NCERT Question and Answer for Class 9 Science Chapter 11 Work Power and Energy

Let's start with a brief and easy explanation of Chapter 11 - Work, Power, and Energy from Class 9 Science NCERT book.

This chapter explores the interesting concepts of work, power, and energy, which are all around us in our daily activities. Whether you're lifting a bag, pushing a book, or riding a bicycle, these principles play a key role.

Understanding Work:

In physics, work happens when a force acts on an object, and the object moves in the direction of the force. For instance, lifting a heavy object or pulling it across the floor involves doing work.

Work Formula:

The formula for work (W) is:

W = Force × Distance × Cos (Θ)

Here, Θ (Theta) is the angle between the force and the direction of motion.

Unit of Work:

Work is measured in joules (J), where

1 Joule = 1 Newton × 1 Meter

Power:

Power is the rate at which work is done, indicating how quickly work is accomplished.

The formula for power (P) is:

The unit of power is the watt (W), where 1 Watt = 1 Joule/ Second

Energy:

Energy is the ability to do work. There are various forms like kinetic, potential, and thermal. The law of conservation of energy states that energy cannot be created or destroyed; it can only change from one form to another.

Kinetic and Potential Energy:

Kinetic Energy (KE): This is the energy a body has due to its motion.

The formula is where m is the mass and v is the velocity.

Potential Energy (PE): This is the energy a body has due to its position. Gravitational potential

energy is

Understanding these concepts gives us insights into how the physical world works and shapes our everyday experiences.

This chapter sets the stage for a deeper exploration of mechanics and the role of energy in various natural phenomena. Let's embark on this exciting journey into the world of work, power, and energy!

Let’s move on to the NCERT Question and Answer section for Chapter 11. Feel free to let me know if you have specific questions or if you'd like a variety of questions and answers.

NCERT Class 9 Science – Page 148

Q1. A force of 7 N acts on an object. The displacement is 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: The work done ( $W$ ) can be calculated using the formula:

W=Force×Distance

W=7N×8m

W=56Joules

NCERT Class 9 Science – Page 149

Q1. When do we say that work is done?

Ans: Work is done in physics when a force causes an object to move.

Q2. 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) x Displacement (d)

Q3. Define 1J of work.

Ans: One joule (1 J) of work is defined as the amount of work done when a force of one newton is applied to move an object one meter in the direction of the force.

For Understanding Only

One joule (1 J) of work is like lifting a small apple (about 100 grams) up to your desk – the effort it takes to move it one meter against the force of gravity.

Q4. A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?

Ans: Work done (W) = Force (F) x Displacement (d) = 140 x 15 = 2,100 J

NCERT Class 9 Science – Page 152

Q1. What is the kinetic energy of an object?

Ans: Kinetic energy is the energy an object possesses due to its motion.

Q2. Write an expression for the kinetic energy of an object.

Ans: The expression for the kinetic energy ( $K E$ ) of an object is given by:

KE=21×mass×velocity2

Ans: Given v= 5m/s, KE= 25J

m=?

We Know

that,

K E = \frac{1}{2} \times m \times v^{2}

(i) When the velocity is doubled:

Initial velocity (v) = 5 m/s New velocity (2 x v) becomes 10 m/s K E = \frac{1}{2} \times m \times v^{2} K E = \frac{1}{2} \times 2 \times (10)^{2} K E = 100 J (ii) When the velocity is increased three times: Initial velocity (v) = 5 m/s New velocity (3 x v) becomes 15 m/s K E = \frac{1}{2} \times m \times v^{2} K E = \frac{1}{2} \times 2 \times 15^{2} K E = 225 J NCERT Class 9 Science - Page 156 Q1. What is power? Ans: Power is how fast work is done or energy is used. It's measured in watts (W). Q2. Define 1 watt of power. Ans: One watt of power is the amount of energy used or work done at a rate of one joule per second. Q3. A lamp consumes 1000 J of electrical energy in 10 s. What is its power? Ans: We know that P = \frac{Energy}{Time}

P = \frac{1000 J}{10 s} P = 100 Watt Q4. Define average power. Ans: Average power is the amount of energy used or work done per unit of time. NCERT Class 9 Science - Page 158 Q1. Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term ‘work’. (a) Suma is swimming in a pond. (b) A donkey is carrying a load on its back. (c) A wind mill is lifting water from a well. (d) A green plant is carrying out photosynthesis. (e) An engine is pulling a train. (f) Food grains are getting dried in the sun. (g) A sailboat is moving due to wind energy. Ans: (a) Suma is swimming: No work is done as swimming doesn't involve a force causing displacement.

(b) A donkey carrying a load: Work is done as the force exerted by the donkey causes the displacement of the load.

(c) Windmill lifting water: Work is done as the windmill exerts force to lift water against gravity, resulting in displacement.

(d) Green plant photosynthesis: Work is done in a biological sense during photosynthesis as the plant converts energy.

