Class 11 Physics Chapter 6 Important Questions Work, Power and Energy
It is important for the students that all the concepts should be very clear for better marks in future. Here, we are providing important conceptual questions and answers for class 11 physics chapter 6 Work, Power and Energy. In this lesson, students will learn about Work, Power and Energy. This will not only help the students to know the important questions but will also help them during revision.
Q.1. What is the condition for two vectors to be
perpendicular to each other?
Ans. The two vectors are perpendicular to each other
if their dot product is zero.
Q.2. What is the work done by a person in holding a
15 kg suitcase, while waiting for a bus for 15 min.
Ans. It is zero. It is because the displacement is zero.
Q.3. What should be the angle between force and
displacement so that no work is done?
Ans. The angle between force and displacement so
that no work is done should be 90°.
Q.4. What is the amount of work done by a mass
pressing a rigid wall with a force of 200 N.
Ans. The amount of work done by the man is zero. It
is because, the displacement is zero.
Q.5. No work is done against gravity, while moving a
body along horizontal. Why?
Ans. It is because the gravity and the displacement
are perpendicular to each other. Therefore,
W = F S cos 90° = 0
Q.6. A man rowing a boat upstream is at rest with respect to the bank. Is he doing work?
Ans. The man applies force for rowing the boat upstream. Since he is at rest with respect to the bank, the displacement is zero. Likewise the work done is also zero.
Q.7. A man carrying a bucket of water walks on a horizontal road with uniform velocity. What is the work done by him?
Ans. Zero. It is because the displacement and the force required to hold the bucket are perpendicular to each other.
Q.8. What is the amount of work done by a force,
when a body moves in a circular path?
Ans. Zero. It is because, the centripetal force needed
to revolve the body is always perpendicular to the
circular path.
Read also: Work, Energy and Power Class 11 Physics Notes Chapter 6
Q.9. What is the work done by Earth's gravitational
force in keeping the moon in its orbit for its one
revolution?
Ans. Zero. It is because the gravitational force
(conservative force) is always perpendicular to the
displacement.
Q.10. What is the amount of work done by
(a) a weightlifter is holding a weight of hundred kg on his shoulders for 40 s ?
(b) the earth's gravitational force in keeping the moon and its orbit (assumed perfectly circular) ?
(c) a locomotive against gravity, if it is travelling on a level plane ?
(d) an electron moving with half the speed of light in
empty space free of electromagnetic field and far
from all matter.
Ans.
- (a) Zero
- (b) Zero
- (c) Zero
- (d) Zero
Q.11. Give one example each of the following:
- (a) zero work
- (b) negative work.
Ans.
- (a) Work done by a man carrying load on his head and walking along horizontal is zero.
- (b) When a body is raised above the surface of the earth, work done by gravity is negative.
Q.12. Does the work done in moving a body depends
upon how fast the body is moved?
Ans. No. The work done in moving a body does not
depend upon how fast the body is moved?
Q.13. Give SI unit of work.
Ans. It is joule (J).
Work done is said to be one joule if a force of one
newton displaces a body through one metre in the
direction of force.
Q.14. Find number of ergs in one joule.
Ans. 1 J = 107 erg.
Q.15. What is gravitational unit of work?
Ans. In SI, gravitational unit of work is kg m.
Q.16. Find the number of joules in the gravitational
unit of work in SI.
Ans. 1kg m = 1kgf × 1m
= 9.8 N × 1m
= 9.8 J
Q.17. Define conservative force. Give its two
examples.
Ans. A force is said to be conservative, if work done
by the force over a closed path is zero.
Examples: gravitational and electrostatic force.
Read also: Class 11 Physics Chapter 6 MCQs with Answer Work, Energy and Power
Q.18. Give one example each of conservative and nonconservative force.
Ans. Gravity is a conservative force, while friction is a
non-conservative force.
Q.19. Friction is a non-conservative force. why?
Ans. Friction is a non conservative force because
work done against friction along a closed path is non-
zero.
Q.20. Can kinetic energy of an object be negative?
