NEET Foundation For Physics Chapter 4 Work And Energy

Physics Chapter 4 Work And Energy

In previous chapter we have discussed about fundamental concepts of physics, which deal with the laws of nature like motion, forces, and Newtons laws of motion and gravitation, etc. Work and energy are the other two basic principles of nature which we come across knowingly or unknowingly in our day to day life.

In this chapter, we will discuss about some of the basic concepts on which work and energy are co-related. Also, energy and power are closely related to work.

All living beings need food to survive. Complex food molecules are broken down by our body and converted into simple soluble form, which in turn is absorbed by the body to get ‘Energy’. Thus, energy can be defined as the element which allows us to perform any function.

For our day to day activities like singing, dancing, running, etc., we need energy. Without energy we cannot perform any work.

For machines, cars, etc., the energy is provided by electricity or the fuel which we provide. Thus, any object whether living or non-living requires energy to do any work.

Chapter 4 Work And Energy

Work

In the language of science, work is said to be done only if the force applied in a specific direction produces motion in the body. In other words, if the body displaces from its original position, work is said to be done otherwise it is not done irrespective of all your efforts. In our everyday life, we consider any useful mental or physical labour as work.

Certain activities like playing in a field, chit chatting with friends, singing a song, watching television, attending a party are sometimes not counted as work. What defines ‘work’ depends on how we use it in science.

From the above mentioned examples, it is clear that how work we usually do is different from the scientific definition of work. To understand the concept better, we would study the concept of work done by a constant force.

We will consider a constant force ’F’ acting on the body and body being displaced in the direction of the force by a distance ’S’ as shown.

Let ’W’ be the work done.

According to the definition,

Work done = force × displacement

Mathematically,

W = F × S

Elaborating the above equation, the work done by a body is equal to the magnitude of the force applied on the body multiplied by the displacement carried in the direction of force. Work has only magnitude and no direction. If either force or displacement in the above equation is zero then the work becomes zero.

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If in the above equation, 1 newton of force is applied on the body producing a ­displacement of 1 metre in the body in the direction of motion, then the work done is said to be 1 Nm or 1 Joule. (1 Nm = 1 Joule).

If force F acts in the direction of displacement at an angle θ, then in the direction of displacement the rectangular component will be represented by F cos θ.

Work done = Force(rectangular component) × displacement

W = F cos θ × S

Therefore, depending on the value of θ (i.e., cos θ) work done can be positive, negative or zero.

Concept of Positive and Negative Work

Work can be positive or negative depending upon the direction of force applied. If the force applied is along the direction of the displacement, work done is positive. Example of such situation is pulling of a toy by a kid (parallel to the ground), pushing the door, etc. In these examples, work done is equal to the product of force and displacement.

On the other hand, when the force applied and displacement act in the opposite directions, the work done is said to be negative. Example of this type of work is, when a body is moving with a uniform velocity and suddenly a retarding force is exerted on the body to halt its motion.

The retarding force is opposite to the direction of motion. The angle between the force applied and the displacement is 180°. The work done in this case is said to be negative. Work done is represented by F × (–S) or (–F) × S. Negative work done is indicated by the minus sign.

 

Chapter 4 Work And Energy Track Your Learning Question And Answers

Question 1. The angle between the force & displacement is_________ in case of negative work.

1. 45°
2. 90°
3. 0°
4. 180°

Answer. 3. 0°

Question 2. Which of the above quantity does not depend upon work done on an object?

1. displacement
2. angle between force and displacement
3. force applied
4. initial velocity of the object

Answer. 4. initial velocity of the object

Question 3. If the force applied is along the direction of the displacement, work done is said to be ______.

1. Negative
2. Positive
3. Neutral
4. Cannot be determined

Answer. 2. Positive

Question 4. Nm is a SI unit of

1. Work
2. Power
3. Acceleration
4. Force

Answer. 1. Work

Question 5. kWh is the unit of

1. Work
2. Power
3. Acceleration
4. energy

Answer. 4. energy

Chapter 4 Work And Energy

Every process on this earth requires energy. Our earth has evolved because of energy. The life processes that keep us alive require energy. Every single process needs energy. Just like work, let us understand what energy is, from where do we get energy in the language of science.

