# WORK ENERGY AND POWER

INTRODUCTION: Here we will study and get idea about scientific conception of work energy and power along with vivid question answer.

## (A) Very short type question on work energy and power

#### 1) Name another term of energy of configuration.

Ans. P.E.and mutual energy.

#### 2)  What is the SI unit of energy?

Ans. The SI unit  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) Mention  the expression of K.E.

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

## (B) Short type question:

#### 1)  What is  P.E.?

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

#### 2)  State the types of P.E.

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

(b) Elastic potential energy

(c) Electrostatic potential energy.

#### 3) Give some examples of P.E.

Ans. Some examples of P.E. are: – (i)  P.E. of water stored in a dam

(ii) P.E. of a stretched bow

(iii) P.E. of a compressed gas

(iv) P.E. 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)  Mention  the law of Conservation of energy.

Ans. According to the law“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 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 electricity. This is the process by which  the mechanical  is converted into electrical energy.

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

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

#### 8) Give an example of transformation of electricity into heat energy.

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

#### 9)  Give an example of the transformation of electricity into sound energy.

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

#### 10)  Give a relationship between gravitation and mass, gravity and height.

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

u (P.E.)= m (MASS) x g (GRAVITY) x h (HEIGHT).

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

Ans. In work energy and power when an electric bulb glows on passing electric current through it then electrical  transforms into heat and light energy.

### (C)  LONG QUESTION TYPE in work energy and power –

#### 1)  Discuss  how gravity is related with various factors ?

Ans. Gravitational P.E. 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 the pendulum starts moving . There are two extreme position where bob of pendulum moves maximum. At extreme position P.E. of pendulum is entirely potential (mgh). When pendulum starts moving towards mean position P.E. changes into K.E.. At mean position KE =   mv2 . Therefore, as the pendulum oscillates it has  P.E.at the extreme position andK.E. 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 in work 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 Energy is what one delivers Power is the rate at which it is delivered c) Storage purpose It can be stored Power cannot store d) Basic unit d) Joule (J) or watt-hour Watt (W) or joule per second

#### (E) NUMARICAL PROBLEMS ON WORK ENERGY AND POWER: –

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.  P.E.  in work energy and power 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 50 kg situated at a height of 10m. What is its P.E.? (given: g=10ms-2)

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

so the P.E. = 50 x 10 x 10 = 5000 J

so the P.E. is 5000 J in work energy and power.

#### 3)  A person throws a ball  from a height of 10 m. If the energy of the ball decreases by 40% after hiting the ground,  upto which height will the ball bounce back in work energy and power? (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  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) P.E. just before dropping.
2. b) K.E. when the mass touches the ground.
3. c) when it hits the ground find the velocity . [ g = 10 ms-2 ] in work energy and power.

Ans.  m = 10 kg h = 50 cm = 0.5 m.

So we have to find out   P. E = ?

1. E =?

V =?

1. a) P.E. = mgh

= 10x10x5 = 50 J.

1. b) Kinetic

#### energy = P.E. 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]

So           a) Potential energy = 50 J

1. b) Kinetic energy = 50 J
2. c) Velocity = 13.16 ms-1

#### 5) A toy car is displaced through 5m on application of a force of 7N in work energy and power.  Let’s imagine that through the displacement the force acts on the object .  In this case what is the work done ?

Ans.       Given F = 7N;      S = 5m

Therefore, work = F x S

= 7 x 5 = 35 J

So  in this case the work  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.  Calculate how much did the porter do work on the luggage in work energy and power? (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  how much did the man work (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.

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