1. Define the term energy.
ANS: Energy of a body is defined as its capacity or ability to do work. When a body is capable of doing more work, it is said to possess more energy.
2. How does the energy of an
object change when it performs work or work is done on it?
ANS: When an object does work, it loses energy. When
work is done on an object, the object gains energy.
3. How does an object with
energy do work?
ANS: An object that possesses energy can exert a force
on another object. When this happens, energy is transferred from the former to
the latter. The second object may move as it receives energy and therefore do
some work. Thus, the first object has a capacity to do work. This implies that
any object that possesses energy can do work.
4. Is energy a scalar or vector
quantity?
ANS: Energy is a scalar quantity as it has only
magnitude and no direction.
5. What is the SI unit of
energy?
ANS: The SI unit of energy is joule (J).
6. Why are the units of energy
the same as those of work?
ANS: Since energy is measured by the amount of work that
a body can do, so units of energy are the same as those of work.
7. Define 1 Joule of energy.
ANS: One joule of energy is the energy required to do 1
joule of work.
8. Which physical quantity has
a unit electron volt (eV)?
ANS: Energy
9. Which physical quantity has
a unit kilo-watt hour (kWh)?
ANS: Energy
10. Calorie is the unit of
_____.
ANS: Energy
11. 1 Joule equals to _____
calorie.
ANS: 4.186 calorie
12. Name the natural source of
energy for us.
ANS: The sun is the natural source of energy for us.
13. What are the various forms
of energy?
ANS: The various forms of energy are, mechanical energy,
heat (thermal) energy, chemical energy, sound energy, chemical energy,
electrical energy, nuclear energy and solar energy.
14. Define heat or thermal
energy.
ANS: The energy possessed by a body due to its
temperature is known as heat or thermal energy.
15. Define chemical energy.
ANS: The energy released in chemical reactions is known
as chemical energy.
16. Define Sound energy.
ANS: The energy of a vibrating body producing sound is
known as sound energy.
17. Define Electrical energy.
ANS: The energy of moving electrons in a conductor
connected with a battery is known as electrical energy.
18. Define Nuclear energy.
ANS: The energy released when two nuclei of light
elements combine with each other to form a heavy nucleus or when a heavy
nucleus breaks into two light nuclei is known as nuclear energy.
19. Define Solar energy.
ANS: The energy radiated by the sun is known as solar
energy.
20. What is mechanical energy?
ANS: The energy produced by mechanical means is called
mechanical energy. (or)
The sum of kinetic and potential energy of a
body is called its mechanical energy.
21. What do you understand by kinetic
energy of a body?
ANS: The energy possessed by a body by virtue of its
motion is called its ‘kinetic energy’.
A moving body can do work. The amount
of work which a moving object can do before coming to rest is equal to its
kinetic energy.
22. Does an object in motion
have ability to do work?
ANS: Yes, because it possesses kinetic energy.
9. Give some examples where an
object possesses kinetic energy?
ANS: (i) A moving vehicle (bus, car, train etc.)
(ii) moving objects (ball, bullet
etc.)
(iii) blowing winds
(iv) running athlete
(v) flowing water or falling water
(vi) a falling object (ball, fruit,
meteors etc.)
(vii) flying aircraft
23. What type of energy is
possessed by a cricket ball just before caught by a fielder?
ANS: Kinetic energy
24. What type of energy is
possessed by flowing water?
ANS: kinetic energy
25. Identify the energy
possessed by a rolling stone.
ANS: kinetic energy
26. Identify the kind of energy
possessed by a running athlete.
ANS: kinetic energy
27. Derive an expression for
the kinetic energy of an object of mass ‘m’ moving with an acceleration ‘a’.
ANS: Mass of the body = m
Let’s consider, the initial velocity
= u
Force applied on the body = F
Acceleration produced in the body in
the direction of force = a
The final velocity of the body = v
Displacement of the object during
this journey = s
As, v2 – u2 =
2as
Therefore, a = v2 – u2/2s
As the force and displacement are in
the same direction, the work done on the body is,
W = F.s
W = ma.s
W = ms (v2 – u2)/2s
W = ½ m(v2 – u2)
This work done appears as the kinetic
energy of the body.
