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Lakhmir Singh solutions for Science for Class 9 Physics chapter 4 - Work and energy [Latest edition]

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Chapter 4: Work and energy

Lakhmir Singh solutions for Science for Class 9 Physics Chapter 4 Work and energyExercise Very Short Answers, very Short Answers, Short Answers, Long Answers, MCQ, Hots Questions [Pages 145 - 149]

Very Short Answers | Q 1 | Page 145

How much work is done when a body of mass m is raised to a height h above the ground?

Very Short Answers | Q 2 | Page 145

State the SI unit of work.

Very Short Answers | Q 3 | Page 145

Is work a scalar or a vector quantity?

Very Short Answers | Q 4 | Page 145

Define 1 joule of work.

Very Short Answers | Q 5 | Page 145

What is the condition for a force to do work on a body?

Very Short Answers | Q 6 | Page 145

Is energy a vector quantity?

Very Short Answers | Q 7 | Page 145

What are the units of (a) work, and (b) energy?

Very Short Answers | Q 8 | Page 145

What is the work done against gravity when a body is moved horizontally along a frictionless surface?

Very Short Answers | Q 9 | Page 145

By how much will the kinetic energy of a body increase if its speed is doubled?

Very Short Answers | Q 10 | Page 145

Write an expression for the kinetic energy of a body of mass m moving with a velocity v.

Very Short Answers | Q 11 | Page 145

If the speed of a body is halved, what will be the change in its kinetic energy?

Very Short Answers | Q 12 | Page 145

On what factors does the kinetic energy of a body depend?

Very Short Answers | Q 13 | Page 145

Which would have a greater effect on the kinetic energy of an object : doubling the mass or doubling the velocity?

Very Short Answers | Q 14 | Page 145

How fast should a man of 50 kg run so that his kinetic energy be 625 J?

Very Short Answers | Q 15.1 | Page 145

State whether the object possess kinetic energy, potential energy, or both :
A man climbing a hill ..................

• kinetic energy

• potential energy

• Both

Very Short Answers | Q 15.2 | Page 145

State whether the object possess kinetic energy, potential energy, or both :
A flying aeroplane ...............

• kinetic energy

• potential energy

• Both

Very Short Answers | Q 15.3 | Page 145

State whether the object possess kinetic energy, potential energy, or both :
A bird running on the ground ..................

• kinetic energy

•  potential energy

• Both

Very Short Answers | Q 15.4 | Page 145

State whether the object possess kinetic energy, potential energy, or both :
A ceiling fan in the off position ....................

• kinetic energy

• potential energy

• Both

Very Short Answers | Q 15.5 | Page 145

State whether the object possess kinetic energy, potential energy, or both :
A stretched spring lying on the ground .....................

• kinetic energy

• potential energy

• Both

Very Short Answers | Q 16 | Page 145

Two bodies A and B of equal masses are kept at heights of h and 2h respectively. What will be the ratio of their potential energies?

Very Short Answers | Q 17 | Page 145

What is the kinetic energy of a body of mass 1 kg moving with a speed of 2 m/s?

Very Short Answers | Q 18 | Page 145

Is potential energy a vector or a scalar quantity?

very Short Answers | Q 19 | Page 145

A load of 100 kg is pulled up by 5 m. Calculate the work done. (g = 9.8 m/s2)

Very Short Answers | Q 20 | Page 145

State whether the following statement is true or false :
The potential energy of a body of mass 1 kg kept at a height of 1 m is 1 J.

Very Short Answers | Q 21 | Page 145

What happens to the potential energy of a body when its height is doubled?

Very Short Answers | Q 22.1 | Page 145

What kind of energy is possessed by the following?
A stone kept on roof-top ......

Very Short Answers | Q 22.2 | Page 145

What kind of energy is possessed by the following?
A running car ...........

Very Short Answers | Q 22.3 | Page 145

What kind of energy is possessed by the following?
Water stored in the reservoir of a dam ...............

Very Short Answers | Q 22.4 | Page 145

What kind of energy is possessed by the following?
A compressed spring ...................

Very Short Answers | Q 22.5 | Page 145

What kind of energy is possessed by the following?
A stretched rubber band ..........

