**and Energy ICSE Class-8th Concise** Selina Physics Solutions Chapter-4. We Provide Step by Step Answer of Objective, True False , Fill in the blanks,Match the following , Type of Exercise-4 **Energy.** Visit official Website CISCE for detail information about ICSE Board Class-8.

## Energy ICSE Class-8th Concise Selina Physics Solutions Chapter-4

–: Select Topics :–

**A.Objective Questions**

**1. Write true or false for each statement**

**(a)** A coolie does no work against the force of gravity while carrying a luggage on a road.

**Answer.** True.

**(b)** The energy stored in water of a dam is the kinetic energy.

**Answer.** False.

The energy stored in water of a dam is the potential energy.

**(c)** The energy of a flying kite is kinetic energy.

**Answer.** True.

**(d)** Work done by a boy depends on the time in which he does work.

**Answer.** False.

**(e)** Power spent by a body depends on the time for which it does work.

**Answer.** True.

**2. Fill in the blanks**

**(a)** Work is said to be done by a forte only when **the body moves.**

**(b)** Work done = Force × **distance moved in direction of force.**

**(c)** The energy of a body is its capacity to do **work.**

**(d)** The S.I. unit of energy is **joule.**

**(e)** The potential energy of a body is due to its **state of rest or position** and kinetic energy of body is due to its **state of motion.**

**(f)** Gravitational potential energy U = mass × force of gravity on unit mass × **height.**

**(g)** Kinetic energy = 1/2 × mass × **(speed) ^{2}**

**(h)**Power P=

**work done**/time taken.

**(i)**The S . i. unit of power is

**watt**

**(j)**I H.P. =

**746 W**

**3. Match the following**

column A |
Column B |

A stone at a height | power |

A Moving ball | joule |

Energy | work done in 1 s |

Power | potential energy |

Watt | kinetic energy |

Answer-

column A |
Column B |

A stone at a height | potential energy |

A Moving ball | kinetic energy |

Energy | joule |

Power | work done in 1 s |

Watt | power |

** **

**4. Select the correct alternative**

**(a)** The S.I. unit of work is

- second
- metre
**joule**- newton

**Answer:**

joule

**(b)** No work is done by a force if the body

- moves in direction of force
**does not move**- moves in opposite direction
- none of the these

**Answer:
**does not move

**(c)** Two coolies A and B do some work in time 1 minute and 2 minute respectively. The power spent is

- same by both coolies
**is more by coolie A than by B**- is less by coolie A than by B
- nothing can be said.

**Answer:**

is more by coolie A than by B

**(d)** The expression of power P is

- P = mgh
- and P = P = 1/2 Mv
^{2} - so P = F × d
**P = F × d/t**

**Answer:
**P = F × d/t

**(e)** I H.P. ¡s equal to

- 1 W
- 1 J
- 746 J
**746 W**

**Answer:
** 746 W

**(f)** When a boy doubles his speed, his kinetic energy becomes

- half
- double
**four times**- no change

**Answer:**

four times

**(g)** A boy lifts a luggage from height 2 m to 4 m. The potential energy will become

- half
**double**- one-third
- one-fourth

**Answer:**

double

**Short/ Long Answer Question Chapter-4 Energy Selina Physics for class-8**

Page-77

**Question-1**

Define work.

**Answer-1**

WORK “When a force is applied on a body and there is displacement of the body, work is said to be done.”

**Question-2**

When does a force perform work?

**Answer-2**

Work is said to be done only when the force applied on a body makes the body move (i.e. if there is some displacement or motion occurred due that **force**).

**Question-3**

State two conditions when no work is done by a force.

**Answer-3**

Two conditions are :

(i) There should be no displacement i.e. S = 0

(ii) The displacement is NORMAL to the direction of FORCE i.e. – θ = 90°

**Question-4**

In which of the following cases is work being done :

(a) A boy pushing a rock

(b) A boy climbing up the stairs

(c) A coolie standing with a box in his head

(d) A girl moving on the road.

**Answer-4**

work being done in the following cases

(b) A boy climbing up the stairs

(d) A girl moving on the road.

**Question-5**

A coolie is moving on a road with a luggage on his head. Does he perform work against the force of gravity? Give reason for your answer.

**Answer-5**

No there is no work done against the force of gravity because the displacement and the direction of force are at an angle of 90°.