Therefore, K.E. = EK = ½
m(v2 – u2)
28. Derive an expression for
the kinetic energy of an object of mass ‘m’ moving with a uniform velocity ‘v’.
ANS:
We have the expression for kinetic energy,
EK = ½ m(v2 – u2)
When it is moving with uniform
velocity, we can put, u=0
Thus the expression for the kinetic
energy of an object moving with a uniform velocity becomes, EK = ½
m(v2 – 02)
EK = ½ mv2
29. State the factors on which
the kinetic energy of a body depends.
ANS: (i) K.E. of a body is directly proportional to its
mass i.e. K.E. a m
(ii) K.E. of a body is directly
proportional to the square of its velocity. K.E. a v2
30. Can the kinetic energy of
an object be negative?
ANS: No, as both m and v2 are always
positive.
31. What will be the kinetic
energy of a body when its mass is made four times and the velocity is doubled?
ANS: We have, KE1= ½ mv2
According to the question, mI
= 4 m and vI = 2 v
Then, KE2= ½ mI(vI)2
KE2 = ½ 4m. (2v)2
KE2 = ½ 4m. 4v2
KE2 =16. ½ mv2
Or,
KE2 =16. (KE1)
Thus the K.E. becomes 16 times its
original value.
32. What change should be
affected in the velocity of a body to maintain the same kinetic energy, if its
mass is increased four times?
ANS: KE1 = ½ mv2
If mass is increased four times, KE2
= ½ 4mv2
= 4. ½mv2
= 4 x KE1
Thus, if mass is increased four
times, the kinetic energy also becomes four times.
Thus, to maintain the same kinetic
energy, the velocity must be reduced to half.
33. What will cause greater
change in kinetic energy of a body? Changing its mass or changing its velocity?
ANS: Change in velocity will cause greater change in
kinetic energy because
34. What is the SI unit of kinetic energy?
ANS: Joule
35. A car and a truck are
moving with the same velocity of 60 km/hr. Which one has more kinetic energy?
ANS: Since mass of truck is greater than the mass of car
and both have same velocity, therefore, K.E. of truck > K.E. of car
36. A bus and a car have same
KE. Which of the two is moving fast? Explain.
ANS: The KE is given by, KE= ½ mv2 => v = √2KE/√m
As KE of both bus and car is same, so
v a 1/√m
Since, mass of car is less than that
of bus, so the car is moving faster than the bus.
37. Give one example, where
mass is large but kinetic energy is less.
ANS: A shot put thrown by an athlete.
38. Give one example, where
mass is small but kinetic energy is high.
ANS: A hockey ball which has been hit hard by a player
and is travelling fast.
39. Does a man standing at rest
on a moving car possess kinetic energy?
ANS: Yes, because he is sharing the motion of the car
and so he possesses kinetic energy.
40. Two bodies of same mass start from rest and move with velocities V and 2V respectively. Find the ratio of their kinetic energies.
41. A blacksmith uses a heavier
hammer than a goldsmith. Why?
ANS: We know that kinetic energy is directly proportional to the mass of the body. Therefore, if the mass of the body is large, the kinetic energy is large. For this reason, a blacksmith uses a heavier hammer than a goldsmith.
42. Derive a relationship
between kinetic energy and linear momentum.
43. If momentum of a body is
doubled, the kinetic energy becomes _____ times.
ANS: As kinetic energy is proportional to the square of
the momentum, so if momentum of a body is doubled, the kinetic energy becomes 4
times.
44. Two bodies of masses m1 and m2 have equal kinetic energies. What is the ratio of their linear momenta?
45. Two balls have different masses but same kinetic energy. Which has the greater momentum? Justify.
Thus, momentum of the ball of large mass is more.
46. A light and a heavy object
have the same momentum, find out the ratio of their kinetic energies. Which one
has a larger kinetic energy?
47. Can a body have momentum
without possessing energy?
ANS: No, if a body has momentum, it will be in motion
and will certainly possess kinetic energy.
