Energy

Objective Questions

Question 1

Write true or false for each statement:

(a) A man going up has potential energy and kinetic energy both.

(b) A gum bottle lying on a table has no energy.

(c) In an electric fan, electrical energy changes into mechanical energy.

(d) Potential energy changes into kinetic energy when it is put to use.

(e) One form of energy cannot be converted into another form.

(f) There is always some loss of energy in conversion from one form of energy to another form, so the total energy is not conserved.

(g) The energy of flowing water can be converted into electric energy (electricity).

Answer

(a) True

(b) False
Correct Statement — A gum bottle lying on a table has potential energy.

(c) True

(d) True

(e) False
Correct Statement — One form of energy can be converted into another form.

(f) False
Correct Statement — The total energy is always conserved in each transformation of energy.

(g) True

Question 2

Fill in the blanks:

(a) An electric fan converts electrical energy into .................... energy.

(b) Cooking gas converts ..................... energy into heat energy.

(c) Energy possessed by a compressed spring is ............... energy.

(d) The ability to do work is called ..................... .

(e) The energy possessed by a body due to its position is called ..................... energy.

(f) The energy possessed by a body due to its motion is called .................... energy.

(g) Green plants convert ...................... energy into chemical energy.

(h) The S.I. unit of energy is ........................... .

(i) An object falling freely from the roof of a multistorey building has ..................... and .................... energy when halfway down the building.

Answer

(a) mechanical

(b) chemical

(c) potential

(d) energy

(e) potential

(f) kinetic

(g) light

(h) joule

(i) potential energy, kinetic

Question 3

Match the following:

Answer

Question 4a

Select the correct alternative:

When we rub our hands

1. kinetic energy changes into potential energy
2. mechanical energy changes into heat energy
3. potential energy changes into kinetic energy
4. heat energy changes into mechanical energy

Answer

mechanical energy changes into heat energy

Reason — When we rub our hands we feel heat i.e. mechanical energy changes into heat energy.

Question 4b

Select the correct alternative:

A ball rolling on the ground possess

1. kinetic energy
2. potential energy
3. no energy
4. heat energy

Answer

kinetic energy

Reason — A body in motion (rolling ball) possess kinetic energy.

Question 4c

Select the correct alternative:

The energy stored in an electric cell is

1. chemical energy
2. electrical energy
3. heat energy
4. mechanical energy

Answer

chemical energy

Reason — The energy stored in an electric cell is chemical energy.

Question 4d

Select the correct alternative:

When a bulb lights up on passing current, the change of energy is

1. from electrical energy to heat energy
2. from electrical energy to light energy
3. from electrical energy to heat and light energy
4. from electrical energy to mechanical energy

Answer

from electrical energy to heat and light energy

Reason — When a bulb lights up, electrical energy changes into heat and light energy.

Question 4e

Select the correct alternative:

The correct statement is

1. Both work and energy have the same units
2. Potential energy of a body is due to its motion
3. Kinetic energy of a body is due to its position or state
4. Kinetic energy can change into potential energy, but potential energy cannot change into kinetic energy

Answer

Both work and energy have the same units

Reason — The S.I. units of both work and energy is joule(J).

Question 4f

Select the correct alternative:

According to law of conservation of energy, energy changes from one form to another form, but the total energy of that system

1. increases
2. decreases
3. alternates
4. remains the same

Answer

remains the same

Reason — According to law of conservation of energy, energy changes from one form to another form, but the total energy of that system remains constant.

Short/Long Answer Questions

Question 1

Define the term energy.

Answer

Energy of a body is defined as its capacity to do work.

Question 2

State the unit of energy and define it?

Answer

The S.I. unit of energy is joule(J).
A body is said to possess an energy of one joule if one joule work is done to bring the body in that state i.e. a force of 1 newton moves the body by a distance of 1 metre in the direction of force.

Question 3

Name five different forms of energy.

Answer

The five different forms of energy are:

1. Mechanical energy
2. Light energy
3. Heat energy
4. Magnetic energy
5. Electrical energy

Question 4

What are the two kinds of mechanical energy?

Answer

The two kinds of mechanical energy are:

1. Potential energy
2. Kinetic energy

Question 5

What is potential energy? State its unit.

Answer

Potential energy of a body is the energy possessed by it due to its state of rest or position. It is equal to the work done in bringing the body to that state of rest or position. Its unit is joule(J).

Question 6

Give one example of a body that has potential energy, in each of the following:
(i) due to its position

(ii) due to its state

Answer

(i) A hammer at a height has potential energy stored in it because of its position at a height.

(ii) A wound up watch spring has potential energy because of the wound up state of its coils.

Question 7

State two factors on which the potential energy of a body at a certain height above the ground depends.

Answer

The two factors on which the potential energy of a body at a certain height above the ground depends are:

1. Mass of the body — Greater the mass of the body, greater is the potential energy of the body.

2. Its height above the ground — Higher the height of the body above the ground, more is the potential energy.

Question 8

Two bodies A and B of masses 10 kg and 20 kg respectively are at the same height above the ground. Which of the two has greater potential energy?

Answer

Body B has greater potential energy.

Reason — Both bodies, A and B are at the same height above the ground. Value of g is also same for both. As the mass of body B (20 kg) is greater than that of body A (10 kg) hence potential energy of body B is greater.

Question 9

A bucket full of water is on the first floor of your house and another identical bucket with same quantity of water is kept on the second floor. Which of the two has greater potential energy?

Answer

The bucket kept on second floor has greater potential energy.