**Explanation**:

Walking involves doing the same amount of work against gravity as standing because the weight of the body is the same.

So, the coolie does work against gravity equal to the total weight he is carrying (his body weight and the weight of whatever he is wearing, carrying, etc).

**Question-6**

The moon is revolving around the earth in a circular path. How much work is done by the moon?

**Answer-6**

The **work done by the moon** during one **circular motion** is zero as the force acting between the **earth** and **moon** is gravitational force is conservative in nature and the **work done** by conservative forces does not depend on **path** followed. Also, the force and displacement are perpendicular to each other. So, the **work done** is zero.

**Question-7**

Write the expression for work done by a force.

**Answer-7**

Work done by applying force F is the product of the force applied on the body and distance moved by the body in the direction of force, work done = Force x distance moved in the direction of the force.

W = F × d

**Question-8**

State the S.I. unit of work and define it.

**Answer-8**

S.I. unit of work is Joule

Joule “Is that much work done when a force of 1 N displaces the body through a distance of l m in the direction of the force.”

**Question-9**

State two factors on which the work done on a body depends.

**Answer-9**

Two factors are :

(i) Magnitude of force applied (F).

(ii) Distance moved by the body in the direction of force (d) or (5)

**Question-10**

Define the term energy.

**Answer-10**

ENERGY : “Capacity of doing work” is called ENERGY

**Question-11**

State the S.I. unit of energy.

**Answer-11**

S.I unit __ is Joule (J).

**Question-12**

Define 1 joule of energy.

**Answer-12**

1 Joule is the amount of work done by a force of 1 Newton on an object that displaces it through a distance of 1 meter in the direction of the applied force.

1 J = 1 N x 1 m.

**Question-13**

How is work related to energy?

**Answer-13**

RELATION BETWEEN WORK AND ENERGY: “Energy is the capacity of doing work” Every form of energy → is work.

i.e. work done on the body is stored in the form of energy. Energy is spent when a body does work. Thus to do more amount of work-more energy is needed.

**Question-14**

What are the two kinds of mechanical energy?

**Answer-14**

Two KINDS OF MECHANICAL ENERGY:

- The Potential energy (P.E.)
- The Kinetic energy (K.E.)

**Question-15**

What is potential energy? State its unit.

**Answer-15**

POTENTIAL ENERGY: (P.E. or U)

“The energy possessed by a body due to its position above the ground. or change in state.”

UNIT: Unit of P.E. = S.I. UNIT OF ENERGY = Joule (J)

**Question-16**

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-16**

(i) Potential energy of a body is the energy possessed by it due to its state of rest or position. It is the energy stored when work is done on the body to bring it to that state or position.

(ii) Potential energy of a body in the raised (or lifted) position depends on two factors:

- the mass of the body greater the mass of the body, greater is the potential energy of the body), and
- the height of the body above the ground (greater the height of the body, greater is its potential energy.)

**Question-17**

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

**Answer-17**

Potential energy = mgh

g is constant.

So, two factors on which P.E. depends:

(i) Mass

(ii) Height above the ground

**Question-18**

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-18**

We know that, PE = mgh

Height of body A and B is same and g is constant, hence the body with greater mass i.e. Body B has greater potential energy.

**Question-19**

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-19**

We know that, PE = mgh

As ‘g’ is constant in both cases and quantity of water is the same in both cases. PE depends on height, the height of the second bucket is greater as it is on the second floor, hence the second bucket has greater PE.

**Question-20**

Write the expression for gravitational potential energy explaining the meaning of the symbols used.

**Answer-20**

Expression for gravitational potential energy = U = mgh

g is acceleration due to gravity

mg is the force acting on body

h is the distance or height moved above the ground level.

**Question-21**

A body of mass m is moved from ground to a height h. If force of gravity on mass of 1 kg is g newton,

(a) find the force needed to lift the body.

(b) find the work done in lifting the body

(c) find the potential energy stored in the body.

**Answer-21**

(a) When a body of mass m at A on ground is raised above ground through height h at B, force is applied.

