Work & Energy Class 9 Science Notes

Please refer to Work & Energy Class 9 Science Notes and important questions below. The Class 9 Science Chapter wise notes have been prepared based on the latest syllabus issued for the current academic year by CBSE. Students should revise these notes and go through important Class 9 Science examination questions given below to obtain better marks in exams

Work & Energy Class 9 Science Notes and Questions

The below Class 9 Work & Energy notes have been designed by expert Science teachers. These will help you a lot to understand all the important topics given in your NCERT Class 9 Science textbook.

Refer to Chapter 11 Work & Energy Notes below which have been designed as per the latest syllabus issued by CBSE and will be very useful for upcoming examinations to help clear your concepts and get better marks in examinations.

1. Work Done By A Constant Force

Work is a scalar quantity equal to the product of the displacement x and the component of the force Fx in the direction of the displacement..
Work is defined as a force acting upon an object to cause a displacement
Mathematically, work can be expressed by the following equation.
W∙F x d cos q ( cos 00∙1)W∙F x d cos q ( cos 00∙1)
where F – force, d – displacement, and the angle (theta) is defined as the angle between the force and the displacement vector
Three things are necessary for the performance of work:
There must be an applied force F.
There must be a displacement x.
The force must have a component along the displacement

Negative Work

Important Questions Class 9 Science Chapter 11 Work & Energy

Positive Work

Force F contributes to displacement x.

2. Energy And Its Forms
The metric system unit of energy is the joule (J), after James Joule.
• Mechanical energy is the energy which is possessed by an object due to its motion or its stored energy of position

Forms of Energy
• Kinetic energy : is the energy of motion Energy which a body possesses because of its motion, which occurs anywhere from an atomic level to that of a whole organism

Examples of Kinetic Energy: This is not an all-inclusive list.

Electrical – The movement of atoms
Electromagnetic or Radiant – The movement of waves
Thermal or Heat – The movement of molecules
Motion – The movement of objects
Sound – The movement through waves

Engineers generally refer to thermal/heat energy as “internal energy” and use “kinetic energy” strictly in reference to motion. Potential Energy (Stored energy or gravitational energy)

The capacity to do work by virtue of position or configuration
an object can store energy as the result of its position or elastic source
Potential Energy is maximum at the maximum HEIGHT

Energy transformation involves the conversion of one form of energy into another form. Examples of energy transformation include:

• Chemical – Food is consumed and converted into motion for playing sports or taking a test.

• Radiant – Sunlight is consumed by plants and converted into energy for growth.

• Electrical – Energy transferred to an oven is converted to thermal energy for heating our food.

Now you know the basic forms of energy. The next question is “What are the energy sources?”

There are renewable and nonrenewable sources of energy. A renewable energy source is a form of energy that is constantly and rapidly replenished by natural processes.

Examples of renewable energy sources include:

• Biomass – The use of a living or once living organism as fuel

• Hydropower – The energy produced from the movement of water

• Geothermal – The use of heat from within the Earth or from the atmosphere near oceans to warm houses or other buildings

• Wind – The use of wind to generate electricity

Solar – The use of the sun as a source of heat; for instance, to heat a room within a house, etc.

Energy Conversion

Fossil fuels Chemical → Heat → Mechanical → Electrical

Solar cells Sunlight → Electrical

Wind turbines Kinetic → Mechanical → Electrical

Hydroelectric Gravitational potential → Mechanical → Electrical Nuclear Nuclear → Heat → Mechanical → Electrical

3. Potential energy of an object at a height

An object increases its energy when raised through a height.

