Students can download Class 8 Science Chapter 10 Energy and its Forms Important Questions, KSEEB Class 8 Science Important Questions and Answers helps you to revise the complete Karnataka State Board Syllabus and to clear all their doubts, score well in final exams.

## Karnataka State Syllabus Class 8 Science Important Questions Chapter 10 Energy and its Forms

Question 1.

What is force?

Answer:

Force is that which can set a stationary body in motion or change the speed and direction of a moving body or. change the shape of a body.

Question 2.

When do we say that work is done?

Answer:

Work is said to be done when the point of application of force is displaced through a distance.

Question 3.

Which are the two conditions under which work is said to be done?

Answer:

Work is said to be done when the following two conditions are satisfied:

- There must be a force acting on the object under consideration.
- The object should be displaced through a distance.

If any one or both conditions are not satisfied, then, work is not done.

Question 4.

On what factors does the amount of work done by a force depend?

Answer:

The amount of work done by a force depends on

- the magnitude of the force, and
- the distance that the body moves.

Question 5.

Give a situation where no work is done even when a force is acting on an object.

Answer:

Imagine a person is pushing a huge rock. Let us say that the rock does not move despite the force exerted on it. Therefore, the object is not displaced. Thus, the displacement is zero. In this situation, work is not done by the force exerted on the rock.

Question 6.

How is work done by a force calculated?

Answer:

Work done by moving a body is equal to the product of force exerted on the body and the distance moved by the body in the direction of the force. That is, Work = Force × Distance moved in the direction of the force.

Question 7.

What is the S.I. unit of work?

Answer:

The S.I. unit of work is called ‘joule’. Its symbol is ‘J’.

Question 8.

Define one joule of work.

Answer:

When a force of one newton moves a body through a distance of one metre in its own direction, then the work done is known as one joule.

1 joule = 1 newton × 1 metre

1 J = 1 Nm

Question 9.

A constant force of 10 N acts on an object through a distance 5 m. The displacement is in the direction of the force. Calculate the work done by the force on the object.

Answer:

Force F = 10 N; Displacement d = 5 m; Work done W = ?

Work done = Force × displacement

W = F × d

= 10 N × 5m

= 50 N m

= 50 J.

Question 10.

A constant force of 100 N does 750 J of work on an object. Find the displacement of the object in the direction of the force.

Answer:

Force F = 100 N; Work done W = 750 J; Displacements d = ?

Work done = Force × displacement

\(d=\frac{W}{F}=\frac{750 \mathrm{J}}{100 \mathrm{N}}=7.5 \mathrm{m}\)

Question 11.

Define energy.

Answer:

The measure of the capacity to do work is called energy.

Question 12.

What is the role of force in doing work?

Answer:

While work is being done, the force transfers energy from one object to another.

Question 13.

What is the S.l. unit of energy?

Answer:

The S.l. unit of energy is ‘joule’ (J). The S.l. unit of energy is same as that of work.

Question 14.

Define 1 joule of energy.

Answer:

One joule is the energy required to do 1 joule of work.

Question 15.

List the various forms of energy.

Answer:

Energy exists in several forms. The various forms of energy are electrical energy, heat energy, mechanical energy, chemical energy, sound energy, etc.

Question 16.

How are energy forms classified?

Answer:

All forms of energy are divided into two namely

- potential energy
- kinetic energy.

Question 17.

What is the basis for classifying energy forms into potential energy and kinetic energy?

Answer:

An agent or a system possesses energy either by virtue of its position or motion. Potential energy is used to describe energy of position. Energy of motion is commonly called kinetic energy.

Question 18.

What is potential energy?

Answer:

The energy possessed by an object by virtue of its position, shape or configuration is called potential energy.

Question 19.

Define kinetic energy.

Answer:

The energy possessed by an object due to its motion is called kinetic energy.

Question 20.

Give examples for various forms of potential energy.

Answer:

Potential energy could be in several forms which include mechanical energy, chemical energy and nuclear energy.

Question 21.

Distinguish between potential energy and kinetic energy of a body.

Answer:

Potential energy:

- Energy possessed by a body by virtue of its position or configuration
- It is the stored up energy.