In simple language, energy is defined as the capacity of doing work. It is the property possessed by objects which allows them to do work or can exert force on another object. Which means energy is transferred from one object to the other.

The latter may move in the direction of force experienced and hence receives the energy and therefore do some work. Thus, the first object has a capacity to do work. This means that any object that can do work, possesses energy.

Commercial Unit of Energy

When we have to express large quantities of energy, joule is not used. We use a bigger unit of energy called kilowatt hour (kWh). Let us understand with the help of an example, suppose we have a machine that uses 1000  J of energy every second. If this machine is used continuously for one hour, it will consume 1 kWh of energy. Thus, 1 kWh is the energy used in one hour at the rate of 1000 J s^-1 (or 1 kW).

1 kW h = 1 kW × 1 h

1000 W × 3600 s = 3600000 J

1 kW h = 3.6 × 10⁶ J

The energy used in households, industries and commercial establishments is usually expressed in kilowatthour. For example, electrical energy used during a month is expressed in terms of ‘units’. Here, 1 ‘unit’ means 1 kilowatt hour.

Classification of Resources

Natural resources are valuable and easily available to us. They are classified on the basis of their ability to replenish or recover. They are broadly classified into:

1. Renewable resources
2. Non Renewable resources

1. Renewable resources: When the energy source used is easily replenished in a short period and there are practically limitless reserves (inexhaustible). An example is the solar energy that is the source of energy from the sun, or the wind used as an energy resource. Other renewable energies are original solar, natural wind (atmospheric flows), natural ­geothermal, oceanic tidal, natural waterfall (hydraulic flows), etc.
2. Non Renewable resources: They are limited sources of energy on earth (exhaustible) in quantity and therefore are ­non-replenishable. The non-renewable energy sources include, ­non-exclusively, fossil source like petroleum, natural gas, coal, etc.

Different Sources of Energy

There are various sources of energy that are used across the world by human beings to generate power and do the work. Over the years, there are other sources being discovered however, none of them has reached the stage where they can fulfill the power requirements of the modern era.

Human beings rely majorly on natural resources, unfortunately some of them are combustible. For this reason, we should look for artificial and alternate sources.

The reason why we are looking for alternate sources of energy is to produce electricity and run machines on a massive scale.

Solar Energy

Solar energy is the chief source of energy generated from the sun in the form of electric or thermal energy. Solar energy is captured in large solar panels made up of silicon and arsenic which convert the sun’s rays into usable electricity. Besides generating electricity, solar energy is also used in thermal applications.

Wind Energy

In wind energy, wind is used to generate electricity with the help of kinetic energy created by moving air. Wind energy is transformed into electrical energy with the help of wind turbines or wind energy conversion systems. The kinetic energy is then changed to rotational energy, by moving a shaft which is further connected to a ­generator which later produces electrical energy.

Geothermal Energy

Geo means ‘earth’ and thermal means ‘energy’;It is the produced from within the earth. Geothermal energy can be used for heating and cooling purposes or to generate clean electricity. However, for the generation of electricity, high or medium temperature resources are required.

Hydrogen Energy

Hydrogen has the ability to power fuel cells in zero-emission electric vehicles and the fuel cell’s potential for high efficiency. Hydrogen is a tremendous source of energy and can be used as a source of fuel to power ships, vehicles, homes, industries and rockets. It is renewable source of energy. It is environment friendly.

Tidal Energy

Tidal energy converts the energy obtained from tides into other forms of energy mainly electricity. Although because of its complications, it is not yet widely used. Tidal energy has potential for future electricity generation. Tidal power is a eco-friendly energy source.

Hydroelectric Energy

Hydroelectric energy involves water flowing through a pipe before pushing against and turning turbine blades connected to an electric generator. Hydropower provides 16 percent of the world’s electricity.