Very Short Answers | Q 23.1 | Page 145

Fill in the blank with suitable word :
Work is measured as a product of __________ and __________

Very Short Answers | Q 23.2 | Page 145

Fill in the blank with suitable word :
The work done on a body moving in a circular path is __________

Very Short Answers | Q 23.3 | Page 145

Fill in the blank with suitable word :
1 joule is the work done when a force of 1 __________ moves an object through a distance of one __________ in the direction of __________.

Very Short Answers | Q 23.4 | Page 145

Fill in the blank with suitable word :
The ability of a body to do work is called __________. The ability of a body to do work because of its motion is called __________.

Very Short Answers | Q 23.5 | Page 145

Fill in the blank with suitable word :
The sum of the potential and kinetic energies of a body is called __________ energy.

Short Answers | Q 24 | Page 145

What are the quantities on which the amount of work done depends? How are they related to work?

Short Answers | Q 25 | Page 145

Is it possible that a force is acting on a body but still the work done is zero? Explain giving one example.

Short Answers | Q 26 | Page 146

A boy throws a rubber ball vertically upwards. What type of work, positive or negative, is done : by the force applied by the boy?
(a) by the force applied by the boy?
(b) by the gravitational force of earth?

Short Answers | Q 27 | Page 146

Write the formula for work done on a body when the body moves at an angle to the direction of force. Give the meaning of each symbol used.

Short Answers | Q 28 | Page 146

How does the kinetic energy of a moving body depend on its (i) speed, and (ii) mass?

Short Answers | Q 29 | Page 146

Give one example each in which a force does (a) positive work (b) negative work, and (c) zero work.

Short Answers | Q 30 | Page 146

A ball of mass 200 g falls from a height of 5 metres. What is its kinetic energy when it just reaches the ground? (g = 9.8 m/s2)

Short Answers | Q 31 | Page 146

Find the momentum of a body of mass 100 g having a kinetic energy of 20 J.

Short Answers | Q 32 | Page 146

Two objects having equal masses are moving with uniform velocities of 2 m/s and 6 m/s respectively. Calculate the ratio of their kinetic energies.

Short Answers | Q 33 | Page 146

A body of 2 kg falls from rest. What will be its kinetic energy during the fall at the end of 2 s? (Assume g = 10 m/s2)

Short Answers | Q 34 | Page 146

On a level road, a scooterist applies brakes to slow down from a speed of 10 m/s to 5 m/s. If the mass of the scooterist and the scooter be 150 kg, calculate the work done by the brakes. (Neglect air resistance and friction)

Short Answers | Q 35 | Page 146

A man drops a 10 kg rock from the top of a 5 m ladder. What is its speed just before it hits the ground? What is its kinetic energy when it reaches the ground? (g = 10 m/s2)

Short Answers | Q 36 | Page 146

Calculate the work done by the brakes of a car of mass 1000 kg when its speed is reduced from 20 m/s to 10 m/s?

Short Answers | Q 37 | Page 146

A body of mass 100 kg is lifted up by 10 m. Find :
(i) the amount of work done
(ii) potential energy of the body at that height (value of g = 10 m/s2)

Short Answers | Q 38 | Page 146

A boy weighing 50 kg climbs up a vertical height of 100 m. Calculate the amount of work done by him. How much potential energy does he gain? (g = 9.8 m/s2)

Short Answers | Q 39 | Page 146

When is the work done by a force on a body : (a) positive, (b) negative, and (c) zero?

Short Answers | Q 40 | Page 146

To what height should a box of mass 150 kg be lifted, so that its potential energy may become 7350 joules? (g = 9.8 m/s2)

Short Answers | Q 41 | Page 146

A body of mass 2 kg is thrown vertically upwards with an initial velocity of 20 m/s. What will be its potential energy at the end of 2 s? (Assume g = 10 m/s2).

Short Answers | Q 42 | Page 146

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

Short Answers | Q 43 | Page 146

A car is being driven by a force of 2.5 ✕ 1010 N. Travelling at a constant speed of 5 m/s, it takes 2 minutes to reach a certain place. Calculate the work done.

Short Answers | Q 44 | Page 146

Explain by an example that a body may possess energy even when it is not in motion.

Short Answers | Q 45.1 | Page 146

On what factors does the gravitational potential energy of a body depend?

Short Answers | Q 45.2 | Page 146

Give one example each of a body possessing : (i) kinetic energy, and (ii) potential energy.