Kinetic energy:

- Energy possessed by a body due to its motion.
- This is not stored up but present only as long as the body is in motion.

Question 22.

Give examples for various forms of kinetic energy.

Answer:

Anything in motion has kinetic energy. The forms of kinetic energy include heat energy, mechanical energy and electrical energy.

Question 23.

Classify the energy possessed by the following into potential energy and kinetic energy

- stretched rubber band
- flowing water
- compressed spring
- water stored in a dam
- wind
- arrow released from the bow or moving arrow
- stretched bow
- a stone at the top of a building
- bird resting on the branch of a tree.

Answer:

- Stretched rubber band: potential energy
- Flowing water: kinetic energy
- Compressed spring: potential energy
- Water stored in a dam: potential energy
- Wind: kinetic energy
- Arrow released from the bow or moving arrow: kinetic energy
- Stretched bow: potential energy
- A stone at the top of a building: potential energy
- Bird resting on the branch of a tree: potential energy.

Question 24.

In the table given below, some activities are listed. Write down your observation in the activity. Also write your inference.

Answer:

Question 25.

On what factors does the kinetic energy of an object depend?

Answer:

- On the mass of the body and
- The velocity with which it is moving.

Question 26.

Describe an activity to show that the kinetic energy of an object depends on its speed

Answer:

Make an inclined plane with a wooden plank. Place it on a smooth surface. Keep a light plastic cube at a fixed distance on the surface at the bottom of the inclined plane as shown in the figure. Mark four positions A, B, C and D on the inclined plane at different heights. Take a heavy cylinder and release it from the position A of the inclined plane.

The cylinder hits the cube and displaces it. Measure the displacement. Bring the plastic cube back to its original position. Repeat the activity by releasing the cylinder from points B, C and D. Measure the displacement in each case. The cylinder released from A will cause the highest displacement of the cube.

When the same is released from point D, the displacement of the cube is least. This is because the cylinder, when released from A, has the highest speed and hence highest kinetic energy when it hits the cube. The displacement decreases in that order when the cylinder is released from B, C and D. This shows that the kinetic energy of an object depends on the speed of the object.

Question 27.

Describe an activity to show that the kinetic energy of a moving object depends on its mass.

Answer:

Make an inclined plane with a wooden plank. Place it on a smooth surface. Keep a light plastic cube at a fixed distance on the surface at the bottom of the inclined plane as shown in the figure. Mark a point A on the inclined plane. Take three cylinders of the same size having mass 50 g, 100 g and 200 g.

Place the 50 g cylinder at A on the inclined plane and release it. The cylinder hits the cube and displaces it. Measure the displacement. Bring the plastic cube back to its original position. Repeat the activity with cylinders of mass 100 g and 200 g starting from the same point A. Measure the displacement of the cube in each case.

Compare the displacement caused by the three different cylinders. It is found that the cylinder with the highest mass causes the highest displacement. The cylinder with the least mass causes the least displacement. This shows that the kinetic energy of an object depends on its mass.

Question 28.

Give the equation for the kinetic energy of an object.

Answer:

The kinetic energy of an object is directly proportional to the product of its mass and square of its velocity. Consider an object of mass m moving with a velocity v. Let E be the kinetic energy of the object.

then, \(E=\frac{1}{2} m v^{2}\)

Question 29.

An object has 20 J of energy. How much work can it do?

Answer:

An object that possesses 20 J of energy can do 20 J of work.

Question 30.

A bus and a car are moving at the same speed. Which of the two has a higher kinetic energy?

Answer:

The kinetic energy of an object is directly proportional to its mass. As the bus has higher mass than a car, the kinetic energy of the bus is higher than that of the car when their velocities are same.

Question 31.

An object moving at a velocity of 2 m s^{-1} is found to possess 400 J of kinetic energy. How much energy does the object possess when its

- mass is doubled
- velocity is doubled?

Answer:

1. When the mass of a moving object is doubled, the kinetic energy also gets doubled. Therefore, the object will possess 2 × 400 J = 800 J of kinetic energy.

2. When the velocity of an object is doubled, its kinetic energy increases 2^{2} = 4 times. Thus, the object will now possess 2^{2} × 400 J = 1600 J of kinetic energy.

