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ENERGY CLASS VIII for ICSE & CLASS IX for CBSE

CHAPTER – ENERGY

 

(A) Very short type question

1) What is the another name of energy of configuration?

Ans. Potential energy and mutual energy.

2)  What is the SI unit of energy?

Ans. The SI unit of energy is Joule (J).

3) State the relation between kilojoule, mega joule and Joule.

Ans.  1 KJ = 103J 1MJ = 106J

4) What do you mean by work?

Ans. Any activity that requires muscular or metal effort is called work.

5) State the expression of kinetic energy.

Ans. K (kinetic energy) =  m (mass of body) x v2(velocity)2

 

(B) Short type question

1)  What is potential energy?

Ans. The energy possessed by a body by virtue of its position or configuration is called potential energy or mutual energy or energy of configuration.

2)  State the types of potential energy.

Ans. There are three types of potential energy (a) Gravitational potential energy

(b) Elastic potential energy

(c) Electrostatic potential energy.

3) Give some examples of potential energy.

Ans. Some examples of potential energy are: – (i) Potential energy of water stored in a dam

(ii) Potential energy of a stretched bow

(iii) Potential energy of a compressed gas

(iv) Potential energy of a system of charges.

4)  Define gravitational potential energy.

Ans. Gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity.

5)   State the law of Conservation of energy.

Ans. According to the law of conversation of energy “The energy in a system can neither be created nor be destroyed; it can simply change from one form into another”. The total energy before and after the transformation remains same.

6)  Give an example of transformation of mechanical energy into electrical energy.

Ans. When water is flown rapidly from reservoir of a dam the potential energy is transferred into kinetic energy which rotates turbine of electric generator resulting production of electric energy. In this way mechanical energy is transformed into electrical energy.

7)  Give an example of transformation of electrical energy into mechanical energy.

Ans. An electric motor is used in various rotating device like fan, washing machine where electrical energy is transformed into mechanical energy.

8) Give an example of transformation of electrical energy into heat energy.

Ans. Electrical heater, oven, geyser, toaster etc convert electrical energy into heat energy when in use.

9)  Give an example of transformation of electrical energy into sound energy.

Ans. A loudspeaker when in use, converts electrical energy into sound energy.

10)  Give a relationship between gravitational potential energy and mass, gravity and height.

Ans. The gravitational potential energy for the earth body system is

u = m x g x h.

11)  Give an example of transformation of electrical into light energy.

Ans. When an electric bulb glows on passing electric current through it then electrical energy is transformed into heat and light energy.

12)  Give an example of transformation of light energy into electrical energy.

Ans. In a photo electric cell, the light energy gets converted into electrical energy.

13)  Give an example of transformation of heat energy into mechanical energy.

Ans. In a steam engine the chemical energy of the coal first changes to the heat energy of steam and then steam helps to move engine that to mechanical energy.

14)  Give an example of transformation of chemical energy into electrical energy.

Ans. In chemical cell, the chemical energy of cell is converted into electrical energy.

15)  Give an example of transformation of chemical energy into heat energy.

Ans. When fossil fuel is burnt then chemical energy is changed into heat energy.

16)  Give an example of transformation of chemical energy into mechanical energy.

Ans. In automobiles, the chemical energy of petrol and diesel changes into mechanical energy.

17)  Give an example of transformation of sound energy into electrical energy.

Ans. A microphone converts sound energy into the electrical energy.

18)  Give an example of transformation of electrical energy into chemical energy.

Ans. Inverter transfers electrical energy into chemical energy during charging. The stored chemical energy later converted into electrical energy.

19)  State two conditions need to be satisfied for work to be done.

Ans.  Two conditions are:

  1. i) A force should act on the object.
  2. ii) The object must get displaced.

20)  Define work.

Ans. The work done by a constant force is defined as the product of the force in the direction of the displacement and the magnitude of the displacement.

21) Show the relation between Work and Force.

Ans. Since F is in the direction of S, hence work done by the force F is given by:

W = Force x displacement.

= F x S.

22) State the type of quantity of work.

Ans.       Work is a scalar quantity.

It has only magnitude.

23) State the different types of work.

Ans.       Work is of three types:

  1. i) Positive work
  2. ii) Negative work

iii) Zero work

 

24) What is positive work?

Ans. Work done is said to be positive if the applied force is in the direction of the displacement.