Short Answers | Q 46 | Page 146

Give two examples where a body possesses both, kinetic energy as well as potential energy.

Short Answers | Q 47 | Page 146

How much is the mass of a man if he has to do 2500 joules of work in climbing a tree 5 m tall? (g = 10 m s−2)

Short Answers | Q 48 | Page 146

If the work done by a force in moving an object through a distance of 20 cm is 24.2 J, what is the magnitude of the force?

Short Answers | Q 49.1 | Page 146

A boy weighing 40 kg makes a high jump of 1.5 m.
What is his kinetic energy at the highest point?

Short Answers | Q 49.2 | Page 146

A boy weighing 40 kg makes a high jump of 1.5 m.
What is his potential energy at the highest point? (g = 10 m/s2)

Short Answers | Q 50.1 | Page 146

What type of energy is possessed :  by the stretched rubber strings of a catapult?

Short Answers | Q 50.2 | Page 146

What type of energy is possessed : by the piece of stone which is thrown away on releasing the stretched rubber strings of catapult?

Short Answers | Q 51.1 | Page 147

A weightlifter is lifting weights of mass 200 kg up to a height of 2 metres. If g = 9.8 m s−2, calculate :
potential energy acquired by the weights.

Short Answers | Q 51.2 | Page 147

A weightlifter is lifting weights of mass 200 kg up to a height of 2 metres. If g = 9.8 m s−2, calculate :
work done by the weightlifter

Long Answers | Q 52.1 | Page 147

Define the term 'work'. Write the formula for the work done on a body when a force acts on the body in the direction of its displacement. Give the meaning of each symbol which occurs in the formula.

Long Answers | Q 52.2 | Page 147

A person of mass 50 kg climbs a tower of height 72 metres. Calculate the work done. (g = 9.8 m s−2).

Long Answers | Q 53.1 | Page 147

When do we say that work is done? Write the formula for the work done by a body in moving up against gravity. Give the meaning of each symbol which occurs in it.

Long Answers | Q 53.2 | Page 147

How much work is done when a force of 2 N moves a body through a distance of 10 cm in the direction of force?

Long Answers | Q 54.1 | Page 147

What happens to the work done when the dispacement of body is at right angles to the direction of force acting on it? Explain your answer.

Long Answers | Q 54.2 | Page 147

A force of 50 N acts on a body and moves it a distance of 4 m on horizontal surface. Calculate the work done if the direction of force is at an angle of 60° to the horizontal surface.

Long Answers | Q 55.1 | Page 147

Define the term 'energy' of a body. What is the SI unit of energy.

Long Answers | Q 55.2 | Page 147

What are the various forms of energy?

Long Answers | Q 55.3 | Page 147

Two bodies having equal masses are moving with uniform speeds of v and 2v respectively. Find the ratio of their kinetic energies.

Long Answers | Q 56.1 | Page 147

What do you understand by the kinetic energy of a body?

Long Answers | Q 56.2 | Page 147

A body is thrown vertically upwards. Its velocity goes on decreasing. What happens to its kinetic energy as its velocity becomes zero?

Long Answers | Q 56.3 | Page 147

A horse and a dog are running with the same speed. If the weight of the horse is ten times that of the dog, What is the ration of their kinetic energies?

Long Answers | Q 57.1 | Page 147

Explain by an example what is meant by potential energy. Write down the expression for gravitational potential energy of a body of mass m placed at a height h above the surface of the earth.

Long Answers | Q 57.2 | Page 147

What is the difference between potential energy and kinetic energy?

Long Answers | Q 57.3 | Page 147

A ball of mass 0.5 kg slows down from a speed of 5 m/s so that of 3 m/s. Calculate the change in kinetic energy of the ball. State your answer giving proper units.

Long Answers | Q 58.1 | Page 147

What is the difference between gravitational potential energy and elastic potential energy? Give one example of a body having gravitational potential energy and another having elastic potential energy.