Force applied = weight of body

Force on mass m = F = mg

‘g’ is the acceleration due to gravity on a mass of 1 kg

(b) Work done = Force $×$ displacement (distance)

W = mgh

(c) The work done is stored in body in the form of potential energy

P.E = mgh

**Question-22**

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

**Answer-22**

- The energy of a body in motion is called its kinetic energy.
- It is defined as follows:

Kinetic energy of a body is the energy possessed by it due to its state of motion. Actually, it is the work done on the body bringing it to the state of motion. - In short form, it is written as K.E. or K.
- Example: In a swinging pendulum moving to and fro, the bob has the kinetic energy.

**Question-23**

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

**Answer-23**

The energy possessed by a body by virtue of its motion is defined as kinetic energy. The factors on which it depends are

(i) Mass of the body.

(ii) Velocity of the body.

**Question-24**

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

**Answer-24**

0.25 v² is greater than 0.1v²

∴ K.E. of car B is greater.

Or

Since speed of both cars is same

∴ The speed of car having greater mass (i.e. of car B), the K.E. is greater

∴ Kinetic energy of car B having greater mass is greater.

**Question-25**

A cyclist doubles his speed. How will his kinetic energy change: increase, decrease or remain same?

**Answer-25**

As Kinetic energy K.E. = ^{1}⁄_{2} MV²

Since speed is doubled, its square will become 4 times

∴ K.E. increases i.e. become 4 times.

Or

K.E j = Mv_{2}

When speed is doubled New speed v_{1} = (2 v)

**Question-26**

Write the expression for the kinetic energy of a body explaining the meaning of the symbols used.

**Answer-26**

Kinetic energy =^{1}⁄_{2} Mv^{2}

Where m is the mass of body v is the speed of body.

**Question-27**

A ball of mass M is moving with a speed v. What is its kinetic energy?

**Answer-27**

Kinetic energy of a ball of mass m and moving with speed v is K.E. = ^{1}⁄_{2} Mv^{2}

**Question-28**

Name the form of energy stored in a wound up spring of a watch.

**Answer-28**

A watch. possesses Potential energy.

**Question-29**

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

**Answer-29**

Yes, a body not in motion can possess energy.

Example: Water stored in the tank on your house except ground floor though not in motion possesses potential energy. Or A stone at rest on the top of a hill possesses potential energy.

**Question-30**

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

(a) A moving cricket ball

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

(c) A compressed spring

(d) A moving bus

(e) A bullet fired from a gun

(f) Water flowing in a river

(g) A stretched rubber band

**Answer-30**

(a) Kinetic energy

(b) Potential energy

(c) Potential energy

(d) Kinetic energy

(e) Kinetic energy

(f) Kinetic energy

(g) Potential energy

**Question-31**

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

**Answer-31**

A stretched bow has the potential energy because of its stretched position. When the stretched bow is released, the potential energy of the bow changes into kinetic energy of the arrow

**Question-32**

State the energy changes that occur in a watch spring while it unwinds.

**Answer-32**

A wound up watch spring has potential energy stored in it, because it is wound up.

The spring unwinds itself, the potential energy changes into kinetic energy with which it moves the hands of the watch.

**Question-33**

Give reasons for the following:

(a) No work is done if a man is pushing against a wall.

(b) Hammer drives a nail into the wood only when it is lifted up and then struck.

(c) A horse and a dog are running with the same speed. Which one of them has more kinetic energy than the other.

(d) A teacher moving around in the class is doing work but a child standing and reading a book is not doing any work.

**Answer-33**

(a) A wall does not move when pushed, hence the object is not moving. No work is done.

(b) When a hammer is lifted, the potential energy stored in it is used in driving the nail into the wood.

(c) When a horse and a dog are running at the same speed. The animal having more mass will have more kinetic energy. A horse has more mass than a dog, hence the kinetic energy of a horse is more than the kinetic energy of a dog.

(d) A child reading a book while standing is not moving from its place i.e. displacement is zero. Hence, no work is done by the child.

**Question-34**

State the energy changes in the following while in use:

(a) An electric bulb

(b) An electric oven

(c) A loud speaker

(d) A microphone

(e) An electric motor

**Answer-34**

(a) Electrical energy to light energy

(b) Electrical energy to heat energy

(c) Electrical energy to sound energy

(d) Sound energy to electrical energy

(e) Electrical energy to mechanical energy

**Numericals of Ch-3 Energy Selina Concise Physics for ICSE Class-8**

Page-78

**Question-1**

A force of 30 N acts on a body and moves it through a distance of 5 m in the direction of force. Calculate the work done by the force.

