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Q 1. What kind of energy is
possessed by the following? (a) A stone kept on the roof top. (b) Water stored
in the reservoir of a dam. (c) A stretched rubber band.
Solution
(a) Potential energy (b)
Potential energy (c) Potential energy
Q 2. Explain how wind mills
are used to generate electricity?
Solution
In a wind mill, kinetic
energy of wind is used to move the blades of fan which in turn move the coil of
generator thereby converting kinetic energy to electrical energy. Thus, in the
above example there is only transformation of energy from one form to another
such that the total energy of the system remains conserved.
Q 3. Explain the principle
behind the wind up toys.
Solution
Wind up toys have a
spring inside it which gets wound up with the help of winding keys. So when we
wind-up the spring of a toy car by using a winding key, then some work is done
by us. As a result, the spring gets coiled up more tightly. The work done in
winding the spring gets stored in the wound up spring in the form of elastic
potential energy (i.e potential energy due to change in shape). When we release
the winding key the spring tries to attain its original shape. During this
process, the potential energy stored in it gets converted to kinetic energy.
This turns the wheels of the toy car.
Q 4. Write the form of
energy possessed by the body in the following situations: (a) a coconut falling
from tree (b) an object raised to a certain height (c) blowing wind (d) a child
driving a bicycle on road
Solution
(a) Kinetic Energy and
potential energy (b) Potential Energy (c) Kinetic Energy (d) Kinetic Energy
Q 5. An engine pumps 40 kg
of water per second. If water comes out with a velocity of 3m/s, what is the
power of the engine?
Solution
Q 6. A porter lifts a
luggage of 15 kg from the ground and puts it on the head 1.7 m above the
ground. Find the work done by the porter on the luggage. Take g = 10 m/s2.
Solution
Work done by the porter
on the luggage = Potential energy of the luggage W = m x g x h
= 15 x 10 x 1.7 = 255 J
Q 7. An electric bulb is
rated 15 watts. What does it mean? What is the energy consumed in joules if it
is used for 10 minutes?
Solution
If power of an electric
bulb is 15 W, it consumes 15 joules of energy per second. Energy consumed by
the bulb in 10 minutes = 15W x 600s = 9000 joules
Q 8. A ball is dropped from
a height of 10m. If the energy of the ball reduces by 40% after striking the
ground, how much high can the ball bounce back? (g = 10ms-2)
Solution
The potential energy at
height of 10 m = mgh = m x 10 x 10 = (100m) J On striking the ground, the
potential energy gets converted into kinetic energy. Kinetic energy = (100m) J
Energy is reduced by 40%, so the remaining energy = 60% x 100m = (60m) J The
height upto which the ball can bounce back is given by h', such that mgh' = 60m
m x 10 x h' = 60 x m h' = 60/10 = 6 m
Q 9. A man whose mass is 70
kg climbs from top of a hill station 'A' to another hill station 'B'. The
height of hill station A is 1200 m above the sea level and that of B is 1800 m
above the sea level. Calculate the work done in going from A to B (g=10m/s2)
Solution
PE at A = mgh1 =
70 x 10 x 1200 = 84x104 J PE at B = mgh2 = 70 x
10 x 1800 = 126x104 J Work done = 126x104 -
84x104 = 42x104 J
Q 10. Is it possible that
an object is in the state of accelerated motion due to an external force acting
on it but no work is being done by the force? Explain with an example.
Solution
Yes, it is possible. A
body in uniform circular motion is an example of accelerated motion. Consider
the motion of the Earth around the Sun. The Earth is constantly moving in a
circular path in a direction perpendicular to the gravitational pull of the Sun.
So, the work done by the gravitational force is zero. Thus, the work done can
be zero for an accelerated body.
Q 11. Give examples of
atmospheric phenomena that have large kinetic energy?
Solution
Hurricanes, tornadoes and
jet stream are some of the atmospheric phenomena having large kinetic energy on
account of their high speeds.