Biomass Energy

Biomass energy is produced from organic material and is commonly used throughout the world. Chlorophyll present in plants captures the sun’s energy by converting ­carbon dioxide present in the air and water from the ground into carbohydrates through the process of photosynthesis. When the plants are burned, the water and carbon dioxide is again released back into the atmosphere.

Biomass energy is used for heating and cooking in homes and also as a fuel in industrial production. This type of energy produces large amount of carbon dioxide into the atmosphere which is a greenhouse gas, thus it is not very efficient.

Nuclear Power

It is the most novel way of getting energy and quantitatively the most important renewable source of energy. Nuclear energy originates from the fission or fusion of uranium atoms. This produces massive heat to generate steam, which is then used by a turbine generator to generate electricity. Since, nuclear power plants do not burn fuel, they are environment friendly.

Fossil Fuels (Coal, Oil and Natural Gas)

Fossil fuels are the world’s dominant source of energy. Oil is converted into many products, the most used of which is gasoline. Natural gas is starting to become more common, vehicles are seen running on it. To get to the fossil fuel and convert it to use there has to be a heavy destruction and pollution of the ­environment.

The fossil fuel reserves are also limited, expecting to last only another 100 years given are the basic rate of consumption. It is estimated by 2040 that maximum of the fossil fuel deposits would get exhausted.

Understanding Energy Better

We come across different forms of energy in our daily lives. The biggest source of energy is the sun and also the tiniest source comes from nuclei of the atoms. Every action that we do involves energy from hitting a cricket ball, cycling, hitting a nail with the hammer, etc. The body which does work loses energy and the one on which work is done gains energy.

The demand for energy is increasing day by day and the pressure of extracting energy possesses a threat to our planet. We will discuss about energy demands and other threats later in the chapter.

A body which has energy has the capability of doing work. A body which has energy can exert a force on other bodies. Due to this, energy is transferred from one body to the other. The body which received energy may move/does work. The units of work are same as that of energy (Joule). Thus, 1 joule of energy is required to do 1 J of work.

Forms of Energy

The energy present around us is available in different forms like heat energy, sound energy, light energy, electricity, chemical, mechanical (kinetic + potential), etc. They can be interchanged from one form to another. When work done on an object is known, upon which energy is acquired then it is called mechanical energy. They are of two types:

Forms of energy

Kinetic Energy

The energy possessed by a body by virtue of its motion is called as kinetic energy. A body moving can do more work than a stable one. A rotating wheel, moving windmill, bullet fired from a gun, speeding car, etc. Greater the speed of the object, greater is the kinetic energy. Thus, objects in motion possess energy called as kinetic energy.

Now, student may ask how much energy (kinetic) does a moving body possess? We will try to devise a ­formula for that.

Consider the above figure:

Consider a body of mass ’m’ starts moving from rest, with uniform velocity ’u’. After a time interval ’t’ its velocity becomes v.

If initial velocity of the body is u or vi = 0, final velocity vf = v and the displacement of body is ’S’. Then

First of all we will find the acceleration of body.

Using the equation of motion

2aS = v_f² – v_i²

Putting the above mentioned values

2aS = v² – 0

a = \(\frac{v^2}{2 S}\)

Now force is given by

F = ma

Putting the value of acceleration

F = \(m\left(\frac{v^2}{2 S}\right)\)

As we know that

Work done = F⋅S

Putting the value of F

Work done = \(\left(\frac{m v^2}{2 S}\right)(S)\)

Work done = \(\frac{m v^2}{2}\)

or

Work done = \(\frac{1}{2} m V^2\)

Since the work done in motion is called ‘kinetic energy’

i.e. K.E. = Work done

or

K.E. = \(\frac{1}{2} m V^2\)

Potential Energy

The energy possessed by the body by virtue of its height or position is called as potential energy. The energy gets stored in the body because of the work done on the object. The energy while doing work is stored in the body as potential energy.

It does not cause any alterations in its speed or velocity. Some of the examples where we come across with potential energy are that of pulling of rubber band, winding the key of a toy car, water in the water tank at the top of the house, etc.

Potential Energy in a Body at Some Height

An object has energy known as potential energy when raised at a certain height. Greater the potential energy, greater is the height. Work is done on the object, against the gravity in bringing an object to a height. The energy gets stored in the object.

