# NCERT Solutions Class 9 Science Chapter 11 Work and Energy

**In-Text Questions Solutions**

**In Text-Questions Page 148**

**A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force (Fig. 11.3). Let us take it that the force acts on the object through the displacement. What is the work done in this case?****Ans:-**When a force F acts on an object to move it in its direction through a distance S, the 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** **149**

**When do we say that work is done?****Ans:-**Work is completed whenever the given conditions are satisfied:

(i) A force acts on the body.

(ii) There’s a displacement of the body caused by the applied force on the direction of the applied force.**Write an expression for the work done when a force is acting on an object in the direction of its displacement.****Ans:-**When a force F displaces a body through a distance S within the direction of the applied force, then the work done W on the body is given by the expression:

W = F × S**Define 1 J of work.****Ans:-**The work done on an object is said to be 1 joule when a force of 1 N displaces it by 1 m along the line of action of the force.**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 by the bullocks is given by the expression:

W= F × d

Where,

Applied force, F = 140 N

Displacement, d = 15 m

W=140 × 15 = 2100 J

Hence, 2100 J of labor is finished in tilling the length of the sector.

**In Text-Questions Page 152**

**What is the kinetic energy of an object?****Ans:-**The energy possessed by an object by virtue of its motion is known as kinetic energy.**Write an expression for the kinetic energy of an object.****Ans:-**If a body of mass m is moving with a speed v, then its K.E. E k is given by the expression,

E_{k}= 2mv

it’s SI unit is Joule (J).**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?****Ans:-**

**In Text-Questions Page 156**

**What is power?****Ans:-**The rate of doing work is called power.**Define 1 watt of power****Ans:-**The power of an agent is said to be 1 watt if it does 1 J work in 1 sec.**A lamp consumes 1000 J of electrical energy in 10 s. What is its power?****Ans:-**Power = Work/Time

P = W/T

Time = 10 s

Work done = Energy consumed by the lamp = 1000 J

Power = 1000/10 = 100 Js^{-1}=100 W**Define average power.****Ans:-**The average Power of an agent could also be outlined because the total work done by it within the total time taken.

**Chapter End Exercises**

**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**Ans:-**Work is done whenever the given conditions are satisfied:

(i) A force acts on a body.

(ii) There is a displacement of the body.

(a) While swimming, Suma applies a force to push the water backwards. Therefore, Suma swims in the forward direction caused by the forward reaction of water. Here, the force causes a displacement. Hence, work is done by Seema while swimming.

(b) While carrying a load, the donkey has to apply a force in the upward direction. But, displacement of the load is in the forward direction. Since, displacement is perpendicular to force, the work done is zero.

(c) A wind mill works against the gravitational force to lift water. Hence, work is done by the wind mill in lifting water from the well.

(d) In this case, there is no displacement of the leaves of the plant. Therefore, the work done is zero.

(e) An engine applies force to pull the train. This allows the train to move in the direction of force. Therefore, there is a displacement in the train in the same direction. Hence, work is done by the engine on the train.

|(f) Food grains do not move in the presence of solar energy. Hence, the work done is zero during the process of food grains getting dried in the Sun.

(g) Wind energy applies a force on the sailboat to push it in the forward direction. Therefore, there is a displacement in the boat in the direction of force. Hence, work is done by wind on the boat.**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 an object depends solely on vertical displacement. Vertical displacement is given by the distinction within the initial and final positions/heights of the thing that is zero

Gravity-related work is expressed as W= mg.

Where, h= Vertical displacement = zero

W = m g × zero = 0 J**A battery lights a bulb. Describe the energy changes involved****in the process****Ans:-**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:**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:-**Mass of the body, m = 20 kg

Initial velocity, u = 5 m/s

Final velocity, v = 2 m/s

Initial kinetic energy of the body,

Final kinetic energy of the body,

Work done by the force = Change in kinetic energy = Final kinetic energy – Initial kinetic energy = E_{kf}– E_{ki}

= 40 J – 250 = -210 J

Work done is negative because force is applied in the direction opposite to that of of displacement.**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:-**Work done on the object by the gravitational force is zero. This is because the force of gravity and displacement of the body are at right angles to each other

W= m g

Where

Vertical displacement, h = 0

∴ W= mg × zero = 0

Therefore the work done on the body by gravity is therefore zero.**The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?****Ans:-**No, the method doesn’t violate the law of conservation of energy. This is because once the body falls from a height, then its mechanical energy changes into kinetic energy increasingly. A decrease within the mechanical energy is capable a rise in the kinetic energy of the body. Throughout the method, total energy of the body remains conserved. Therefore, the law of conservation of energy isn’t desecrated.-
**What are the various energy transformations that occur when you are riding a bicycle?****Ans:-**During riding a bicycle, the muscular energy of the rider is regenerate into heat and mechanical energy. Kinetic energy provides rate to the bicycle and warmth energy heats our body.