The potential energy of an object at a height depends on the ground level or the zero level

4. Law Of Conservation Of Energy

The principle of Conservation of Mechanical Energy

The total mechanical energy (E = KE+PE)(E = KE+PE) of an object remains constant as the object moves, provided that the net work done by external non-conservative forces is zero, W_nc=0JW_nc=0J Total mechanical energy: the sum of kinetic energy and gravitational potential energy

E=KE+PE

W_nc=(KEf−KE0)+(PEf−PE0)W_nc=(KEf−KE0)+(PEf−PE0)

W_nc=(KEf+PEf)−(KE0+PE0)Wnc=(KEf+PEf)−(KE0+PE0)

W_nc=Ef−E0Wnc=Ef−E0

Ef=−KEf+PEfEf=−KEf+PEf

E=KE0+PE0E=KE0+PE0

5. Rate of Doing Work & Commercial Unit Of Energy POWER
Rate at which work is performed or energy is expended P=WtP=Wt
Watt is the base unit of Power .One watt is equal to 1 joule of work per second

Types of Power

Electrical Power
Uses electrical energy to do work

Mechanical Power
Uses mechanical energy to do work (linear, rotary)

Fluid Power
Uses energy transferred by liquids (hydraulic) and gases (pneumatic)

• Power is the rate that we use energy.

• Power = Work or Energy / Time

• P = W/t = F × d/t = F vP = W/t = F × d/t = F v

• The unit joule is too small .The bigger unit of energy called kilowatt hour (kW h)

1 kW h is the energy used in one hour at the rate of 1000 J s−−1 (or 1 kW).1000 J s−−1 (or 1 kW).

1 kW h = 1 kW ∗1 h1 kW h = 1 kW ∗1 h

= 1000 W∗3600 s= 3600000 J1 kW h = 3.6 x 106J.

Questions for Class 9 Science Chapter 11 work And Energy

Question. Name the transformation of energy involved in the following cases :
(a) When a body is thrown upwards.
(b) When a body falls from the top of a hill.
(c) When coal burns.
(d) When a gas burns.
(e) When water falls from a height.
Ans : (a) Kinetic energy into potential energy.
(b) Potential energy into kinetic energy.
(c) Chemical energy into heat energy.
(d) Chemical energy into heat energy.
(e) Potential energy into kinetic energy.

Question. What are the factors on which the work done depends?
Ans : The work done by a force depends upon :
(i) The magnitude of the force.
(ii) The magnitude of the displacement.
(iii) The angle between force and displacement.

Question. What is power? What is its SI unit?
Ans : It is defined as the rate of doing work. Its unit is watt.

Question. Which type of energy is present in a battery?
Ans : Chemical energy.

Question. Name the energy present in an object due to its position or configuration.
Ans : Potential energy.

Question. Give one example of potential energy due to position.
Ans : Water stored in the reservoir of a dam has potential energy.

Question. (a) An arrow moves forward when released from a stretched bow. Explain the transformation of energy in the process.
(b) A boy of mass 50 kg climbs up a vertical height of 100 m. Calculate the amount of potential energy he gains.
Ans : (a) When the bow is stretched it stores potential energy. When the arrow is released the potential energy stored in the bow gets transformed into the kinetic energy of the arrow.
(b) Given m = 50 kg, h = 100 m,
g = 10 ms–1, PE = ?
PE = mgh = 50 × 10 × 100 = 5000 J

Question. Give an example of potential energy due to change in shape.
Ans : A stretched bow has potential energy due to change of shape.

Question. What type of energy is possessed by a flying bird and a flying aeroplane?
Ans : Both potential energy and kinetic energy.

Question. When do we say that work is done?
Ans : Work is said to be done when a force causes displacement of an object in the direction of applied force.

Question. Write an expression for the work done when a force is acting on an object in the direction of its displacement.
Ans : Work done = Force × Displacement

Question. Define : (a) power (b) work done (c) kinetic energy.
Give SI unit of each.
Ans : (a) The rate of doing work is called power. Its SI unit is watt.
(b) Work is the product of force and displacement. Its SI unit is joule.
(c) It is the energy possessed by a body by virtue of its motion. Its SI unit is joule.

Question. Find the energy in kWh consumed in 10 hours by a machine of power 500 W.
Ans : W = P × t
= 500 × 10 = 5000 Wh = 5 kWh.