**Answer-1**

F = 30 N

d= 5m

Work done = w = F × d

W = 30 × 5= 150 J

**Question-2**

A man lifts a mass of 20 kg to a height of 2.5 m. Assuming that the force of gravity on 1 kg mass is 10 N, find the work done by the man.

**Answer-2**

Mass = 20 kg h = 2.5 m

Force of gravity on a mass of 1 kg = 10 N

Force of gravity on a mass of 20kg

F = mg = 20 × 10 = 200 N

Work done in lifting the mass to height h = 20 m is

W = F × h

= 200 N × 2.5 m

= 200 × 25/10 =500 J

**Question-3**

A body, when acted upon by a force of 10 kgf, moves to a distance 0.5 m in the direction of force. Find the work done by the force. Take 1 kgf = 10 N.

**Answer-3**

F= 10 kgf = 10 × 10N= 100 N,

Displacement S = 0.5 m

Work done

(i) When displacement is in the direction of force

W = F × S

W= 100 × 0.5 = 50 J

**Question-4**

Two bodies of same masses are placed at heights h and 2h. Compare their gravitational potential energy

**Answer-4**

Gravitational potential energy = mgh

Gravitational Potential energy of body 1 = mgh

Gravitational Potential energy of body 2 = mg2h

On comparing,

= ^{ mgh}⁄_{2mgh}

= ^{1}⁄_{2}

The gravitational potential energy of object 2 is twice that of object 1

**Question-5**

Find the gravitational potential energy of 2.5 kg mass kept at a height of 15 m above the ground. The force of gravity on mass 1 kg is 10 N.

**Answer-5**

Gravitational potential energy = U = mgh

m = 2.5 kg

g = 10 N

h = 15 m

U = 2.5 x 10 x 15

U = 375 J

**Question-6**

The gravitational potential energy stored in a box of weight 150 kgf is 1.5 x 10^{4} . Find the height of the box. Take 1 kgf = 10 N

**Answer-6**

Gravitational potential energy = U = mgh

1.5 x 10^{4} J = 150 kgh $×$ h

1.5 x 10^{4} J = 150 $×$ 10 N $×$ h

h = (15 $×$ 1000) $÷$ 1500

h = 10 m

**Question-7**

The potential energy of a body of mass 0.5 kg increases by 100 J when it is taken to the top of a tower from ground. If force of gravity on 1 kg is 10 N, what is the height of the tower?

**Answer-7**

Potential energy = Force $×$ height

U = (mg)h

100 J = (0.5$×$ 10) N $×$ h

h = 100 $÷$ 5 = 20 m

**Question-8**

A body of mass 60 kg is moving with a speed 50 m/s. Find its kinetic energy.

**Answer-8**

mass m = 60 kg

Speed v = 50 mS^{1}

K.E = 1/2 MV^{2}

1/2 × 60 × 50 × 50 = 75000 kgm/s or Joule

K.E = 75/10 × 1000 × 10 = 7.5 × 10^{4} J

**Question-9**

A truck of mass 1000 kg, increases its speed from 36 km h^{-1} to 72 km h^{-1}. Find the increase is its kinetic energy.

**Answer-9**

Weight of truck = Force = 1000 kgf

**Question-10**

A car is moving with a speed of 15 km h^{-1} and another identical car is moving with a speed of 30 km h^{-1}. Compare their kinetic energy.

**Answer-10**

Two identical cars mean, they have equal mass

K.E = 1/2 mv^{2}

Let m be the mass of each car

K.E. of car B is 4 times K.E. of car A.

**Question-11**

A pump raises water by spending 4×10^{5}J of energy in 10s. Find the power of the pump.

**Answer-11**

Energy spent = w = 4 × 10^{5}J

Time taken = 10s

Power = Energy / Time

P =( 4×10^{5})/10 = 4×10^{4} W

**Question-12**

It takes 20 s for a girl A to climb up the stairs while girl B takes 15 s for the same job.

(a) Compare the work done.

(b) Compare the power spent by then.

**Answer-12**

(a) As height is same for both girls A and B, work done is same

Work done by A : Work done by B = 1 : 1

(b) Power = Energy/Time

= 15 : 20

= 3 : 4

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