Ans. No.
Q.21. Give SI unit of energy.
Ans. It is joule (J).
Work done is said to be one joule, if a force of one newton displaces a body through one metre in the direction of force.
Q.22. Define one kilowatt hour.
Ans. It is the energy consumed by an appliance of
power one kilowatt in one hour.
Q.23. Give the number of joules in 1 kWh.
Ans. 1kWh = 3.6 × 10,6 J.
Q.24. how many joules are in 1 MeV.
Ans. 1MeV = 1.6 ×10-13 J.
Q.25. Give SI unit of power.
Ans. It is watt (W).
The power of an agent is said to be one watt, if one joule of work is done in one second.
Q.26. What is horse power?
Ans. It is the practical unit of power used in
engineering.
Q.27. How many watt are in 1 horse power?
Ans. 1 horse power (h.p.) = 746 W.
Q.28. What type of energy is stored in the spring of a
watch?
Ans. Potential energy is stored in the spring of a
watch.
Q.29. What is spring constant of a spring? Give its SI
unit.
Ans. The spring constant of a spring is numerically
equal to the restoring force set up in the spring per
unit extension. In SI, the unit of spring constant is
Nm-1.
Q.30. Which spring has greater value of spring
constant - a delicate or a stout spring?
Ans. A stout spring.
Q.31. A spark is produced, when two stones are struck
against each other. Why?
Ans. Work done in striking the two stones gets
converted into heat energy, which appears as spark.
Q.32. What are elastic collisions?
Ans. Those collisions, in which both momentum and
kinetic energy of the system are conserved, are called
elastic collisions.
Q.33. What are inlastic collisions?
Ans. Those collisions, in which the momentum of the
system is conserved but the kinetic energy is not
conserved, are called inelastic collision.
Q.34. After collision, two bodies stick together. What
type of collision, the two bodies are undergoing?
Ans. Perfectly inelastic collision.
Q.35. In a collision process, the law of conservation of
energy does not hold. What type of collision it is?
Ans. Inelastic collision, the law of conservation of
energy does not hold.
Q.36. State principle of conservation of energy.
Ans. Principle of conservation of energy states that
the energy can neither be created nor be destroyed
but can only be converted from one form to another.
Q.37. State work-energy theorem.
Ans. Work-energy theorem states that the work done
by a force acting on a body is equal to the change
produced in the kinetic energy of the body.
Q.38. State the conditions under which a force does
no work.
Ans. A force does no work, when
- (a) the displacement is zero or
- (b) the displacement is perpendicular to the direction of force or (c) when a body moves under the effect of a conservative force over a closed path.
Q.39. "The earth moving round the sun in a circular
orbit is acted upon by a force and hence work must
be done on the earth by the force." Do you agree with
this statement?
Ans. The gravitational force on the Earth due to the
sun is a conservative force. Since the work done by a
conservative force or a closed path is always zero
(irrespective of the nature of the path), the work done
by the gravitational force over every complete orbit
of the Earth is zero.
Q.40. Define kinetic energy. Give its units and
dimensional formula.
Ans. Kinetic energy: The energy possessed by a body
by virtue of its motion is called Kinetic energy.
Units of kinetic energy: Joule in SI and erg in cgs
system.
Dimensional formula of kinetic energy: [ML2T-2].
Q.41. "Chemical, gravitational and nuclear energies
are nothing but potential energies for different types
of force in nature." Explain this statement clearly
with examples.
Ans. A system has potential energy, when the objects
constituting the system are held at certain distance
against the force between them. Chemical energy
results from the chemical bonding between the
atoms. Gravitational energy results, when the objects
are held at some separation against the gravitational
force of attraction. Nuclear energy is the result of the
nuclear force between the nucleons.
Q.42. When is potential energy of a body said to be
positive or negative?
Ans. The potential energy of a body is said to be
positive, if they are held at some distance against the
force of repulsion. On the other hand, the potential
energy is said to be negative, if the bodies are held
against the force of attraction.