48. Define potential energy.
ANS: The energy possessed by a body by virtue of its
position or shape or configuration is called its ‘potential energy’.
49. Explain by an example that
a body may possess energy even when it is not in motion.
ANS: A stone lying on the roof of the building has
potential energy. When the stone is lifted up, work has to be done on it
against the force of gravity which is stored in it as potential energy. When
allowed to fall, it has capacity to do an equal amount of work.
50. Give some examples where an
object possesses potential energy?
ANS: (i) Stretched strings of catapult
(ii) stretched bow and arrow has
potential energy due to its shape
(iii) a stretched or compressed
spring or a rubber band due to its shape
(iv) energy possessed by the water in
a tank on the roof of a building
(v) water stored in a dam has potential energy due to its position
(vi) a stone lying on the top of a hill or a mountain has potential energy due to its position51. Write an expression for potential energy.
ANS: The potential energy of a body is given by, PE =
mgh.
52.
Write the SI unit of potential energy.
ANS: The SI unit of potential energy is joule (J).
53. Give one example of body
having potential energy due to change of shape.
ANS: Spring of a watch or stretched bow and arrow
54. Name the type of energy possessed
by a raised hammer.
ANS: Potential energy
55. In what form, energy is
stored in a wrist watch?
ANS: Potential energy
56. How is energy stored in a
watch?
ANS: In winding a watch, work is done in compressing the
spring which is stored in it as potential energy. As the spring expands, it
does work and moves the hands of the watch.
57. Can the potential energy of
an object be negative?
ANS: Yes, potential energy is negative when forces
involved are attractive.
58. What are the types of potential
energies?
ANS: Potential energy is generally of three types:
Elastic potential energy, gravitational potential energy and chemical potential
energy.
59. Define elastic potential
energy.
ANS: The energy possessed by an object due to change in
shape or configuration, is called as elastic potential energy.
60. Give an example of an
object having elastic potential energy?
ANS: A stretched or compressed spring has elastic
potential energy.
61. Define gravitational
potential energy.
ANS: The energy possessed by an object due to its height
from the surface of the earth, is called gravitational potential energy.
62. What are the properties of
gravitational potential energy?
ANS: (i) Gravitational potential energy of a body lying
on the surface of earth is taken as zero.
(ii) The gravitational potential
energy of the body increases as the height of the body from the surface of the
earth increases. (i.e. when the body moves away from the surface of the earth)
(iii) The gravitational potential
energy of the body decreases as the height of the body from the surface of the
earth decreases. (i.e. when the body falls towards the surface of the earth)
63. Derive an expression for
the potential energy of a body of mass ‘m’ at a height ‘h’ from the ground.
ANS: The object is being raised to the height ‘h’
against gravity.
Minimum force required to lift the
object,
F = Weight of body = mg
Distance travelled by the object, S =
h
Therefore, work done by the force on
the object,
W = F.S = mgh
This work done against force of
gravity is equal to the potential energy or gravitational potential energy of
the object.
Therefore, P.E. = mgh
64. State the factors on which
the gravitational potential energy of a body depends.
ANS: The gravitational potential energy of a body
depends upon,
(i) the weight of the object (mg)
(ii) the height of the object from
the surface of the earth (h)
65. Does the work done depend
upon how fast an object is raised?
ANS: Work done depends on the force applied and distance
travelled by an object. It does not depend on how fast an object is raised.
66. Does the gravitational potential
energy of a body depend on the path along which the body is moved?
ANS: No. The gravitational potential energy depends only
on the initial and final positions of the body.
67. “The gravitational
potential energy of a body does not depend on the path along which the body is moved.”-
Prove this.
ANS: Consider a block of mass ‘m’ raised through height
h from position A to B along path (a) and path (b).
In case of path (a),
When the block is raised through a height h, from A to B, the work done is given by
W = mg.h
This work done on the block is stored
inside the block as its gravitational potential energy.