(e) Engine pulling a train: Work is done as the force exerted by the engine results in the displacement of the train.

(f) Food grains drying in the sun: No work is done as the drying process is a natural occurrence, not involving force and displacement.

(g) Sailboat moving due to wind energy: Work is done as the force of the wind on the sail causes the sailboat to move, involving displacement.

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: The work done by the force of gravity is zero because the displacement and the force act in perpendicular directions.

For Understanding Only

The work done by the force of gravity on the object is zero.

This is because the force of gravity acts vertically downward, while the displacement (change in position) of the object is horizontal. In physics, work is calculated as the dot product of force and displacement. When the force is perpendicular to the direction of displacement, as is the case here, the work done is zero.

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

Ans: The battery initially stores chemical energy, which it transforms into electrical energy. This electrical energy, upon passing through the bulb, is then converted into both light and heat energy.

Q4. 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.

Ans: The work done (

W

) by a force is calculated using the formula:

$�$ W=Force×Distance

(However, if the force and distance are not directly given, we can use an alternative formula involving mass and initial and final velocities:)

So, the work done by the force is −210 Joules−210Joules. The negative sign indicates that
work is done against the direction of motion.

Q5. A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer.
Ans: The work done by the gravitational force is zero because the object is moved
horizontally, and the force of gravity acts vertically.

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

Ans: No, the decrease in potential energy of a falling object doesn't violate the law of conservation of energy because it's converted into kinetic energy.

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

Ans: Riding a bicycle involves converting muscular energy to mechanical energy for motion (kinetic energy), overcoming potential energy on inclines, and some energy losses due to friction.

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: Yes, energy is transferred when you push a rock, even if it doesn't move. The energy you spend is often transformed into heat or deformation, indicating the work done despite the lack of visible movement.

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

Q10. An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy?

Ans: The potential energy (PE) of an object raised to a height is calculated using the formula:

PE=m×g×h

Where

$�$

$�$ is the mass of the object (40 kg),
$�$ is the acceleration due to gravity (approximately $9.8 {m/s}^{2}$ ),
$ℎ$ is the height above the ground (5 m).

Substitute these values into the formula:

PE=40kg×9.8m/s2×5m

$� � = � \times � \times ℎ$

$PE = 1960 Joules$

So, the potential energy of the object is $1960 Joules$ .

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

Ans: The work done by gravity on a satellite moving around the Earth is zero because the force of gravity and the displacement of the satellite are perpendicular in its circular orbit.

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: In the absence of any force acting on an object, there can still be displacement under certain conditions. According to Newton's First Law of Motion, an object at rest will remain at rest, and an object in motion will continue to move with a constant velocity unless acted upon by a net external force.

If the object is initially at rest and no external force is applied, it will stay at rest. Similarly, if the object is already in motion and no net external force is acting on it (meaning all forces are balanced), it will continue moving with a constant velocity.

However, for an object to change its state of motion (start moving or stop), or change its velocity, a net external force is required according to Newton's laws. Therefore, in the absence of any force, the object will not experience any acceleration or deceleration, and its displacement will remain constant.

NCERT Class 9 Science – Page 159

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: No, the person hasn't done work in the way physics defines it. Even though they feel tired, work involves both force and movement, and the hay hasn't moved vertically.

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

Ans: We know that,

                                       P=W/t
       where,
       Power rating of the heater, P = 1500 W = 1.5 kW
       Time for which the heater has operated, t = 10 h
         Work done = Energy consumed by the heater
       Therefore, energy consumed = Power × Time
                                                        = 1.5 × 10 = 15 kWh
Hence, the energy consumed by the heater in 10 h is 15 kWh or 15 units.

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: The process of drawing and releasing a pendulum bob demonstrates the conversion between potential and kinetic energy, showcasing the conservation of mechanical energy in an ideal scenario. As the pendulum oscillates, energy transitions between these two forms, maintaining a constant total mechanical energy in the absence of external forces. However, in reality, external factors like air resistance and friction at the pivot point introduce non-conservative forces. These forces gradually dissipate the system's energy, causing the pendulum bob to lose its mechanical energy and eventually come to rest. This loss in mechanical energy is compensated by an increase in non-mechanical forms of energy, such as heat and sound. While the pendulum bob's eventual rest may seem like a violation of the conservation of mechanical energy, it aligns with the broader principle of the conservation of energy. The total energy, encompassing both mechanical and non-mechanical forms, remains constant despite the changes in the pendulum's motion.

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 work done to bring an object of mass

m

and constant velocity

v

�

�

$� = Force \times Distance$

NCERT Question and Answer for Class 9 Science Chapter 11 Work Power and Energy

NCERT Question and Answer for Class 9 Science Chapter 11 Work Power and Energy

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NCERT Question and Answer for Class 9 Science Chapter 11 Work Power and Energy

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