Question 32.

A boy who weighs 40 kg is sitting inside a car. If the car is moving at a uniform velocity of 15 m s^{-1}, what is the kinetic energy of his body?

Answer:

Mass of the boy m = 40 kg

Velocity of the boy v = 15 m s^{-1}

Kinetic energy of the boy E = ?

Question 33.

A car is moving with uniform velocity of 15 msr1. What is the kinetic energy of a boy of mass 40 kg sitting in the car?

Answer:

Velocity of the boy = Velocity of the car

Here m = 40 kg,

v = 15 m s^{-1}.

Question 34.

Two bodies having the same mass are moving with velocity V and ‘3v’ respectively. What is the ratio of their kinetic energy?

Answer:

Let the mass of each body be m and their kinetic energy be E_{1} and E_{2} respectively.

Mass of body 1 = m

Velocity of body 1 = v

Kinetic energy of body 1, E_{1} = \(\frac{1}{2} \times m \times(3 v)^{2}\)

Mass of body 2 = m

Velocity of body 2 = 3v

The ratio of kinetic energies of the two bodies is given by,

The ratio of the kinetic energies of the two bodies is 1:9.

Question 35.

What is potential energy? Give two examples.

Answer:

The energy that gets stored in an object by virtue of its position, shape or configuration is called potential energy. eg: Energy possessed by water stored in an overhead tank, energy possessed by a wound toy car.

Question 36.

The following table gives the potential energy passed by a few objects. this is due to either their position or shape. identify in each case whether the potential energy is due to shape or position and put a ( √ ) mark in the space provided in the table.

Answer:

Question 37.

Obtain an expression for the potential energy of an object held at a height above the ground.

Answer:

Consider an object of mass ‘m’ placed at a height ‘h’ above the ground. The object possesses potential energy by virtue of its position. Let this energy be ‘E’. This means the object can do work equal to ‘E’. The force acting on the object is the gravitational force equal to ‘mg’. Here ‘g’ is the acceleration caused by the gravitational attraction of the earth.

Energy possessed E = Work done

= Force × displacement

= mg × h

E = mgh.

Question 38.

Two masses of 1 kg and 3 kg are held at the same height above the ground. Which of them has higher potential energy? Why?

Answer:

The object with higher mass, namely 3 kg mass, possesses higher potential energy. This is because potential energy increases with the mass of the object.

Question 39.

Calculate the potential energy present in a body of mass 1 kg when it is at a height of 5 m above the ground. [Acceleration due to gravity of the earth g = 10 m s^{-2}]

Answer:

Mass of the object m = 1 kg

Height of the object h = 5 m

Acceleration due to gravity g = 10 m s^{-2}

Potential energy E = ?

Potential energy E = mgh ..

= 1 kg × 10 m s^{-2} × 5 m

= 50 J.

Question 40.

A ball of mass 200 g falls from a height of 5 m. What is its kinetic energy when it just reaches the ground? [g = 9.8 m s^{-2}]

Answer:

Mass of the ball m = 200

g = 0.2 kg

Initial velocity u = 0

Acceleration due to gravity g – 9.8 m s^{-2}

Distance travelled/height h = 5 m

Final velocity v = ?

Question 41.

If the acceleration due to gravity is 10 ms~2, what is the potential energy of a body of mass 1 kg kept at a height of 5 m?

Answer:

Potential energy = mgh (here m = 1 kg, g = 10 m s^{-2}, h = 5 m)

Potential energy = 1 kg × 10 m s^{-2} × 5 m

Potential energy = 50 J.

Question 42.

A bag containing wheat weighs 200 kg. To what height should it be raised so that its potential energy would be 9800 J? [g = 9.8 m s^{-2}]

Answer:

Mass of the bag m = 200 kg

Potential energy E = 9800 J

Acceleration due to gravity g = 9.8 m s^{-2}

Height of the bag from the ground h = ?

Potential energy E = mgh

The bag of wheat should be raised to a height of 5 m.

Question 43.

What is meant by transformation of energy?

Answer:

The change of energy from one form to another is known as transformation of energy.