Gravitational potential energy of an object above the ground at a certain point is defined as raising the body to that point against the gravity.

Let us try to devise a formula to calculate the potential energy.

Consider an object of mass, m. Let it be raised through a height h from the ground. A force is required to do this. The minimum force required to raise the object is equal to the weight of the object mg. The object gains energy equal to the work done on it. Let the work done on the object against gravity be W. That is, work done,

W = force × displacement

= mg × h

= mgh

Since work done on the object is equal to mgh, energy equal to mgh units is gained by the object. This is the potential energy (EP) of the object.

E_p = mgh

The potential energy of an object at a height depends on the ground level or the zero level. An object in a given position can have a certain potential energy with respect to one level and a different value of potential energy with respect to another level. At ground potential energy is zero and at some height it is equal to mgh.

The potential attained by the body is independent of the path followed. The potential energy depends only on the initial and final positions. Energy is independent of the path followed.

Conservative and Non-Conservative Forces

Work done by gravity in moving object from one place to another depends only on the initial and final positions and doesn’t depend on the path taken. The work done by gravity from A to B is same by path 1, 2 and 3.

Hence the work done by such forces depends only on initial and final position and not on the path taken. Hence such forces are called conservative forces. Examples of conservative forces are Gravitational forces, spring force, etc.

On the other hand non conservative forces are those in which work done depends on path taken. For example fiction force is non conservative force.

Law of Conservation of Energy

This law states that energy can neither be created nor destroyed. It can only be changed from one form to the other. The total energy of the system remains constant.

According to the law of conservation of energy, energy can only be converted from one form to another; it can neither be created nor destroyed. The total energy before and after the transformation remains the same.

The law of conservation of energy is valid in all situations and for all kinds of transformations.

Consider a simple example. Let an object of mass m be made to fall freely from a height h. At the start, the potential energy is mgh and kinetic energy is zero. Why is the kinetic energy zero? It is zero because its velocity is zero. The total energy of the object is thus mgh.

As it falls, its potential energy will change into kinetic energy. If v is the velocity of the object at a given instant, the kinetic energy would be \(\frac{1}{2} m V^2\). As the fall of the object continues, the potential energy would decrease while the kinetic energy would increase.

When the object is about to reach the ground, h = 0 and v will be the highest. Therefore, the kinetic energy would be the largest and potential energy will be the least. However, the sum of the potential energy and kinetic energy of the object would be the same at all points. That is,

Potential energy + kinetic energy = constant

\(m g h+\frac{1}{2} m v^2=\mathrm{constant}\)

The sum of kinetic and potential energies gives the total mechanical energy of the system.

Efficient use of Energy

Energy is very important to us. We need energy in every form or the other to sustain or to carry out any work. From the naturally gifted resources, we get energy in one form or the other, for example, coal, petroleum, sunlight, etc. It is our duty to preserve and protect these resources. Judicious use of energy is very important to keep the resources available for future generations.

 

Chapter 4 Work And Energy Track Your Learning Question And Answers

Question 1. ______ is defined as the capacity of doing work.

1. Work
2. Energy
3. Power
4. None

Answer. 2. Energy

Question 2. The main source of energy is ______, which gives energy to all the entities of the universe either directly or indirectly.

1. Sun
2. Sea
3. Ocean
4. Coal

Answer. 1. Sun

Question 3. The energy used in households, industries and commercial establishments is usually expressed in:

1. Joules
2. Watt
3. Kilowatt hour
4. None of the options

Answer. 3. Kilowatt hour

Question 4. The largest group of geothermal power plants in the world are located in ______.

1. Germany
2. India
3. United States
4. UK

Answer. 3. United States

Question 5. Tidal energy uses rise and fall of tides to convert ______ energy of incoming and outgoing tides into electrical energy.

1. Potential
2. Mechanical
3. Electrical
4. Kinetic

Answer. 4. Kinetic

Question 6. When the speed of a bike increases by 120%, then its kinetic energy increases by:

1. 4.84 times
2. 3 times
3. 1.44 times
4. By 240 %

Answer. 1. 4.84 times

Question 7. How fast should a man of 50 kg run so that his kinetic energy reaches 500 J?