Long Answers | Q 58.2 | Page 147

If 784 J of work was done in lifting a 20 kg mass, calculate the height through which it was lifted. (g = 9.8 m/s2)

MCQ | Q 59 | Page 148

A car is accelerated on a levelled road and acquires a velocity 4 times of its initial velocity. During this process, the potential energy of the car :

• does not change

• becomes twice that of initial potential energy

• becomes 4 times that of initial potential energy

•  becomes 16 times that of initial potential energy

MCQ | Q 60 | Page 148

A car is accelerated on a levelled road and attains a speed of 4 times its initial speed. In this process, the kinetic energy of the car :

• does not change

• becomes 4 times that of initial kinetic energy

•  becomes 8 times that of initial kinetic energy

• becomes 16 times that of initial kinetic energy

MCQ | Q 61 | Page 148

In case of negative work, the angle between the force and displacement is :

• 45°

•  90°

• 180°

MCQ | Q 62 | Page 148

An iron sphere of mass 10 kg has the same diameter as an aluminium sphere of mass 3.5 kg. Both the spheres are dropped simultaneously from a tower. When they are 10 m above the ground, they have the same :

• acceleration

• momentum

• potential energy

• kinetic energy

MCQ | Q 63 | Page 148

A girl is carrying a school bag of 3 kg mass on her back and moves 200 m on a levelled road. If the value of g be 10 m/s2, the work done by the girl against the gravitational force will be :

• 6000 J

• 0.6 J

• 0 J

• 6 J

MCQ | Q 64 | Page 148

The work done on an object does not depend on the :

• displacement

• angle between force and displacement

• force applied

• initial velocity of the object

MCQ | Q 65 | Page 148

Water stored in a dam possesses :

• no energy

•  electrical energy

• kinetic energy

• potential energy

MCQ | Q 66 | Page 148

The momentum of a bullet of mass 20 g fired from a gun is 10 kg.m/s. The kinetic energy of this bullet in kJ will be :

• 5

• 1.5

• 2.5

• 25

MCQ | Q 67 | Page 148

Each of the following statement describes a force acting. Which force is causing work to be done?

• the weight of a book at rest on a table

• the pull of a moving railway engine on its coaches

• the tension in an elastic band wrapped around a parcel

• the push of a person's feet when standing on the floor

MCQ | Q 68 | Page 148

A girl weighing 400 N climbs a vertical ladder. If the value of g be 10 m s−2, the work by her after climbing 2 m will be :

• 200 J

•  800 J

• 8000 J

• 2000 J

MCQ | Q 69 | Page 148

Which of the following does not posses the ability to do work not because of motion?

• a sparrow flying in the sky

• a sparrow moving slowly on the ground

• a sparrow in the nest on a tree

• a squirrel going up a tree

MCQ | Q 70 | Page 148

A stone is thrown upwards as shown in the diagram. When it reaches P, which of the following has the greatest value of the stone?

•  its acceleration

• its kinetic energy

• its potential energy

•  its weight

Hots Questions | Q 71 | Page 148

A boy tries to push a truck parked on the roadside. The truck does not move at all. Another boy pushes a bicycle. The bicycle moves through a certain distance. In which case was the work done more : on the trick or on the bicycle? Give a reason to support your answer.

Hots Questions | Q 72 | Page 149

The work done by a force acting obliquely is given by the formula : W = F cos θ ✕ s. What will happen to the work done if angle θ between the direction of force and motion of the body is increased gradually? Will it increase, decrease or remain constant.

Hots Questions | Q 73 | Page 149

What should be the angle between the direction of force and the direction of motion of a body so that the work done is zero?

Hots Questions | Q 74 | Page 149

In which of the following case the work done by a force will be maximum : when the angle between the direction of force and direction of motion is 0° or 90°?

Hots Questions | Q 75 | Page 149

How much work is done by the gravitational force of earth acting on a satellite moving around it in a circular path? Give reason for your answer.

Hots Questions | Q 76.1 | Page 149

A man is instructed to carry a package from the base camp at B to summit A of a hill at a height of 1200 metres. The man weighs 800 N and the package weighs 200 N. If g = 10 m/s2,
how much work does man do against gravity?

Hots Questions | Q 76.2 | Page 149

A man is instructed to carry a package from the base camp at B to summit A of a hill at a height of 1200 metres. The man weighs 800 N and the package weighs 200 N. If g = 10 m/s2,
what is the potential energy of the package at A if its is assumed to be zero at B?

Hots Questions | Q 77 | Page 149

When a ball is thrown vertically upwards, its velocity goes on decreasing. What happens to its potential energy as its velocity becomes zero?

