence, the work done on the satellite by the force of gravity is zero.

Question 12: 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.

Answer 12: Yes, there be displacement of an object in the absence of any force acting on it by moving with uniform velocity. Suppose an object is moving with constant velocity, then net force acting on it is zero. Hence, there can be a displacement without a force.

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.

Answer 13: When the two requirements are met, the work is finished. (i) The body is subject to a force. (ii) The body is moved by exerting force in the force’s direction or the opposite direction. There isn’t any movement in a hay bundle when someone holds it above their head. The bundle is being affected by the force of gravity, but the individual is not exerting any effort on it. Consequently, the person’s work on the bundle is zero in the absence of force.

Question 14: An electric heater is rated 1500 W. How much energy does it use in 10 hours?

Answer 14: Energy = Power×Time

Given:

• Power = 1500 W = 1.5 kW
• Time = 10 hours

Energy used:

Energy = 1.5kW×10h = 15kWh

Energy used = 15 kilowatt-hours (kWh)
(or 54,000,000 joules if converted: (15×3.6×10⁶)

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 15: 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 right, then its PE changes into KE such that in mean position, it has maximum KE, and Zero PE. As the pendulum moves towards 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 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 in 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.

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?

Answer 16: Kinetic energy of an object of mass, moving with a velocity, v is given by the expression, E_k= 1/2 mv²

To bring the object to rest, 1/2 mv² amount of work is required to be done on the object.

Question 17: Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?

Answer 17: Given data:

The mass of the body = 1500kg

Velocity v = 60km/hr

\(\frac{60 × 1000 \text{ m}}{3600 \text{ s}}\)

The work required to stop the moving car = change in kinetic energy

\(= \frac{1}{2}mv^{2}\)

\(=\frac{1}{2}\times 1500\times \left ( \frac{50}{3} \right )^{2}\)

=-208333.3 J

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.

Answer 18: 1st Diagram:

• Force FF: Acting upward (vertical)
• Displacement ss: Toward the right (horizontal, east)

🔸 Force is perpendicular to displacement
✅ Work done = 0

2nd Diagram:

• Force FF: Acting in the same direction as displacement (both to the right)

🔸 Force is in the direction of displacement
✅ Work done is positive

3rd Diagram:

• Force FF: Acting in the opposite direction of displacement
• Displacement ss: Toward the right, but force is to the left

🔸 Force is opposite to displacement
✅ Work done is negative

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 her? Why?

Answer 19: Acceleration in an object could be zero even when many forces work on it. This happens when all the forces get rid of one another, i.e., the online force working on the object is zero. For a uniformly moving object, the online force working on the it is zero. Hence, the acceleration of the thing is zero. Hence, Soni is correct.

Question 20: Find the energy in joules consumed in 10 hours by four devices of power 500 W each.

Answer 20: Total power of 4 devices = 4 × 500 = 2,000 W
Energy consumed in 10 hours = Pt
E = 2,000 W × 10 h = 20,000 Wh
E = 20 kWh.

Question 21: A freely falling object eventually stops on reaching the ground. What happenes to its kinetic energy?

Answer 21: Its kinetic energy has been converted to heat and sound energy on impact with the ground. We hear some sound of impact. Also there is an increase in temperature at the place of impact.