1. 5 m/s
2. 10 m/s
3. 20 m/s
4. 2(5)1/2 m/s

Answer. 4. 2(5)1/2 m/s

Chapter 4 Work And Energy Rate of Doing Work

We all consume energy at some rate. Therefore, rate of consuming energy is called power. Power measures the speed of work done, that is, how fast or slow the work is done. Power is defined as the rate of doing work or the rate of transfer of energy. If an agent does a work W in time t, then power is given by:

\(\text { Power }=\frac{\text { work }}{\text { time }} \text { or } P=\frac{W}{t}\)

The unit of power is watt having the symbol W. 1 watt is the power of an agent, which does work at the rate of 1 joule per second. We can also say that power is 1 W when the rate of consumption of energy is 1 J s^-1. 1 watt = 1 joule/second or 1 W = 1 J s^-1. We express larger rates of energy transfer in kilowatts (kW).

1 kilowatt = 1000 watts

1 kW = 1000 W

1 kW = 1000 J s^-1

Key Points to Remember

1. Work done on an object is defined as the magnitude of the force multiplied by the distance moved by the object in the direction of the applied force. The unit of work is joule: 1 joule = 1 newton × 1 metre.
2. Work done on an object by a force would be zero if the displacement of the object is zero.
3. An object having capability to do work is said to possess energy. Energy has the same unit as that of work.
4. An object in motion possesses what is known as the kinetic energy of the object. An object of mass, m moving with velocity v has a kinetic energy of \(\frac{1}{2} m V^2\).
5. The energy possessed by a body due to its position called the potential energy. The gravitational potential energy of an object of mass, m raised through a height, h from the earth’s surface is given by mgh.
6. According to the law of conservation of energy, energy can only be transformed from one form to another; it can neither be created nor destroyed. The total energy before and after the transformation always remains constant.
7. Energy exists in nature in several forms such as kinetic energy, potential energy, heat energy, chemical energy etc. The sum of the kinetic and potential energies of an object is called its mechanical energy.
8. Power is defined as the rate of doing work. The SI unit of power is watt. 1 W = 1 J/s.
9. The energy used in one hour at the rate of 1kW is called 1 kW h.

Chapter 4 Work And Energy Very Short Question and Answers

Question 1. Define power. What is the SI unit of power?
Answer:

Power:

Power is defined as total work done divided by total time taken. The SI unit of power is Watt.

Question 2. What is the commercial unit of energy? Convert it into Js^-1
Answer:

Commercial unit of energy:

The commercial unit of energy is kilowatt-hour (kWh).

Question 3. A boy of mass 60 kg runs up a staircase of 50 steps in 10 s. If the height of each step is 10 cm, find his power. Take g = 10 m s^-2.
Answer:

Given

A boy of mass 60 kg runs up a staircase of 50 steps in 10 s. If the height of each step is 10 cm

Weight of the boy = mg = 60 × 10 ms^-2

= 600 N

Height of the staircase,

h = 50 × 10/100 m

= 5 m

Time taken to climb = 10 s

P = work done/time

P = mgh/t

P = 600 × 5/10 s

P = 300 W

Thus, power is 300 W.

Question 4. An electric bulb of 60 W is used for 6 h per day. Calculate the ‘units’ of energy consumed in one day by the bulb.
Answer:

Given

Power of electric bulb = 60 W

= 0.06 kW.

Time used,

t = 6 h

Energy = power × time taken

= 0.06 kW × 6 h

= 0.36 kWh

= 0.36 ‘units’.

The energy consumed by the bulb is 0.36 ‘units’.

Chapter 4 Work And Energy

Question 1. A pump transfers 500 L of water to the overhead tank of a building of height 12 m in 15 minutes. Calculate the power of motor pump.
(Take g = 10 ms^-2) (Mass of 1 L of water = 1 kg)

1. P = 66.7 W
2. P = 68 W
3. P = 72 W
4. P = 83 W

Answer. 1. P = 66.7 W

Question 2. A machine does 192 J of work in 12 Sec. What is the power of the machine?