Question 44.

Give some examples to show that conversion of energy from one form to another is possible.

Answer:

Conversion of energy from one form to another is taking place in nature. We have also learned to convert energy in one form to other forms of energy. A few examples are

- Green plants convert solar energy (light energy) into chemical energy (starch).
- When water falls from a height, potential energy is changed into kinetic energy.
- A loud speaker changes electrical energy into sound energy.
- Our body converts chemical energy (in the form of glucose) into heat energy.

Question 45.

State the law of conservation of energy.

Answer:

The law of conservation of energy states that energy can neither be created nor destroyed. However, energy in one form can be converted into other forms.

Question 46.

In the following table, some situations involving conversion of energy from one form to another are given. Write in the table the form of energy before the occurrence of transformation and the form of energy after the transformation.

Answer:

Question 47.

We use a number of devices that change energy from one form to another. Name the devices that change

- electrical energy into light energy
- electrical energy into mechanical energy
- electrical energy into heat
- sound energy into electrical energy
- chemical energy into electrical energy.

Answer:

Energy conversion from what form to what form | Name of the device |

1. Electrical energy into light energy | CFL, LED lamps, Tubelight |

2. Electrical energy into mechanical energy | Electric motor |

3. Electrical energy into heat | Electric stove, Electric boiler |

4. Sound energy into electrical energy | Microphone |

5. Chemical energy into electrical energy | Electric cell |

Question 48.

What kind of energy transformation takes place in a thermal power plant?

Answer:

In a thermal power plant, coal is burned. During this process chemical energy is transformed into heat energy. The heat energy contained in steam is used to rotate a turbine. Here, heat energy is changed into mechanical energy.

Question 49.

Describe the mode of energy transformation in

- a mechanical clock
- a quartz clock.

Answer:

1. In a mechanical clock, we wind the clock. During this process muscular energy is changed into potential energy and gets stored in the spring. When the clock works, potential energy is changed into kinetic energy.

2. In a quartz clock, electrical energy is changed into mechanical energy (kinetic energy).

Question 50.

Why is the sun considered as the ultimate source of energy to us?

Answer:

The sun is the biggest natural source of energy to us. Further, most of the other sources of energy such as wind energy, hydel energy, energy of fuels, energy of tides and heat energy from the ocean are all due to the sun. Therefore, the sun is called the ultimate source of energy to us.

Question 51.

It is marked on an electric bulb as 60 W. What is it?

Answer:

The mark ‘60 W’ on a bulb indicates the rate at which the bulb consumes energy. It is the power of the bulb.

Question 52.

Define power. Mention its S. I. unit.

Answer:

The rate of consumption of energy is known as power. The S.l. unit of power is called ‘watt’. Its symbol is W.

Question 53.

How is power calculated?

Answer:

The power of an agent is calculated by dividing the energy consumed by it by the time it takes to consume that much of energy. Consider a device that consumes energy ‘E’ in time ‘t’. Let its power be ‘P’. Then,

Power is also calculated by dividing the work done by the time taken. If an agent does ‘W’ units of work in time ‘t’, then, its power ‘P’ is given by

Question 54.

Define 1 watt of power.

Answer:

One watt is the power of an agent that consumes energy at the rate of 1 joule per second.

1 watt = 1 joule/second

1 W = 1 J s^{-1}

Question 55.

What is the commercial unit of power? Define it.

Answer:

The commercial unit of power is called ‘kilowatt’. Its symbol is kW.

One kilowatt is defined as the power of an agent which uses 1000 J of energy per second.

1 kilowatt = 1000 watt

l kW = 1000 W

1 kW = 1000 J s^{-1}.

Question 56.

body does 20 J of work in 5 s. Calculate its power.

Answer:

Work done W = 20 J ; Time taken t = 5 s; Power P = ?

Question 57.

Describe the changes in energy that occur when a simple pendulum oscillates.

Answer:

In a simple pendulum, the bob oscillates between two extreme points B and C. The point A is the central position of the bob. As the pendulum oscillates, it reaches a certain maximum height. This height is reached when the bob is in extreme positions. At these points the bob will be at rest for a short period of time.