Q.43. In a thermal station, coal is used for the
generation of electricity. Mention how energy
changes from one form to another before it is
transformed into electrical energy.
Ans. The heat energy produced by burning the coal is
used to convert water into steam. The heat energy of
the steam is converted into mechanical energy, when
the steam is made to rotate the blades of the turbine.
The mechanical energy so obtained is converted by
the generators into electrical energy.
Q.44. Coal is burnt in a city electric power plant to
make steam to run a steam turbine that drives an
electric generator. The city water department uses
electric supply to run an electric motor to pump
water from a well into a stand pipe on a hill top. List,
in order, all the energy changes that take place.
Ans. When coal is burnt, the chemical energy of the
coal is changed to thermal energy of the flame. The
energy of the flame is transferred to water, which
takes the form of steam. The thermal energy of the
steam (which resides in the steam molecules in the
form of their kinetic energy) is used to impart kinetic
energy to the turbine rotor due to the collision of the
steam molecules with its vanes. The rotation of the
turbine shaft turns the generator rotor and thus
produces electrical energy.
The electric energy from the generator is converted into kinetic energy by the electric motor, which runs the pump. The kinetic energy of the pump is transferred to the water, causing it to flow up into the stand pipe on the hill top. As the water rises into the stand pipe, it acquires potential energy.
Q.45. Nuclear fission and fusion reactions are
examples of conversion of mass into energy. Can we
say that strictly speaking, mass is converted into
energy even in an exothermic chemical reaction?
Ans. Like Nuclear fission and fusion reactions, in
exothermic reaction also, mass is converted into
energy. The only difference is that mass converted
into energy in a chemical reaction is much less (about
a million times less).
Q.46. The energy released in fusion reaction of light
nuclei is much less than the energy released in a
fission reaction of a heavy nucleus why is then
hydrogen bomb (based on nuclear fusion) far more
powerful than an atomic bomb (based on nuclear
fission) ?
Ans. In a fusion reaction, light nuclei take part; while
in fission reaction, heavy nuclei undergo the reaction.
A hydrogen bomb (based on nuclear fusion) is more
powerful than an atom bomb (based on nuclear
fission), as in the former case, the energy released per
nucleon (common name for proton and neutron) or
per unit mass is much greater than that in the case of
nuclear fission.
Q.47. Distinguish between a head-on and an oblique
collision.
Ans. If the colliding objects move along the straight
line joining their centres, the collision is said to be a
head-on collision.
If the colliding objects do not move along the
straight line joining their centres, the collision is said
to be an oblique collision.
Q.48. Throwing mud on the Wall is an example of
perfectly inelastic collision. Explain.
Ans. A perfect inelastic collision is that in which there
is a total loss of kinetic energy of the system. It will
happen so, if the two colliding objects stick together
and do not move at all. Hence, throwing of mud on
the wall is an example of perfectly inelastic collision.
Q.49. Explain how fast moving neutrons can be
quickly slow down by passing through water or
heavy water?
Ans. When an incident particle in motion strikes
against a stationary target particle, the transfer of
energy is maximum, when the two have the same
mass. In water or heavy water, mass of a hydrogen
atom is almost the same as that of the neutron.
Therefore, when fast neutrons pass through water or
heavy water, they transfer most of their energy to the
hydrogen atom and are slowed down.
Q.50. What are the characteristics of elastic collisions?
Ans. Following are the characteristics of elastic
collisions:
- (a) the momentum is conserved.
- (b) the total energy is conserved.
- (c) the kinetic energy is conserved.
- (d) the mechanical energy is not converted into any other form (sound, heat, light) of energy.
- (e) Forces involved during the interaction are of conservative nature.
Q.51. What are the characteristics of inelastic
collisions?
Ans. Following are the characteristics of inelastic
collisions:
- (a) the momentum is conserved.
- (b) the total energy is conserved.
- (c) the kinetic energy is not conserved.
- (d) a part or whole of the mechanical energy may be converted into other forms (heat, light, sound) of energy.
- (e) some or all of the forces involved are nonconservative in nature.