In case of path (b),
When the block is raised through
height h1, from A to C, work done is given by,
WAC = mgh1 …….. (ii)
When the block is taken from C to D, then no work is done against gravity
because the force of gravity acts perpendicular to the displacement of the
ball.
Thus, WCD = 0 ……. (iii)
Now, when the block is raised through
a height h2 from position D to E, then the work done is given by, WDE
= mgh2 …….. (iv)
When the block is taken from E to B,
then no work is done against gravity because the force of gravity acts
perpendicular to the displacement of the ball.
Thus, WEB = 0 ……. (v)
Now the total work done in raising
the block from A to B is given by,
WAB = WAC + WCD
+ WDE + WEB
WAB = mgh1 + 0
+ mgh2 + 0
WAB = mg(h1 + h2)
WAB = mgh
This work done on the object is stored as its gravitational potential energy.
Hence, gravitational potential energy of the block at B when taken along path (b) = mgh …(vi)
From equation (i) and (vi), we
conclude that the gravitational potential energy of a body does not depend upon
the path followed by the body in going from initial position to final position.
68. At the bottom of the
waterfall, water is warmer than at the top. Give reason.
ANS: When water falls on the ground, its mechanical
energy (KE+PE) is converted into heat energy, due to which the temperature of
water at the bottom of the waterfall increases.
69. When an arrow is shot, from
where does the arrow acquire its kinetic energy?
ANS: A stretched bow has potential energy due to the
change in its shape. To shot the arrow, the bow has to be released. Therefore,
its potential energy is converted into the kinetic energy of the arrow.
70. What is meant by
transformation of energy? Explain with the help of two suitable examples.
ANS: One form of energy can be converted into other
forms of energy, this phenomenon is called transformation of energy.
(i) When we throw a ball, muscular
energy which is stored in our body, gets converted into kinetic energy of ball.
(ii) The wound spring in the toy car
possesses potential energy. As the spring is released, its potential energy
changes into kinetic energy due to which, toy car moves.
71. Does the transfer of energy
take place when you push a huge rock with all your might and fail to move it?
Where is the energy you spend going?
ANS: Yes, our muscular energy is spent in doing work against friction between the rock and the ground. Thus, our muscular energy is converted into heat energy. The heat energy is transferred to atmosphere.
72. A body is thrown vertically
upwards, its velocity goes on decreasing. What happens to its kinetic energy
when it stops at the top.
ANS: The whole of its kinetic energy gets converted into
potential energy.
73. Can any object have
mechanical energy even if its momentum is zero?
ANS: Since, mechanical energy is the sum of kinetic
energy and potential energy. And as given that, momentum of the body is zero,
it means velocity of the body is zero, so it has kinetic energy equals to zero.
But it may have potential energy. So, even if the momentum of the body is zero,
it may have mechanical energy.
74. Can any object have
momentum even if its mechanical energy is zero?
ANS: Since, mechanical energy = kinetic energy +
potential energy
If mechanical energy = 0
Potential energy = - kinetic energy
So, we can say that body may have
momentum in case mechanical energy is zero.
75. The head of a nail becomes
warm when it is hammered into a plank of wood. Explain stating the series of
energy transformation taking place.
76. Why does a driver speed up
his vehicle when he moves up a hill?
ANS: As the vehicle moves up the hill, its kinetic
energy gradually changes into potential energy, so to move it up further, it
needs more of kinetic energy which is obtained by increasing its speed.
77. Give two examples in which
a body possesses both K.E. and P.E. at the same time.
ANS: (i) An aeroplane flying at a certain height has
both K.E. and P.E.
(ii) A body falling freely from a
certain height possesses both K.E and P.E.
78. A battery lights up a bulb.
Describe the energy change involved in the process.
ANS: The chemical energy of the battery changes into
electrical energy which is then converted into light energy and heat energy.
79. When a ball is thrown upwards, its velocity goes on decreasing. What happens to its potential energy as its velocity becomes zero?
ANS: Potential energy of the ball goes on increasing when its kinetic energy decreases. Potential energy becomes maximum and equal to initial kinetic energy when the velocity of the ball becomes zero.