At positions B and C, the bob is at maximum height and hence its potential energy is highest. At that instant, the bob is at rest. Therefore, its kinetic energy is zero. As the bob begins to swing from its extreme position, its potential energy changes into kinetic energy.

At the central position A, the bob acquires highest velocity. Therefore, at point A, the kinetic energy is highest. Since A is the lowest possible position for the bob, the potential energy of the bob is zero at A.

Multiple Choice Questions

Question 1.

Work done = Force ×

(a) displacement

(b) acceleration

(c) velocity

(d) speed

Answer:

(a) displacement

Question 2.

The type of energy possessed by a simple pendulum, when it is at the mean position is

(a) only potential energy

(b) only kinetic energy

(c) both potential energy and kinetic energy

(d) neither potential energy nor kinetic energy

Answer:

(b) only kinetic energy

Question 3.

The type of energy that can be easily converted to other forms is

(a) sound energy

(b) light energy

(c) heat energy

(d) electrical energy

Answer:

(d) electrical energy

Question 4.

An object of mass 2 kg is dropped from a height of I m. Its kinetic energy as it touches the ground is

(a) 19.6 N

(b) 19.6 J

(c) 19.6 kg

(d) 19.6 m

Answer:

(b) 19.6 J

Question 5.

Which one of the following has kinetic energy?

(a) Water stored in a dam

(b) Stretched rubber band

(c) Water released from a dam

(d) Compressed spring

Answer:

(c) Water released from a dam

Question 6.

Power is a measure of the

(a) rate of change of momentum

(b) force which produces motion

(c) change of energy

(d) rate of change of energy

Answer:

(d) rate of change of energy

Question 7.

Which one of the following is a unit of power?

(a) joule per second

(b) watt per second

(c) joule

(d) kilojoule

Answer:

(a) joule per second

Question 8.

If an object of mass ‘m’ moving with velocity ‘v’ has kinetic energy ‘k’.then the velocity of the object can be calculated using the formula

(a) \(v=\frac{2 k}{m}\)

(b) \(v=\sqrt{\frac{2 k}{m}}\)

(c) \(v=\frac{4 k^{2}}{m}\)

(d) \(v=\frac{k m}{2}\)

Answer:

(b) \(v=\sqrt{\frac{2 k}{m}}\)

Question 9.

A 1 kg mass has a kinetic energy of 1 joule when its speed is

(a) 0.45 m s^{-1}

(b) 1 m s^{-1}

(c) 1.4 m s^{-1}

(d) 2 m s^{-1}

Answer:

(c) 1.4 m s^{-1}

Question 10.

Which one of the following is not the unit of power?

(a) joule per second

(b) newton metre

(c) kilowatt

(d) watt

Answer:

(b) newton metre

Question 11.

Which object with the mass indicated has the highest potential energy?

(a) mass = 10 kg, g = 9.8 m s^{-1}, h = 10 m

(b) mass = 5 kg, g = 9.8 m s^{-1}, h = 12 m

(c) mass = 8 kg, g = 9.8 m s^{-1}, h = 100 m

(d) mass = 6 kg, g = 9.8 m s^{-1}, h = 20 m

Answer:

(c) mass = 8 kg, g = 9.8 m s^{-1}, h = 100 m

Question 12.

The correct sequence of energy changes in a hydroelectric power station is

(a) kinetic energy → mechanical energy → electrical energy

(b) potential energy → kinetic energy → mechanical energy → electrical energy

(c) potential energy → electrical energy → mechanical energy

(d) potential energy → electrical energy → kinetic energy → mechanical energy

Answer:

(b) potential energy → kinetic energy → mechanical energy → electrical energy

Fill In The Blanks

- The rate of doing work is called power
- The S.I. unit of energy is joule
- Energy possessed by a rolling stone is kinetic energy
- The ultimate source of energy to us is Sun
- The energy possessed by an object due its shape is potential energy
- The kinetic energy of an object is directly proportional to the square of its velocity
- A unit equivalent to J s
^{-1}is watt - 1 kilowatt = 1000 watt
- The physical quantity which has ‘newton metre’ as its unit is work
- The change in the kinetic energy of an object is a measure of work done