Question. When is work said to be done against the force of gravity?
Ans : When a body is lifted, the work is done against the force of gravity.

Question. Calculate the cost of using a 2 kWh immersion rod for heating water in a house for one hour each day for 60 days if the rate is 1.50 per unit kWh.
Ans : E = Pt = 2 kW × 60 × 1 h
= 120 kWh = 120 unit
The cost of using a 2 kWh immersion rod for heating
water = 120 × 1.5 = Rs 180

Question. A work of 4900 J is done on load of mass 50 kg to lift it to a certain height. Calculate the height through which the load is lifted.
Ans : Work done on road to lift = mgh
4900 = 50 × 9.8 h
h=10m

Question. What is the SI unit of work done and power?
Ans : Joule and Watt.

Question. When is 1 joule of work said to be done?
Ans : When a force of 1 newton acting on a body displaces it 1m in its own direction.

Question. Does work done depend upon the velocity of the body.
Ans : No.

Question. State the law of conservation of energy.
Ans : It states that energy can neither be created nor destroyed. It can only change its form.

Question. In a tug-of-war one team gives way to the other. What work is being done and by whom?
Ans : The winning team does work. The work is equal to the product of the resultant force and the displacement undergone by the losing team.

Question. What will cause greater change in kinetic energy of a body? Changing its mass or changing its velocity?
Ans : Changing its velocity.

Question. List two essential conditions for work to be done.
Ans : (i) A force must act and
(ii) There should be displacement in the body.

Question. A force of 7 N acts on an object. The displacement is, say 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 : Given,
displacement = 8 m,
Force = 7 N
Now, Work done = Force × Displacement
= 7 × 8 = 56 J

Question. What is the work done by a coolie walking on a horizontal platform with a load on his head?
Ans : In order to balance the load on his head, the coolie applies a force on it in the upward direction, equal to its weight. His displacement is along the horizontal direction. Thus, the angle between force F and displacement is 90°. Therefore, work done W = Fs cos q = Fs cos 90° = 0.

Question. We wind our watch once a day, what happens to the energy?
Ans : When we wind our watch, we wind the spring inside the watch. As a result, energy is stored in the spring in the form of elastic potential energy. This elastic potential energy is used to make the watch work the whole day.

Question. Will work be done by a man who pushes a wall?
Ans : No.

Question. Name the form of energy associated in each case :
(i) A flying bird.
(ii) A man climbing the stairs.
(iii) A compressed watch spring.
(iv) A fast moving object.
Ans : (i) Mechanical energy
(ii) Mechanical energy
(iii) Potential energy
(iv) Kinetic energy

Question. Name the various forms of energy.
Ans : The various forms of energy are :
(i) Potential energy
(ii) Kinetic energy
(iii) Mechanical energy
(iv) Heat energy
(v) Chemical energy
(vi) Electrical energy
(vii) Light energy.

Question. What is the work done when the force acting on the body and the displacement produced in the body are at right angles to each other?
Ans : Zero.

Question. Write an expression for the work done in lifting a body of mass ‘m’ through a vertical height ‘h’.
Ans : Work done W = mgh, where g is acceleration due to gravity.

Question. What is the work done on a body moving in a circular path?
Ans : Zero, because force and displacement are perpendicular to each other.

Question. When a book is lifted from a table, against which force work is done?
Ans : Work is done against the force of gravity.

Question. Distinguish between work, energy and power. State the SI units for each of these quantities.
Ans : Work : It is defined as the product of force applied and the distance moved by the body on the application of the force. In SI unit it is measured in joule.
Energy : It is defined as the capacity of a body to do work. In SI unit it is measured in joule.
Power : It is defined as the rate of doing work. It measures how fast or slow the work is done. In SI unit it is measured in watt unit.