80. The potential energy of a
freely falling object decreases progressively. Does this violate the law of
conservation of energy? Why?
ANS: No. As the potential energy of a freely falling
object decreases, the object acquires more and more speed. So, the kinetic
energy of the object increases at the cost of its potential energy. However,
total energy of the object remains the same at every point.
81. A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?
ANS: A freely falling object just before hitting the ground has maximum kinetic energy. After falling it rolls on the rough ground and finally comes to rest. The kinetic energy of the object is used up in doing work against friction; which finally appears as heat energy.82. What are the various energy
transformations that occur when you are riding a bicycle?
ANS: Our muscular energy is converted into the kinetic energy of the bicycle. A part of the muscular energy is used in doing work against friction of the road. This part of the muscular energy changes into heat.
83. When we switch on the
electric bulbs, one form of energy is converted into another form. Name these
forms of energy.
ANS: Electrical energy is converted into light and heat energy.
84. Write the energy
transformation that takes place at thermal power station.
ANS: The Chemical energy stored inside the coal converts into heat energy on combustion. This heat converts water into steam. The heat energy of the steam is converted into mechanical energy when it turns blades of a turbine. The mechanical energy so obtained is converted into electrical energy by the generators.
85. State the transformation of
energy taking place when green plants prepare their food.
ANS: The green leaves convert the solar energy into
chemical energy (stored as food).
86. State the transformation of
energy taking place when head of a nail is hammered hard and it becomes hot.
ANS: Kinetic energy of the hammer is converted into the
heat energy.
87. Our hands become warm when
rubbed against each other. Explain.
88. State the law of conservation
of energy.
ANS: When one form of energy is changed or transformed into
other forms of energy, the total energy of an isolated system remains the same
i.e.,
The total energy before
transformation = the sum of the different energies transformed
Thus, the law of conservation of
energy states that, “energy can neither be created nor be destroyed, but can be
changed from one form to another form.”
89. Prove that – “mechanical
energy of a freely falling body is conserved.”
ANS: Consider a body of mass ‘m’ at a height h above
the ground. Suppose this position of the body is A (reference level). Suppose
the body is at rest at A i.e. v = 0
At position A
Potential energy of the body, P.E. =
mgh
Kinetic energy of the body, K.E. = 0
Therefore, Total mechanical energy of
the body at A = P.E. + K.E. = mgh + 0 = mgh
Let the body now falls freely under
the action of gravity to position B through a height x.
The height of the body from the
ground = (h-x)
The velocity of the ball after time t
can be found by using equation of motion.
u = 0, a =g, s = x, v = vB
Using, vB2 – u2
= 2as
vB2 – u2
= 2gx
vB2 = 2gx
At position B
Potential energy of the body, P.E. =
mg (h-x) {as the height of B
from ground= (h-x)}
Kinetic energy of the body, K.E. = ½ m
vB2
= ½ m. 2gx
= mgx
Therefore, Total mechanical energy of
the body at B = P.E. + K.E. = mg(h-x) + mgx = mgh
Finally let the body touches the
ground at C, so that the distance through which it falls = h
The velocity of the ball just before
touching the ground can be found by using equation of motion.
u = 0, a =g, s = h, v = vc
Using, vc2 – u2
= 2as
vc2 – u2
= 2gh
vc2 = 2gh
At position C
Potential energy of the body, P.E. =
mg (0) = 0
Kinetic energy of the body, K.E. = ½ m
vc2
= ½ m. 2gh
= mgh
Therefore, Total mechanical energy of
the body at B = P.E. + K.E. = 0 + mgh = mgh
Hence, it is proved that the total
mechanical energy of a body at any instant during free fall remains constant.
90. Name some man-made devices
which convert one form of energy into another form.