1. 8 W
2. 2 W
3. 16 W
4. 10 W

Answer. 3. 16 W

Question 3. A weighting 500 kg runs up a hill rising himself vertically 10 m in 40 Sec. Calculate power. given g = 9.8 m^-1

1. 1220 W
2. 2000 W
3. 1623 W
4. 1225 W

Answer. 2. 2000 W

Question 4. A rickshaw puller pulls the rickshaw by applying a force of 200 N. If the rickshaw moves with constant velocity of 10 ms^-1. Find the power of rickshaw puller.

1. 2220 W
2. 2000 W
3. 2623 W
4. 1005 W

Answer. 2. 2000 W

Question 5. Calculate the time taken 60 W bulb to consume 3600 J of energy.

1. 3 min
2. 60 min
3. 6 min
4. 1 min

Answer. 4. 1 min

Chapter 4 Work And Energy Practice Exercises

Question 1. When a body falls freely towards the earth, then its total energy

1. increases
2. decreases
3. remains constant
4. first increases and then decreases

Answer. 3. remains constant

Question 2. A car is accelerated on a levelled road and attains a velocity 4 times of its initial velocity. In this process the potential energy of the car

1. does not change
2. becomes twice to that of initial
3. becomes 4 times that of initial
4. becomes 16 times that of initial

Answer. 1. does not change

Question 3. In case of negative work the angle between the force and displacement is

1. 0°
2. 45°
3. 90°
4. 180°

Answer. 4. 180°

Question 4. An iron sphere of mass 10 kg has the same diameter as an aluminium sphere of mass is 3.5  kg. Both spheres are dropped simultaneously from a tower. When they are 10m above the ground, they have the same

1. acceleration
2. momenta
3. potential energy
4. kinetic energy

Answer. 1. acceleration

Question 5. A girl is carrying a school bag of 3 kg mass on her back and moves 200 m on a levelled road. The work done against the gravitational force will be (g = 10 m s^-2)

1. 6 × 103 J
2. 6 J
3. 0.6 J
4. zero

Answer. 4. zero

Question 6. Which one of the following is not the unit of energy?

1. joule
2. newton metre
3. kilowatt
4. kilowatt hour

Answer. 3. kilowatt

Question 7. The work done on an object does not depend upon the

1. displacement
2. force applied
3. angle between force and displacement
4. initial velocity of the object

Answer. 4. initial velocity of the object

Question 8. Water stored in a dam possesses

1. no energy
2. electrical energy
3. kinetic energy
4. potential energy

Answer. 4. potential energy

Question 9. A body is falling from a height h. After it has fallen a height h/2 it will possess

1. only potential energy
2. only kinetic energy
3. half potential and half kinetic energy
4. more kinetic and less potential energy

Answer. 3. half potential and half kinetic energy

Chapter 4 Work And Energy Fill in the Blanks

Question 1. If force and displacement are in the same direction, the work would be ______.
Answer. Positive

Question 2. The work done on a 5 kg body to displace it by ______ is 5 J, given that force applied is 1 N.
Answer. 5 m

Question 3. Work done on a body get stored as ______.
Answer. Potential energy

Question 4. A bird sitting at a height has only ______ energy.
Answer. Potential

Question 5. Kinetic energy is a ______ quantity.
Answer. Scalar

Question 6. If the speed of a body is doubled, its kinetic energy must become ______.
Answer. Four times

Question 7. The sum of potential energy and kinetic energy is called ______ energy.
Answer. Mechanical

Question 8. When a ball is thrown up, ______ energy is converted into potential energy.
Answer. Kinetic

Question 9. Electricity is measured by electric meters installed in our homes that measure electric energy in units of ______.
Answer. Kilowatt-hour (kWh)

Question 10. Energy is a ______ quantity.
Answer. Scalar

 

Chapter 4 Work And Energy Match the Columns

Question 1. Match the column 1 with 2.