Question. What is the relationship between megawatt and watt?
Ans : 1 megawatt = 106 watt

Question. What is the relationship between megawatt and joules per second?
Ans : 1 megawatt = 106 joule/second
1 MW = 106 js^–1

Question. Define work. How is work measured? When is work done by a force negative?
Ans : Work is said to be done if force acting on an object displaces it through a certain distance.
It is measured as the product of force and displacement.
Work done is negative if force and displacement are in the opposite direction.

Question. What is the work done by the force of gravity in the following cases?
(a) Satellite moving around the Earth in a circular orbit of radius 35000 km.
(b) A stone of mass 250 g is thrown up through a height of 2.5 m.
Ans :
(a) Zero, as the displacement in one complete revolution is zero.
(b) Given m = 250 g = 0.25 kg, h = 2.5 m, g = 10
ms–2, W = ?
Now, W = Fs = mg × h
= 0.25 × 10 × 2.5 = 6.25 J

Question. Define 1 J of work.
Ans : Work done = Force x Displacement Force, F = 1 N and displacement, s = 1 m, then the work done by the force will be 1 Nm or 1 J. Thus, 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. What is the amount of work done by a force when a body moves in a circular path?
Ans :
Work done is given by the expression W = Fs cos q
. When a body moves in a circular path, then the
displacement (s) is zero. Therefore, work done is
W = F × 0 = 0.

Question. Name the common forms of the mechanical energy.
Ans : The common forms of the mechanical energy are :
(i) Kinetic energy
(ii) Potential energy

Question. Does every change in energy of the body involve work?
Ans : Yes.

Question. Is it possible that some force is acting on a body but still the work done is zero?
Ans : Yes, when force acts at an angle of 90° with the displacement.

Question. The masses of scooter and bike are in the ratio of 2 : 3 but moving with same speed of 108 km h^–1. Compute the ratio of their kinetic energy.
Ans : The energy possessed by a body by virtue of its motion.
Given, m₁/m₂=2/3
2 , the ratio of KE is equal to the ratio of their masses if they have the same velocity, therefore,ratio of their KE is also 2 : 3.

Question. Define power. Write commercial unit and SI unit of electrical energy. An electrical geyser of 1.5 kW works for 2 hours. Find the electrical energy units consumed in a day.
Ans : Power is defined as the rate of doing work. SI unit s joule and kWh is the commercial unit of electrical energy.
Given, P = 1.5 kW,
t = 2 hours,
E = P × t = 1.5 × 2 = 3 kWh

Question. A force of 10N causes a displacement of 2 m in a body in its own direction. Calculate the work done by force.
Ans : The work done by force = F × s = 10 N × 2 m = 20 J

Question. A body of 5 kg raised to 2 m find the work done.
Ans : The work done to raise a body
= PE
= mgh
= 5 kg × 9.8 × 2 = 98 joule

Question. A moving body of 30 kg has 60 J of KE. Calculate the speed.
Ans :
KE = 0.5 mv₂
60 = 0.5 × 30 × v₂
60 = 15 v₂
v = 2 m/s

Question. What is meant by the term horsepower (hp)?
Ans : Horsepower is another commercial unit power
1 hp = 746 W
1W = 746
1 hp = 0.0013 hp

Question. What is the kinetic energy of an object?
Ans : The energy possessed by a body by virtue of its motion is called kinetic energy.

Question. Write an expression for the kinetic energy of an object.
Ans : The expression is KE = 2
1 mv₂ where ‘m’ is the mass
and v is the velocity of the body.

Question. Define 1 watt of power.
Ans : When a work of 1 joule is done in 1 s, the power is said to be one watt.

Question. Define average power.
Ans : When a machine or person does different amounts of work or uses energy in different intervals of time, the ratio between the total work or energy consumed to the total time is average power.