Q 12. (a) "An arrow
and the stretched string on the bow is said to possess energy". Comment.
(b) A bag of wheat weighs 50 kg. Calculate the height to which it should be
raised so that its potential energy is 5000J.
Solution
(a) (i) The stretched
string possesses potential energy. When the bow string is released, arrow flies
off the bow. The potential energy stored in the bow is used in the form of
kinetic energy of the arrow. (b) P.E = mgh 5000 = 50 x 10 x h h = 5000/500 = 10
m
Q 13. When is work said to
be done against the force of gravity? State and define SI unit of work.
Solution
When the body is raised
to a height, the work is said to be done against the force of gravity. S.I.
Unit of work is joule. One joule of work is said to be done when 1N force moves
a body through a distance of 1m.
Q 14. What is the work done
by frictional force on an object when dragged along a rough surface?
Solution
Frictional force is
always opposing the relative motion of the body. When a body is dragged along a
rough surface, the frictional force will be acting in the direction opposite to
the displacement. The angle between the frictional force and the displacement
of the body will be 180o. Thus, the work done by the frictional
force will be negative.
Q 15. If the velocity of an
object is doubled, what is the change in its kinetic energy?
Solution
Kinetic energy is
proportional to the square of velocity. Hence, kinetic energy will become 4
times when velocity is doubled.
Q 16. When an object moves
on a circular path, what is the work done?
Solution
Work done is zero,
because displacement (along tangent) is always perpendicular to the direction
of force (along centre).
Q 17. State the energy
transformation in the following : (i) heat engine (ii) electric motor
Solution
(i) In a heat engine heat
energy gets converted into mechanical energy (ii) In an electric motor
electrical energy gets converted into mechanical energy
Q 18. What kind of energy
does a flying bird or a flying airplane has during their course of action?
Solution
A flying bird or a flying
airplane has kinetic energy as well as potential energy during their course of
action. The flying bird and airplane have kinetic energy because they are
moving. They have potential energy at any point of time in their flight due to
their height above ground at that particular point of time. Thus, a body can
have both potential and kinetic energy at the same time.
Q 19. When we burst a
cracker, is it correct to say that the chemical energy of crackers get used up
or are destroyed as we are left with just ashes after bursting cracker?
Solution
Its wrong to say that
energy gets used up or destroyed. According to law of conservation of energy,
energy can neither be created nor destroyed it just converts from one form to
other. On bursting a cracker the chemical energy of crackers transform into heat
and light energy.
Q 20. Given below are a few
situations. Study them and state in which of the cases, work is said to be
done. Give reason for your answer. (i) A person pushing a huge rock but the
rock does not move. (ii) A bullock pulling a cart upto 1 km on road. (iii) A
girl pulling a trolley for about 2 m distance. (iv) A person standing with a
heavy bag on his head.
Solution
(i) No work is done, as
there is zero displacement (ii) Yes, the work is said to be done as the cart
moves through a distance. (iii) Yes, the work is said to be done as the trolley
moves through a distance. (iv) No work is done, as there is no displacement of
the person or the bag.
Q 21. Two masses m and 2m
are dropped from heights h and 2h. On reaching ground, which will have
a greater kinetic energy and why?
Solution
Body with mass 2m will
have a greater kinetic energy, because it has greater height and mass, and
hence greater potential energy. This potential energy is converted to kinetic
energy as the mass falls down. Hence, mass 2m has greater kinetic energy.
Q 22. A mass m is dropped
from a height h. At half way to the ground, what is the energy?
Solution
At maximum height h,
potential energy is maximum and kinetic energy is zero. PE = mgh Now, according
to conservation of energy, the energy at all the points is same along the line
of motion. Thus, at midway also, the total energy is mgh.
Q 23. Which formula do we
use to calculate the work done when a body is moved against gravity ?