ANS:
|
SL. NO |
DEVICE NAME |
INPUT ENERGY |
OUTPUT ENERGY |
|
1 |
Fan |
Electrical energy |
Kinetic energy |
|
2 |
Electric lamp |
Electrical energy |
Light energy |
|
3 |
Electrical heaters |
Electrical energy |
Heat energy |
|
4 |
Radio |
Electrical energy |
Sound energy |
|
5 |
Water pump |
Electrical energy |
Kinetic energy of impeller to
potential energy of water |
|
6 |
Cell |
Chemical energy |
Electrical energy |
|
7 |
Microphone |
Sound energy |
Electrical energy to again sound
energy |
|
8 |
Rechargeable cell (During discharging) |
Chemical energy |
Electrical energy |
|
9 |
Rechargeable cell (During charging) |
Electrical energy |
Chemical energy |
|
10 |
Loudspeaker |
Electrical energy |
Sound energy |
|
11 |
Elevator moving up |
Electrical energy |
Potential energy |
|
12 |
Television |
Electrical energy |
Sound energy, Light energy |
|
13 |
Thermal power plant |
Chemical energy of coal |
Electrical energy |
|
14 |
Car |
Chemical energy of petrol/diesel |
Mechanical energy |
|
15 |
Nuclear power plant |
Nuclear energy |
Electrical energy |
|
16 |
Solar cell |
Solar energy |
Electrical energy |
|
17 |
Watch |
Potential energy of wound spring |
K.E of hands of a watch |
|
18 |
Generator |
Mechanical energy |
Electrical energy |
90. State the physical quantity
which will be affected by changing the rate of doing work.
ANS: Power
92. Define power.
93. A boy and a girl do the
same work in 6 minutes and 9 minutes respectively. Which of these two has more
power and why?
ANS: As,
So, as the boy takes less time to do
the same work as compared to the girl, so power of the boy will be more than
that of the girl.
94. Two persons A and B do same
amount of work. The person A does that work tA seconds and person B
in tB seconds. Find the ratio of power delivered by them
95. Why do we say that a lift
delivers more power in taking a man up than that delivered by the man climbing
stairs through the same height?
ANS: It is true to say that a lift delivers more power
in taking a man up than that delivered by him climbing the stairs through same
height. It is because, lift takes less time in taking the man up, so it
delivers more power.
96. Define average power.
ANS: The power of an agent may vary with time. This means that the agent may be doing work at different rates at different intervals of time. Therefore, the concept of average power is useful.
97. What is the SI unit of
power?
ANS: Watt (or) joule/second
98. Define 1 watt.
ANS: If 1 Joule of work is done by a machine or an agent
in 1 second, then its power is 1 watt.
(or) If 1 Joule of energy is consumed
by a machine in 1 second, then its power is 1 watt.
99. Name some bigger units of
power.
ANS: Some bigger units of power are, kilowatt, megawatt
and gigawatt.
1 kilowatt (KW) = 103 watt
1 megawatt (MW) = 106 watt
1 gigawatt (GW) = 109 watt
100. Name the practical unit of power.
ANS: horse power (h.p.)
101. How the SI unit of power
and the practical unit of power are related?
ANS: 1 horse power (h.p.) = 746 W
102. Express Kilowatt in terms
of joules per second.
ANS: 1 Kilowatt = 1000 W
Since, 1 W = 1 Js-1
Therefore, 1 Kilowatt = 1000 J s-1
103. Deduce the relationship
between power, force and velocity.
104. Derive some other units of
power.
ANS: As, P = F.V
So, unit of power = N.m/s
105. Name the commercial unit
of electric energy.
ANS: Commercial unit of energy is kilowatt-hour (kWh) or
Board of Trade (B.O.T.) Unit.
106. Define one kilowatt-hour.
ANS: One kilowatt-hour is defined as the electric energy
consumed by an appliance of power 1000 watt in one hour.
107. Establish a relationship
between SI unit of energy (Joule) and commercial unit of energy (kWh).
ANS: 1 kilowatt-hour = 1000 Watt hour
= 1000
J/s x 3600 s
=
3600000 J
Therefore, 1 kilowatt-hour = 3.6 x 106
J
108. Differentiate between
kilowatt and kilowatt hour.
ANS: Kilowatt is the unit of power. 1 kilowatt = 1000 JS-1
Kilowatt-hour is the unit of electric
energy. 1 Kilowatt-hour = 3.6 x 106 J
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