Question. What is the relationship between horse power and watt?
Ans : 1 horsepower = 746 watt
1 hp = 746 W

Question. An electric kettle of 500W is used to heat water everyday for 2 hours. Calculate the number of unit of electrical energy consumed by it in 10 days.
Ans : E = Pt = 500 W × 10 × 2h
= 10000 Wh
= 10 kWh = 10 unit

Question. How does the kinetic energy of a body change if the mass of the body is halved?
Ans : If the mass of a body is heaved the kinetic energy is also halved.

Question. A car and a truck are moving with the same velocity of 60 km/hr–1, which one has more kinetic energy?
Ans : Truck has more kinetic energy as kinetic energy is directly proportional to the mass.

Question. By how much will the speed of a body, of fixed mass, increase if its kinetic energy becomes four times its initial kinetic energy?
Ans : The speed is doubled.

Question. Can a body possess energy even if it is not in motion?
Ans : Yes, it can possess potential energy.

Question. What is the relationship between watt and joules per second.
Ans : 1 Watt = 1 joule/second
1 W = 1 Js^–1

Question. What will happen to the kinetic energy of a body if its mass is doubled?
Ans : Its kinetic energy will be doubled.

Question. What will happen to the kinetic energy of a body if its velocity is halved?
Ans : The kinetic energy of the body will become one-fourth.

Question. When a constant force is applied to a body moving with constant acceleration, is the power of the force constant? If not, how would force have to vary with speed for the power to be constant?
Ans :We know that,
power (p) = force (f) × velocity (v)
Since the body is moving with accleration, V changes
and as a result of that P also changes, F being constant.
For P to be constant, FV = constant or F \ V
Thus, as V increases, F should decrease to keep P constant.

Question. What is the SI unit of energy?
Ans : The SI unit of energy is joule.

Question. Does a body at rest possess any kinetic energy?
Ans : No.

Question. A spring which is kept compressed by tying its ends together is allowed to be dissolved in an acid. What happens to the potential energy of the spring?
Ans : The potential energy of the spring gets converted into heat energy (kinetic energy of acid molecules). Due to this heat, the temperature of the acid rises.

Question. How much energy will an electric motor of 1 horsepower consume in one second?
Ans : An electric motor will consume 746 joule of energy per second.

Question. Give the mathematical relation between power, force and velocity.
Ans : Power (P) = force (F) × velocity (V) = (F × V)

Question. A hammer of mass 1 kg falls freely from a height of 2 m. Calculate (I) The velocity and (II) The kinetic energy of the hammer just before it touches the ground. Does the velocity of hammer depend on the mass of hammer?
Ans : PE = mgh = 1 × 9.8 × 2 = 19.6 J
PE = KE = 0.5 mv₂
19.6 = 0.5 × 1 × v₂
39.2 = v2
v = 6.29 m/s
No, velocity of hammer does not depend on the mass of the hammer as v = u + at

Question. Give the SI unit of potential energy.
Ans : The SI unit of potential energy is joule.

Question. What do you mean by transformation of energy?
Ans : It is the change of energy from one form of energy into another form of energy.

Question. When an arrow is shot from its bow, it has kinetic energy. From where does it get the kinetic energy?
Ans : A stretched bow possesses potential energy on account of a change in its shape. To shoot an arrow; the bow is released. The potential energy of the bow is converted into the kinetic energy of the arrow.

Question. Name at least three commonly used units of energy.
Ans : (i) Joule, (ii) Erg, (iii) Kilowatt hour.

Question. Name the practical unit of power in engineering.
Ans : Horsepower.

Question. Name at least six forms of energy.
Ans : (i) Chemical energy
(ii) Heat energy
(iii) Light energy
(iv) Electrical energy
(v) Sound energy
(vi) Solar energy

Question. How much work is done by a force of 10 N in moving an object through a distance of 4 m in the direction of the force?
Ans :
Work done = Force × Displacement
= F × s
= (10 N) × (4 m)
= 40 joule or 40 J.

Question. Define potential energy.
Ans : It is defined as the energy possessed by a body by virtue of its position or change in shape.