Solution
When the work is done
against gravity, then the amount of work done is equal to the weight of the
body times the vertical distance through which it is lifted. Work done = Weight
of body x vertical distance against gravity W = mg x h= mgh
Where, W - Work m - Mass g - acceleration due to gravity h - vertical distance
Q 24. An object of mass 20
kg is dropped from a height ‘h’ metres as shown in the tables below. (g = 10
m/s2) Table A Height from which an object is dropped ‘h’ (m)
Velocity of an object falling from the height ‘v2’ (m/s2)
10 0 2 80 Table B Height from which an object is dropped ‘h’ (m) Velocity of an
object falling from the height ‘v2’ (m/s2) 10 0 2 40
Which of the above tables has wrong data in terms of the law of
conservation of energy of an object falling from height ‘h’ metres?
Solution
Table A: Height
from which an object is dropped ‘h’ (m) Velocity of an object falling from the
height ‘v2’ (m/s2) Potential energy mgh (J) Kinetic
energy ½mv2 (J) TE = PE + KE 10 0 20 × 10 ×10
= 2000 ½ × 20 × 0 = 0 2000 2 80 20 × 10 ×2 = 400 ½ × 20 × 80
= 800 1200 Table B: Height from which an object is dropped ‘h’ (m)
Velocity of an object falling from the height ‘v2’ (m/s2)
Potential energy mgh (J) Kinetic energy ½mv2 (J) TE = PE + KE
10 0 20 × 10 × 10 = 2000 ½ × 20 × 0 = 0 2000 2 40 20 × 10 × 2 = 400 ½ × 20 × 40
= 400 800 According to the law of conservation of energy, the sum of the
potential energy and kinetic energy of the object should remain the same at
every point during its fall.
Q 25. Define the SI unit of
Work.
Solution
The SI unit of work is
Joule (J) Work = Force x Distance 1 Joule = 1 newton x 1 metre
Therefore, we define 1 Joule as the amount of work done when a force of one
newton moves a body through a distance of 1 metre in the direction of force
applied.
Q 26. A microwave uses 450
KJ of electric energy in 5 minutes. Calculate its power rating.
Solution
Electric energy consumed,
E = 450 KJ
= 450 x 1000 J = 450000 J Time taken, t = 5 minutes = 5 x 60 sec = 300 sec
Power, P = E/t
= 450000/300 = 1500 watts
Q 27. Two bodies of same
mass start from rest and move with velocities of v and 2v respectively. Find
the ratio of their kinetic energies.
Solution
For body1: K.E.1 =
½ mv2 For body 2: K.E.2 = ½ m(2v)2 K.E.2 =
4 (½ mv2) Thus, we have K.E.1: K.E.2 =
1:4
Q 28. State the factors on
which work done depends.
Solution
Work done by a force
depends on: 1. Magnitude of the force applied. 2. The distance through which
the body moves in the direction of force.
Q 29. Derive an expression
for the work done when a force is acting on an object in the direction of its
displacement. A pair of bullocks exerts a force of 150 N on a plough. The field
being ploughed is 15 m long. How much work is done in ploughing the length of
the field?
Solution
When a force F displaces
a body through a distance s in the direction of the applied force, then the
work done W on the body is given by the expression:Work done = Force ×
DisplacementW = F × s Work done by the bullocks is given by
the expression:Work done = Force × DisplacementW = F × sWhere,Applied
force, F = 150 NDisplacement, s = 15 mW = 150
× 15 = 2250 J Hence, 2250 J of work is done in ploughing
the length of the field.
Q 30. Home electricity
bills come in units called Kilowatt-hour. How many joules is equal to 1
kilowatt hour. Why kilowatt hours are used instead of joules in the bill?
Solution
1 Kilowatt hour = 36,
00,000 joules = 3.6 x 106 J One kilowatt hour is the amount of
electrical energy consumed when an electrical appliance having a power rating
of 1 kilowatt is used for 1 hour. Kilowatt hour is used for measuring
electricity consumed, as joule represents a very small quantity of energy and
therefore it is inconvenient to use joules where large quantities of energy is
involved.
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