Select the correct option:

1. A-3, B-1, C-4, D-2
2. A-4, B-3, C-2, D-1
3. A-3, B-4, C-1, D-2
4. A-1, B-3, C-2, D-4

Answer. 2. A-4, B-3, C-2, D-1

Question 2. Match the column 1 with 2.

Select the correct option:

1. A-2, B-3, C-4, D-1
2. A-4, B-3, C-2, D-1
3. A-3, B-4, C-1, D-2
4. A-1, B-3, C-2, D-4

Answer. 1. A-2, B-3, C-4, D-1

Chapter 4 Work And Energy Assertion Reasoning

Direction: Choose the correct answer from the following choices:

1. Assertion and reason are both correct statements and reason is explanation for assertion.
2. Assertion and reason are both correct statements but reason is not the correct explanation for assertion.
3. Assertion is correct statement but reason is incorrect statement.
4. Assertion is incorrect statement but reason is correct.

Question 1. Assertion (A): Humans are machines, they can do lot of work.
Reason (R): People are able to do work because of energy.
Answer. 1. Assertion and reason are both correct statements and reason is explanation for assertion.

Question 2. Assertion (A): Thus, work done by force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force.
Reason (R): Work has both magnitude and direction.
Answer. 3. Assertion is correct statement but reason is incorrect statement.

Question 3. Assertion (A): Work done is negative when the force acts opposite to the direction of displacement.
Reason (R): Work done is positive when the force is in the direction of displacement.
Answer. 2. Assertion and reason are both correct statements but reason is not the correct explanation for assertion.

Question 4. Assertion (A): The energy possessed by an object is measured in terms of its capacity of doing work.
Reason (R): The unit of energy is, therefore, the same as that of work, that is, joule (J).
Answer. 1. Assertion and reason are both correct statements and reason is explanation for assertion.

Question 5. Assertion (A): Flowing water, ­blowing wind, a running athlete, etc., possess kinetic energy.
Reason (R): Kinetic energy is the energy possessed by an object due to its motion.
Answer. 1. Assertion and reason are both correct statements and reason is explanation for assertion.

Chapter 4 Work And Energy Comprehension Passage

An object at a certain height possess potential energy. Now, work is done on it, against the gravity by bringing the object to some other point.

Gravitational potential energy of an object above the ground at a certain point is defined as raising the body to that point against the gravity.

Let us try to devise a formula to calculate the potential energy.

Consider an object of mass m. Let it be raised through a height h from the ground. A force is required to do this. The minimum force required to raise the object is equal to the weight of the object mg.

The object gains energy equal to the work done on it. Let the work done on the object against gravity be W. That is, work done.

W = force × displacement

= mg × h

= mgh

Since work done on the object is equal to mgh, energy equal to mgh units is gained by the object. This is the potential energy (EP) of the object.

E_p = mgh

Question 1. When a body is raised above the ground against gravity, which energy is used?

1. Potential energy
2. Kinetic energy
3. Gravitational potential energy
4. Gravitational kinetic energy

Answer. 3. Gravitational potential energy

Question 2. ‘Mgh’ is the formula for

1. Kinetic energy
2. Potential energy
3. Both (a) and (b)
4. None of the above

Answer. 2. Potential energy

Chapter 4 Work And Energy Integer Type Question And Answers

Question 1. Calculate the kinetic energy of the body of mass 4 kg moving with the velocity of 0.1 metre per second.
Answer. 0.02 J

Question 2. How much work is done by a force 20 N in moving an object through a distance of 2 m in direction of the force?
Answer. 40 J

Question 3. What is the work to be done to increase the velocity of a car from 20 km h-1 to 40 km h^-1 if the mass of the car is 1200 kg?
Answer. 2.22 x 105 J

Question 4. 10 tube lights each of 50 W are operated for 15 hours. Calculate electrical energy consumed in ‘units’.
Answer. 7.5 kWh

Question 5. Rahul, having her own mass 50 kg, climbs 20 m height along with 30 kg mass in 40 s. Calculate her power and work done. (take g = 10 m/s²)
Answer. W = 16 kJ P = 400 W