Reason — The two buckets are identical containing the same amount of water so their mass is same. Value of g is also same for the two buckets. As the bucket on second floor is at a greater height hence its potential energy is also greater.

Question 10

Define the term kinetic energy. Give one example of a body which possesses kinetic energy.

Answer

Kinetic energy of a body is the energy possessed by it due to its state of motion. Example: A fast moving stone has kinetic energy which has the capacity of breaking a window pane when it strikes on it.

Question 11

State two factors on which the kinetic energy of a moving body depends.

Answer

The two factors on which the kinetic energy of a moving body depends are:

1. The mass of the body — Greater the mass of the body, higher is its kinetic energy.
2. The speed of the body — More the speed of the body, higher is its kinetic energy.

Question 12

Two toy cars A and B of masses 500 g and 200 g respectively are moving with the same speed. Which of the two has greater kinetic energy?

Answer

The toy car A has greater kinetic energy than toy car B.

Reason — The two toy cars A and B are moving with the same speed. As the toy car A has greater mass (500 g) than toy car B (200 g) hence its kinetic energy is also greater.

Question 13

A cyclist doubles his speed. How will his kinetic energy change: increases, decreases or remains the same?

Answer

Kinetic energy of cyclist will increase as speed is doubled because more the speed of the body, more is its kinetic energy.

Question 14

Name the form of energy which a wound up watch spring possesses.

Answer

Potential Energy.

Question 15

Can a body possess energy even when it is not in motion? Explain your answer with an example.

Answer

Yes, a body can possess energy even when it is not in motion.

Example — A stone at rest placed at a height above the ground has potential energy. Even though the body is not in motion (kinetic energy = 0), but when a body is raised to a height say h possess potential energy because of its raised position.

Question 16

Name the type of energy (kinetic or potential) possessed by the following:

(i) A moving cricket ball.

(ii) A stone at rest on the top of a building.

(iii) A compressed spring.

(iv) A moving bus.

(v) A bullet fired from a gun.

(vi) Water flowing in a river.

(vii) A stretched rubber band.

Answer

(i) A moving cricket ball has kinetic energy due to its state of motion.

(ii) A stone at rest on the top of a building has potential energy due to its raised position.

(iii) A compressed spring has potential energy due to its compressed state.

(iv) A moving bus has kinetic energy due to its state of motion.

(v) A bullet fired from a gun has kinetic energy due to its state of motion.

(vi) Water flowing in a river has kinetic energy due to its state of motion.

(vii) A stretched rubber band has potential energy because of its stretched position.

Question 17

Give one example to show the conversion of potential energy to kinetic energy when put in use.

Answer

A stone placed at a top of building has potential energy due to its height and when it is dropped from the top of the building it begins to fall. When it begins to fall the potential energy stored in it changes into kinetic energy. So the falling stone has kinetic energy.

Question 18

State the energy changes that occur in the following:

(i) The unwinding of a watch spring.

(ii) Burning coal while operating a steam engine

(iii) Lighting of a torch bulb.

(iv) An electric generator (or dynamo).

Answer

(i) During the unwinding of a watch spring, the potential energy changes into kinetic energy.

(ii) In a steam engine chemical energy of coal first changes into heat energy which is transferred to steam. Then the heat energy of steam changes into mechanical energy which causes the train to move.

(iii) When torch bulb is lighted, chemical energy of batteries change into light and heat energy.

(iv) In an electric generator (dynamo) mechanical energy changes into electrical energy.

Question 19

Energy can exist in several forms and may change from one form to another. Give two examples to show the conversion of energy from one form to another.

Answer

The two examples that show the conversion of energy from one form to another are:

1. In an electric fan, the electrical energy changes into mechanical energy that rotates the blades of a fan.

2. In a glowing bulb, the electrical energy changes into heat energy and light energy.

Question 20

Give one relevant example for each of the following transformations of energy:

(i) Electrical energy to heat energy

(ii) Electrical energy to mechanical energy

(iii) Electrical energy to light energy

(iv) Chemical energy to heat energy

(v) Chemical energy to light energy

Answer

(i) Electric iron

(ii) Electric motor

(iii) Glowing bulb

(iv) Burning of wood

(v) Bursting of crackers

Question 21

What do you mean by conservation of mechanical energy? State the condition when it holds.

Answer

Whenever there is an interchange between the potential and kinetic energy, the total mechanical energy remains constant. This is the principle of conservation of mechanical energy. The condition when it holds is that there should be no frictional forces.

Question 22

Give one example to show that the sum of potential energy and kinetic energy remains constant if friction is ignored.

Answer

During the vertical fall of a ball downwards from a height, the sum of potential energy and kinetic energy remains constant at each point of its path, if friction is ignored.

Question 23

A ball is made to fall freely from a height. State the kind/kinds of energy possessed by the ball when it is:

(a) at the highest point

(b) just in the middle

(c) at the ground.

Answer

(a) Potential energy

(b) Potential energy and kinetic energy

(c) Kinetic energy

Question 24

State the changes in form of energy while producing hydro-electricity.

Answer

Energy changes while producing hydro-electricity are:

Potential energy (Water stored in dam) ⟶ Kinetic energy (Water from dam falls on turbine) ⟶ kinetic energy (of blades of turbine) ⟶ Electrical energy (Rotating turbine rotates the generator)

Question 25

A truck starts from rest on a plane road. What are the possible energy changes taking place while the truck is in motion?

Answer

The possible energy changes taking place while the truck is in motion are:

1. Burning of diesel causes the truck to run so chemical energy changes into kinetic energy.
2. When the truck runs on the road, due to friction kinetic energy changes to heat and sound energy.