Muscular energy mechanical energy + heat **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 we push a large rock, there’s no transfer of muscular energy to the stationary rock. Also, there’s no loss of energy as a result of muscular energy is transferred into energy, which causes our body to become hot.**A certain household has consumed 250 units of energy during a month. How much energy is this in joules?****Ans:-**1 unit of energy is up to one B.T.U. (kWh).

1 unit = one kWh

1 kWh = 3.6 x 10^{6}J

Therefore, 250 units of energy = 250 × 3.6 × 10^{6}= 9 × 10^{8}J.**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:-**Gravitational mechanical energy is given by the expression,

W = mgh

Where,

h = Vertical displacement = 5 m, m = Mass of the item = 40 kg

g = Acceleration because of gravity = 9.8 m s^{−2}

∴ W = 40 × 5 × 9.8 = 1960 J.

At half-way down, the mechanical energy of the item are going to be 1960/2

At this time, the item has an equal quantity of potential and K.E.

This can be due to law of conservation of energy. Hence, half-way down, the K.E. of the item can be 980 J.**What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.****Ans:-**Work is completed whenever the given 2 conditions are satisfied:

→ a force acts on the body.

→ there’s a displacement of the body by the appliance of force in or opposite to the direction of force.

If the force direction is perpendicular to the displacement, the work performed is zero. When a satellite moves round the Earth, then the direction of force of gravity on the satellite is perpendicular to its displacement. Hence, the work done on the satellite by the planet is zero.**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:-**Yes, consider a uniformly moving object,

Suppose an object is moving with constant rate. The web force performing on its zero. But, there is a displacement on the motion of the article. Hence, there will be a displacement while not a force.**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 completed whenever the given two conditions are satisfied.

(i) A force acts on the body.

(ii) There’s a displacement of the body by the applying of force in or opposite to the direction of force.

When an individual holds a bundle of fodder over his head, then there’s no displacement within the bundle of fodder. Although, force of gravity is functioning on the bundle, the person isn’t applying any force thereon. Hence, within the absence of force, work done by the person on the bundle is zero.**An electric heater is rated 1500 W. How much energy does it use in 10 hours?****Ans:-**With the help of the expression, energy consumed by an electric heater will be obtained,

P = T

Where,

Power rating of the heater,

P = 500 W = 1.5 power unit Time that the heater has operated,

T = ten 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 10h is 15 kWh.**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:-**Consider the case of oscillation pendulum.**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:-**Kinetic energy of an object of mass*m*moving with a velocity*v*is given by the expression ½mv^{2}. To bring the object to rest, an equal amount of work i.e. ½mv^{2}is required to be done on the object.**Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?****Ans:-**The mass of the body = 1500kg

Velocity v = 60km/hr

The work required to stop the car = kinetic energy change of the car

**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.****Ans:-**Case I

In this case, the direction of force functioning on the block is perpendicular to the displacement. Therefore, work done by force on the block are going to be zero.

Case II

In this case, the direction of force functioning on the block is within the direction of displacement. Therefore, work done by force on the block are going to be positive.

Case III

In this case, the direction of force on the block is contrary to the direction of displacement. Therefore, work done by force on the block are going to be negative.**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:-**Acceleration in associate object might be zero even once many forces are working on it. This happens once all the forces get rid of one another i.e., the online force working on the thing is zero. For a uniformly moving object, the online force working on the thing is zero. Hence, the acceleration of the thing is zero. Hence, Soni is correct.**Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.****Ans:-**Energy consumed by an electrical device will be obtained with the assistance of the expression for power,

P = T

Where,

Power rating of the device,

P = five hundred W = 0.50 power unit Time that the device runs,

T= ten h Work d consumed by the device thus, energy c × Time

= 0.50 × 10 = 5 kWh

Hence, the energy consumed by four equal rating devices in 10 h = 4 × 5 kWh = 20 kWh = 20 Units.**A freely falling object eventually stops on reaching the ground. What happenes to its kinetic energy?****Ans:-**When the object falls freely towards the bottom, its mechanical energy decreases and K.E. will increase, because the object touches the bottom, all its mechanical energy gets reborn into K.E. Because the object hits the laborious ground, all its K.E. gets reborn into heat and sound energy. It may also deform the bottom relying upon the character of the ground and therefore the quantity of K.E. possessed by the thing.

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