Question. Name the energy possessed by a stretched rubber band lying on the table.
Ans : Potential energy.

Question. What is power? How do you differentiate kilowatt from kilowatt hour?
Ans : Power is the rate of doing work. Kilowatt is the unit of power and kilowatt hour is the unit of energy.

Question. Calculate the work done in lifting 200 kg of water through a vertical height of 6 m.
Ans : (i) Work done in lifting a body
= Weight of body × vertical distance
(ii) The work done in lifting
= W = mgh
= 200 kg × 10 m/s₂ × 6 m = 1200 J

Question. How many watt are there in 1 horsepower?
Ans : 746 watt.

Question. A light and a heavy body have equal kinetic energy. Which one is moving fast?
Ans : The lighter body is moving fast.

Question. State the relation between kWh and joule. Define 1 watt.
Ans : 1 kWh – 1000 Wh = 1000 Js^–1 × 60 × 60 s
= 3.6 × 106 J
1 watt is the power of an agent which can do one joule of work in one second.

Question. Can kinetic energy of a body be negative?
Ans : No as mass and velocity cannot be negative

Question. What is the energy of a body due to its motion called?
Ans :Kinetic energy.

Question. What is the SI unit of kinetic energy?
Ans : Joule.

Question. Can any object have mechanical energy even if its momentum is zero? Explain.
Ans : Yes, mechanical energy comprises of both potential energy and kinetic energy. Zero momentum means that velocity is zero. Hence, there is no kinetic energy but the object may possess potential energy.

Question. Give one example each of potential energy (i) due to position (ii) due to shape.
Ans : (i) Potential energy due to position : Water stored in dam has potential energy.
(ii) Potential energy due to shape : In a toy car, the wound spring possesses potential energy and as the spring is released, its potential energy changes into kinetic energy due to which the car moves.

Question. What kind of energy transformation takes place when a body is dropped from a certain height?
Ans : When a body falls, its potential energy gradually gets converted into kinetic energy. On reaching the ground, the whole of the potential energy of the body gets converted into kinetic energy.

Question. Can energy be destroyed? Can energy be created?
Ans : No, energy can neither be created nor be destroyed.

Question. A cell converts one form of energy into another. Name the two forms.
Ans : It converts chemical energy into electrical energy.

Question. Name one unit of power bigger than watt.
Ans : A unit bigger than watt is kilowatt.

Question. A spring which has been kept compressed by tying its ends together is allowed to be dissolved in an acid. What happens to the potential energy of the spring?
Ans : The PE of the spring gets converted into KE of acid molecules whose temperature rises.

Question. Justify giving proper reasoning whether the work done in the following case is positive or negative – Work done by resistive force of air on a vibrating pendulum
in bringing it to rest.
Ans : Work done is negative because the resistive force of air always acts opposite of the direction of motion of the vibrating pendulum.

Question. The work done in lifting a box on to a platform does not depend upon how fast it is lifted up. Explain your answer giving proper reasoning.
Ans : The work done (W) in lifting a box through a distance
(s) against the gravitational force (F) is given by W =
Fs. Hence, it is obvious that it is independent of the
rate at which the box is lifted.

Question. A body of mass 2 kg is moving with a speed of 20 m/s.
Find the kinetic energy.
Ans : KE = 0.5 mv²
= 0.5 × 2 × 20 × 20 = 400 J

Question. At what rate is electrical energy consumed by a bulb of 60 watt?
Ans : A 60 watt bulb consume electrical energy at the ratio of 60 joule per second.

Question. Which type of energy is present in an atom?
Ans : Nuclear energy.

Question. Define energy.
Ans : Energy is the ability of a body to do work. It is also defined as the capacity to do work.

Question. A body performs no work. Does it imply that the body possesses no energy?
Ans : When a body does not perform any work, it never implies that the body has no energy. The body may have energy but still does not perform any work, e.g., a book placed on a table has potential energy but is not performing any work.