Hots Questions | Q 78 | Page 149

A max X goes to the top of a building by a vertical spiral staircase. Another man Y of the same mass goes to the top of the same building by a slanting ladder. Which of the two does more work against gravity and why?

Hots Questions | Q 79 | Page 149

When a ball is thrown inside a moving bus, does its kinetic energy depend on the speed of the bus? Explain.

Hots Questions | Q 80 | Page 149

A bullet of mass 15 g has a speed of 400 m/s. What is its kinetic energy? If the bullet strikes a thick target and is brought to rest in 2 cm, calculate the average net force acting on the bullet. What happens to the kinetic energy originally in the bullet?

Lakhmir Singh solutions for Science for Class 9 Physics Chapter 4 Work and energyExercise Very Short Answers, Short Answers, Long Answers, MCQ, Hots Questions [Pages 162 - 165]

Very Short Answers | Q 1 | Page 162

Name the commercial unit of measurement of energy.

Very Short Answers | Q 2 | Page 162

Define kilowatt-hour.

Very Short Answers | Q 3 | Page 162

Name two units of power bigger than watt.

Very Short Answers | Q 4 | Page 162

Define the term 'watt'.

Very Short Answers | Q 5 | Page 162

How many watts equal one horse power?

Very Short Answers | Q 6 | Page 162

Name the physical quantity whose unit is watt.

Very Short Answers | Q 7 | Page 162

What is the power of a body which is doing work at the rate of one joule per second?

Very Short Answers | Q 8 | Page 162

A body does 1200 joules of work in 2 minutes. Calculate its power.

Very Short Answers | Q 9 | Page 162

How many joules are there in one kilowatt-hour?

Very Short Answers | Q 10 | Page 162

Name the quantity whose unit is :
(a) kilowatt
(b) kilowatt-hour

Very Short Answers | Q 11 | Page 162

What is the common name of '1 kWh' of electrical energy?

Very Short Answers | Q 12 | Page 162

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

Very Short Answers | Q 13 | Page 162

Name the device which converts electrical energy into mechanical energy.

Very Short Answers | Q 14.1 | Page 162

Name the device or machine which convert :
Mechanical energy into electrical energy.

Very Short Answers | Q 14.2 | Page 162
Name the device or machine which convert :
Chemical energy into electrical energy.
Very Short Answers | Q 14.3 | Page 162
Name the device or machine which convert :
Electrical energy into heat energy.
Very Short Answers | Q 14.4 | Page 162

Name the device or machine which convert :
Light energy into electrical energy.

Very Short Answers | Q 14.5 | Page 162

Name the device or machine which convert :
Electrical energy into light energy.

Very Short Answers | Q 15.1 | Page 162

Name the device or machine which convert :
Electrical energy into sound energy.

Very Short Answers | Q 15.2 | Page 162

Name the device or machine which convert :
Heat energy into kinetic energy (or mechanical energy).

Very Short Answers | Q 15.3 | Page 162

Name the device or machine which convert :
Chemical energy into kinetic energy (or mechanical energy).

Very Short Answers | Q 15.4 | Page 162

Name the device or machine which convert :
Chemical energy into heat energy.

Very Short Answers | Q 15.5 | Page 162

Name the device or machine which convert :
light energy into heat energy.

Very Short Answers | Q 16.1 | Page 162

Fill in the blank with suitable word :
Power is the rate of doing ___________.

Very Short Answers | Q 16.2 | Page 162

Fill in the blanks with suitable words :
One watt is a rate of working of one ___________ per ___________.

Very Short Answers | Q 16.3 | Page 162

Fill in the blank with suitable word :
The electricity meter installed in our homes measure electrical energy in the units of ___________.

Very Short Answers | Q 16.4 | Page 162

Fill in the blanks with suitable words :
The principle of ___________ of energy says energy can be transformed from one form to another, but it cannot be ___________ or ___________.

Very Short Answers | Q 16.5 | Page 162

Fill in the blanks with suitable words :
When a ball is thrown upwards, ___________ energy is transformed into ___________ energy.

Short Answers | Q 17 | Page 163

A trolley is pushed along a road with a force of 400 N through a distance of 60 m in 1 minute. Calculate the power developed.