25) Give three examples of positive work.

Ans.  Examples of positive work are:

i)Work done by gravity if a body falls towards the earth.

  1. ii) Work done by a horse pulling a cart.

iii) Work done by the stretching force acting on a spring.

26) What is negative work?

Ans. Work done is said to be negative if the applied force is in a direction opposite to that of displacement.

27) Give three examples of negative work.

Ans.  Examples of negative work are:

  1. i) Work done by the braking force when brakes are applied to a vehicle.
  2. ii) Work done by friction.

iii) Work done by the force of gravity when a body is projected away from the surface of the earth.

28) What is kinetic energy?

Ans. The energy possessed by a body by virtue of its motion is called kinetic energy.

29) How is kinetic energy dependent?

Ans. The kinetic energy is dependent upon the square of the speed.

 

(C)  LONG QUESTION TYPE: –

1)  Discuss how gravity is related with various factors?

Ans. Gravitational potential energy for the earth body system = u = mgh. Work done against gravity depends only upon the difference between the initial and the final position of the body. It is independent of the way the body is moved between the two points.

2)  Explain how law of conservation of energy is maintained in pendulum?

Ans. A pendulum has one mean position where pendulum starts moving. There are two extreme position where bob of pendulum moves maximum. At extreme position the energy of pendulum is entirely potential (mgh). When pendulum starts moving towards mean position potential energy changes into kinetic energy. At mean position KE =   mv2 . Therefore, as the pendulum oscillates it has potential energy at the extreme position and kinetic energy at the mean position. At all other positions the energy is partially kinetic and partially potential. In this way pendulum confirms the law of conservation of energy.

3)  Differentiate between energy and power.

 

Key point Energy Power
a) Definition a) Ability to do work a) Measurement which calculates the time by which the energy has been used.
b) Description b) Energy is what one delivers b) Power is the rate at which it is delivered
c) Storage purpose c) It can be stored c) Power cannot store
d) Basic unit d) Joule (J) or watt-hour d) Watt (W) or joule per second

4)            Explain kinetic energy with examples.

Ans. Kinetic energy is the total work that has to be done on a particle to accelerate it from rest to its present speed. A falling coconut, a speeding car, a rolling stone, blowing wind, a running athlete all possess kinetic energy. The kinetic energy of an object increases with its speed. Example of kinetic energy are: a) A bullet fire from a gun penetrates a target due to the kinetic energy it possesses.

  1. b) Energy possessed by a moving hammer as it drives a nail into a piece of wood.

5)            Explain zero work with examples.

Ans. Work done is said to be zero if the applied force is zero or displacement is zero or the angle between displacement and force is 900.  Examples of zero work are:

  1. a) Work done by the tension in the string of a simple pendulum.
  2. b) Work done by centripetal force
  3. c) Work done by a coolie carrying load on his head and walking straight on a level platform.

6)            What is erg? How is it related to dine?

Ans. In cgs system, the absolute unit of work is erg. Work done is said to be one erg if a force of one dyne displaces a body through a distance of one centimetre in its own direction. We can say, 1 erg = 1 dyne x 1cm

7)  What is the absolute unit of work? How is related to erg?

Ans. In SI system, the absolute unit of work is Joule (J). Work done is said to be one joule if a force of one newton displaces a body through a distance of one metre in its own direction.

We can say: 1 joule = 1 newton x 1 m

Further, 1 joule = 1 N x 1 m

= 105 dyne x 102 cm = 107 erg.

In this way joule is related with erg.

(D) FILL IN THE BLANKS

1)  Write the proper term KE ________.

Ans.   mv2

2)  Blowing air has ________ energy.

Ans. Kinetic.

(E) NUMARICAL PROBLEMS: –

1) Find the energy possessed by an object of mass 10 kg when it is raised to a height of 6m above the ground (given g = 10ms-2).

Ans.  Potential energy will be the work done by the object i. e.

W = mgh                                              [ m=10kg

= 10 x 10 x 6                                   g=10ms-2

= 600 J                                                h=6m.]

So the energy is 600 J.

2) A body of mass 5okg situated at a height of 10m. What is its potential energy? (given: g=10ms-2)

Ans. We know:  m = 50 kg ; g = 10 ms-2  ;  h = 10 m

so the potential energy = 50 x 10 x 10 = 5000 J

so the potential energy is 5000 J.

