# Class 11 Physics Chapter 6 Important Questions Work, Power and Energy

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 produc

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.

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.

Q.18. Give one example each of conservative and nonconservative 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 nonconservative in nature.