Short Answers | Q 18 | Page 163

What kind of energy transformation take place at a hydroelectric power station?

Short Answers | Q 19 | Page 163

What kind of energy transformations take place at a coal-based thermal power station?

Short Answers | Q 20 | Page 163

A man weighing 500 N carried a load of 100 N up a flight of stairs 4 m high in 5 seconds. What is the power?

Short Answers | Q 21 | Page 163

The power output of an engine is 3kW. How much work does the engine do in 20 s?

Short Answers | Q 22 | Page 163

An electric heater uses 600 kJ of electrical energy in 5 minutes. Calculate its power rating.

Short Answers | Q 23.1 | Page 163

How much electrical energy in joules does a 100 watt lamp consume : in 1 second?

Short Answers | Q 23.2 | Page 163

How much electrical energy in joules does a 100 watt lamp consume :  in 1 minute?

Short Answers | Q 24 | Page 163

Five electric fans of 120 watts each are used for 4 hours. Calculate the electrical energy consumed in kilowatt-hours.

Short Answers | Q 25 | Page 163

Describe the energy changes which take place in a radio.

Short Answers | Q 26 | Page 163

Write the energy transformations which take place in an electric bulb (or electric lamp).

Short Answers | Q 27 | Page 163

Name five appliances or machines which use an electric motor.

Short Answers | Q 28 | Page 163

A bulb lights up when connected to a battery. State the energy change which takes place :

Short Answers | Q 29 | Page 163

The hanging bob of a simple pendulum is displaced to one extreme position B and then released. It swings towards centre position A and then to the other extreme position C. In which position does the bob have :

(i) maximum potential energy?
(ii) maximum kinetic energy?

Short Answers | Q 30.1 | Page 163

A car of weight 20000 N climbs up a hill at a steady speed of 8 m/s, gaining a height of 120 m is 100 s. Calculate : work done by the car.

Short Answers | Q 30.2 | Page 163

A car of weight 20000 N climbs up a hill at a steady speed of 8 m/s, gaining a height of 120 m is 100 s. Calculate : power of engine of car.

Long Answers | Q 31.1 | Page 163

What do you understand by the term 'transformation of energy"? Explain with an example.

Long Answers | Q 31.2 | Page 163

Explain the transformation of energy in the following cases :
(i) A ball thrown upwards.
(ii) A stone dropped from the roof of a building.

Long Answers | Q 32.1 | Page 163

State and explain the law of conservation of energy with an example.

Long Answers | Q 32.2 | Page 163

Explain how, the total energy a swinging pendulum at any instant of time remains conserved. Illustrate your answer with the help of a labelled diagram.

Long Answers | Q 33.1 | Page 163

What is the meaning of the symbol kWh? What quantity does it represent?

Long Answers | Q 33.2 | Page 163

How much electric energy in kWh is consumed by an electrical appliance of 1000 watts when it is switched on for 60 minutes?

Long Answers | Q 34.1 | Page 163

Derive the relation between commercial unit of energy (kWh) and SI unit of energy (joule).

Long Answers | Q 34.2 | Page 163

A certain household consumes 650 units of electricity in a month. How much is this electricity in joules?

Long Answers | Q 35.1 | Page 163

Define power. Give the SI unit of power.

Long Answers | Q 35.2 | Page 163

A boy weighing 40 kg carries a box weighing 20 kg to the top of a building 15 m high in 25 seconds. Calculate the power. (g = 10 m/s2)

MCQ | Q 36 | Page 164

When an object falls freely towards the earth, then its total energy :

• increases

• decreases

• remains constant

• first increases and then decreases

MCQ | Q 37 | Page 164

Which one of the following is not the unit of energy?

• joule

• newton-metre

• kilowatt

• kilowatt-hour

MCQ | Q 38 | Page 164

Which of the following energy change involves frictional force?

• chemical energy to heat energy

• kinetic energy to heat energy

• potential energy to sound energy

• chemical energy to kinetic energy

MCQ | Q 39 | Page 164

Which one of the following statements about power stations is not true?