3)  A ball is dropped from a height of 10m. If energy of the ball reduces by 40% after striking the ground, how high can the ball bounce back? (g = 10 ms-2)

Ans. Given that, h = 10kg; g = 10 ms-2 ;

So PE1 = mgh = m x 10 x 10 =100m.

Energy is reduced by 40%, the remaining energy is 60m.

Therefore, 60m = m x 10 x h

h = 60m/10m = 6 mt.

So at the height 6 metre the ball can bounce back.

4) A mass of 10 kg is dropped from a height of 50 cm. Finds its

  1. a) Potential energy just before dropping.
  2. b) Kinetic energy just on touching the ground.
  3. c) Velocity with which it hits the ground. [ given g = 10 ms-2 ]

Ans.  m = 10 kg h = 50 cm = 0.5 m.

So we have to find out   P. E = ?

  1. E =?

V =?

  1. a) Potential energy = mgh

= 10x10x5 = 50 J.

  1. b) Kinetic energy = Potential energy as we know by law of conservation of energy.

So kinetic energy = 50 J.

  1. c) We know: KE = 1/2 mv2

=> v2 = 2KE/m

=> v = √[(2×50)/10]

= 3.16 ms-1

So           a) Potential energy = 50 J

  1. b) Kinetic energy = 50 J
  2. c) Velocity = 3.16 ms-1

5) A toy car is displaced through 5m on application of a force of 7N. Let us take it that the force acts on the object through the displacement. What is the work done in this case?

Ans.       Given F = 7N;      S = 5m

Therefore, work = F x S

= 7 x 5 = 35 J

So the work done in this case is 35 J.

6) A porter lifts a luggage of 20kg from the ground and puts it on his head 1.7m above the ground. Find the work done by the porter on the luggage. (g = 10ms-2)

Ans.       Given    m = 20kg; h = 1.7m; g= 10ms-2

W = F x S = m x g x h

= 20 x 10 x 1.7 = 340 J.

So, the work done on luggage is 340 J.

7)  A man weighing 70 kg carries a weight of 10 kg on the top of a tower 100m high. Calculate the work done by the man (g = 10ms-2)
Ans.       Given    F = (70 + 10) = 80 kg wt.

= 80 x 10 = 800N

S = 100m.

W = F x S

= 800 x 100 = 80,000 J.

So, the work done by the man is 80, 000 J.

8) An object of mass 10 kg is moving with a uniform velocity of 5ms-1. Calculate the kinetic energy possessed by the object.

Ans.       Mass of the object m = 10 kg.

Velocity of the object V = 5ms-1

Kinetic energy (KE) = 1/2mv 2 = 1/2 x 10 x (5)2

= 125J

9)  Two bodies have equal masses and are moving with uniform velocities of 2v and 3v. What is the ratio of their kinetic energies?

Ans.      Given    m1 = m

m2 = m

V1 = 2v

V2 = 3v

We have to find E1/E2 =?

We know that E =  mv2, therefore we have

E1/E2  = 1/2m1v12/1/2m2v22 = v12/v22 = (2v)2/(3v)2 = 4/9

So, the ratio of kinetic energies of two bodies are 4/9 .

10)  What is the kinetic energy of a 45 kg object moving at 13 ms-1?

Ans.       Given    m = 45 kg

V = 13 ms-1

                                                So K =?

We know that K.E or kinetic energy

K = 1/2 mv2

                                                K = 1/2 x 45 x (13)2 = 3802.5 J.

So the kinetic energy is 3802.5J.

11)  What is the mass of a baseball that has a kinetic energy of 100 J and is travelling at 5 ms-1?

Ans.       Given that K = 100 J

V = 5 ms-1

So                m =?

We know that    K =  mv2

                                                Or           m = 2k/v2= (2×100)/(5×5) = 8 kg.

Therefore, the mass of the baseball is 8 kg.

12)  A student is hit with a 1 kg cake, the kinetic energy of the cake was 32 J. What was the speed of the cake?

Ans.       Given that           K = 32 J

m = 1 kg

We know that    K = 1/2 mv2

Or    v2 = 2k/m

Or   v =√(2k/m) = √[(2×32)/1} = 8 ms-1

Therefore, the speed of the cake is 8 ms-1.

 

 

 

 

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