Question. (a) What is meant by potential energy? Is potential energy vector or scalar quantity?
(b) Give one example of a body having potential energy.
Ans : (a) The energy possessed by a body by virtue of its position or configuration. It is a scalar quantity.
(b) Stretched string of a bow.

Question. When is the work done by a force said to be negative?
Give one situation in which one of the forces acting on the object is doing positive work and the other is doing negative work.
Ans : We know that work done W = Fs cos q , where q is the angle between F and S. Clearly, W will be – ve, if q is between 90° and 180° because then cos q will be –ve. Consider the case of a body falling under gravity. The body experiences an upward frictional force and downward force due to gravity. Since, the body is moving downwards, the work done by force
to gravity will be +ve but that is against the upward thrust will be –ve.

Question. A light and a heavy body have the same momentum.
Which one will have greater kinetic energy?
Ans :
As K =p²/2m and p is same for both. Hence, K∝ m/1 Thus, the lighter body has more kinetic energy thanbthe heavier body.

Question. (i) What is a closed system?
(ii) State the law of conservation of energy.
Ans : (i) A system that does not interact with another is a closed system. Energy in a closed system is neither taken out of it nor taken in from outside. However, energy transformation may occur inside a closed system.
(ii) The law of conservation of energy states that “the energy can neither be created nor destroyed, but can be transformed from one form to another. In other words, the total energy of a closed system remains constant. Therefore, whenever one from of energy disappear, an equivalent amount of another form appears.

Question. Is it possible that a body be in accelerated motion under a force acting on the body, yet no work is being done by the force? Explain your answer giving a suitable example.
Ans : Yes, it is possible, when the force is perpendicular to the direction of motion. The Moon revolving round the Earth under the centripetal force of attraction of the Earth, but Earth does no work on the motion.

Question. A body moves along a circular path. How much work is done in doing so? Explain.
Ans : In case of a body moving along a circular path, the force (centripetal) is always along the radius while displacement is tangential. Hence, work done W = FS
cos 90° = 0 as angle between F and S is 90°.

Question. A battery lights a bulb. Describe the energy changes involved in the process.
Ans : Within the electric cell of the battery the chemical energy changes into electrical energy. The electric energy on flowing through the filament of the bulb, first changes into heat energy and then into the light energy.

Question. What kind of energy transformations take place at a thermal power station?
Ans : At a thermal power station, the chemical energy of coal is changed into heat energy which is further changed into electrical energy with the help of an electric generator.

Question. (a) How much work is done when a force of 1 N moves a body through a distance of 1 m in its direction?
(b) Is it possible that a force is acting on a body but still the work done is zero? Explain giving one example.
Ans : (a) 1 J of work is done.
(b) Yes, it is possible when force acts at right angles to the direction of motion of the body. Example :
Gravitational force of Earth acts on a satellite at right angles to its direction of motion.

Question. Is it possible that a body is in accelerated motion tinder a force acting on the body, yet no work is being done by the force? Explain your answer giving a suitable example.
Ans : Yes, it is possible, when the force is perpendicular to the direction of motion. The Moon revolving round the Earth under the centripetal force of attraction of the Earth but Earth does not do any work on the motion of the Moon.

Question. Does the potential energy of a spring increase or decrease when it is compressed?
Ans : The potential energy of the spring increases because work is done on it when it is compressed.

Question. A spring is compressed, what change is expected in the potential energy of the spring
Ans : When a spring is compressed, its potential energy is used up to changing its shape.

Question. A man rowing a boat upstream is at rest with respect to the shore. Is he doing work?
Ans : The man is doing work relative to the stream because he is applying force to produce relative motion between the boat and the stream. But he does zero work relative to the shore as the displacement relative to the shore is zero.

Question. What type of energy is stored in the spring of a watch?
Ans : When we wind a watch, the configuration of its spring is changed. The energy stored in the spring is obviously potential in nature (elastic potential energy to be more accurate).