• hydroelectric power stations use water to drive turbines

• in a power station, turbines drive generators

• electricity from thermal power stations differs from that produced in hydroelectric power stations

• in hydroelectric power stations and thermal power stations, alternators produce electricity

MCQ | Q 40 | Page 164

An electric motor raises a load of 0.2 kg, at a constant speed, through a vertical distance of 3.0 m in 2 s. If the acceleration of free fall is 10 m/s2, the power in W developed by the motor in raising the load is :

• 0.3

• 1.2

• 3.0

• 6.0

MCQ | Q 41 | Page 164

An object is falling freely from a height x. After it has fallen a height pi/2 , it will possess :

• only potential energy

• only kinetic energy

• half potential and half kinetic energy

• less potential and more kinetic energy

MCQ | Q 42 | Page 164

The commercial unit of energy is :

• watt

• watt-hour

•  kilowatt-hour

• kilowatt

MCQ | Q 43 | Page 164

How much energy does a 100 W electric bulb transfer in 1 minute?

•  100 J

• 600 J

• 3600 J

• 6000 J

MCQ | Q 44 | Page 164

The device which converts mechanical energy into energy which runs our microwave oven is :

• electric motor

• alternator

• turbine

• electric heater

MCQ | Q 45 | Page 164

A microphone converts :

• electrical energy into sound energy in ordinary telephone

• microwave energy into sound energy in a mobile phone

• sound energy into mechanical energy is a stereo system

• sound energy into electrical energy in public address system

Hots Questions | Q 46 | Page 164

The following data was obtained for a body of mass 1 kg dropped from a height of 5 metres :

 Distance above ground Velocity 5 m 0 m/s 3.2 m 6 m/s 0 m 10 m/s

Show by calculations that the above data verifies the law of conservation of energy (Neglect air resistance). (g = 10 m/s2).

Hots Questions | Q 47.1 | Page 164

A ball falls to the ground as shown below :

What is the kinetic energy of ball when it hits the ground?

Hots Questions | Q 47.2 | Page 164

A ball falls to the ground as shown below :

What is the potential energy of ball at B?

Hots Questions | Q 47.3 | Page 164

A ball falls to the ground as shown below :

Hots Questions | Q 48 | Page 165

In an experiment to measure his power, a student records the time taken by him in running up a flight of steps on a staircase. Use the following data to calculate the power of the student :

Number of steps = 28
Height of each step = 20 cm
Time taken = 5.4 s
Mass of student = 55 kg
Acceleration = 9.8 m s−2
due to gravity

Hots Questions | Q 49.1 | Page 165

In loading a truck, a man lifts boxes of 100 N each through a height of 1.5 m.
How much work does he do in lifting one box?

Hots Questions | Q 49.2 | Page 165

In loading a truck, a man lifts boxes of 100 N each through a height of 1.5 m.
How much energy is transferred when one box is lifted?

Hots Questions | Q 49.3 | Page 165

In loading a truck, a man lifts boxes of 100 N each through a height of 1.5 m.
If the man lifts 4 boxes per minute, at what power is he working?
(g = 10 m s−2)

Hots Questions | Q 50.1 | Page 165

Name the energy transfers which occur when :
an electric ball rings

Hots Questions | Q 50.2 | Page 165

Name the energy transfers which occur when :
someone speaks into a microphone

Hots Questions | Q 50.3 | Page 165

Name the energy transfers which occur when :  there is a picture on a television screen

Hots Questions | Q 50.4 | Page 165

Name the energy transfers which occur when : a torch is on

Lakhmir Singh solutions for Science for Class 9 Physics chapter 4 - Work and energy

Lakhmir Singh solutions for Science for Class 9 Physics chapter 4 (Work and energy) include all questions with solution and detail explanation. This will clear students doubts about any question and improve application skills while preparing for board exams. The detailed, step-by-step solutions will help you understand the concepts better and clear your confusions, if any. Shaalaa.com has the CBSE Science for Class 9 Physics solutions in a manner that help students grasp basic concepts better and faster.

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Concepts covered in Science for Class 9 Physics chapter 4 Work and energy are Work and Energy (Introduction ), Scientific Conception of Work, Work Done by a Constant Force, Work Done by a Energy, Work Done by a Power, Different Forms of Energy, Kinetic Energy, Potential Energy, Potential Energy of an Object at a Height, Are Various Energy Forms Interconvertible?, Law of Conservation of Energy, Rate of Doing Work, Commercial Unit of Energy, Work and Energy (Numericals).

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