The document Lakhmir Singh & Manjit Kaur: Work and Energy, Solutions- 3 Notes | EduRev is a part of the Class 9 Course Class 9 Physics Solutions By Lakhmir Singh & Manjit Kaur.

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__Page 163__**Solution 17**

Force, F = 400 N

Distance, s = 60 m

Time taken, t = 1 minute = 60 s

Work done, W = F x s = 400 x 60

**Solution 18**

At a hydroelectric power station, the potential energy of water is transformed into kinetic energy and then into electrical energy.**Solution 19**

At a coal-based thermal power station, the chemical energy of coal is transformed into heat energy, whhich is further converted into kinetic energy and electrical energy.**Solution 20**

Weight of the man = 500 N

Weight of the load = 100 N

Total weight = 600 N

Height of stairs = 4 m

Time taken = 5 s

Work done = mg x h = weight X h = 600 x 4

**Solution 21**

Power = 3 kW

Time = 20 s

Work done = power x time = 3 x 20 kWs = 60 kJ

**Solution 22**

Energy consumed = 600 kJ

Time taken = 5 minutes = 300 s

**Solution 23**

Power = 100 W

a) time = 1 s

energy = power x time = 100 J

b) time = 1 minute = 60 s

energy = power x time= 100 x 60 = 6 kJ**Solution 24**

Power of 1 fan = 120 W

Power of 5 fans = 5 x 120 = 600 W = 0.6 kW

Time = 4 hours

Electrical energy = 0.6 x 4 = 2.4 kWh**Solution 25**

A radio first converts electrical energy into kinetic energy and then into sound energy**Solution 26**

In an electric bulb, electrical energy is first converted into heat energy and then into light energy**Solution 27**

Fan, washing machine, mixer grinder, water pump, hair dryer use electric motor**Solution 28**

i) chemical energy to electrical energy

ii) electrical energy to heat and light energy**Solution 29**

i) Maximum potential energy is present in the bob at point C as at point C bob is at maximum height.

ii) Maximum kinetic energy is present in the bob at point A as at point A bob is at maximum speed**Solution 30**

Weight of the car = 20000 N= 20 kN

Speed = 8 m/s

Distance s= 120 m

Time = 100 s

a)Work done W = f x s = 20 x 120 = 2400 kJ

b) Power

**Solution 31**

a)The change of one form of energy into another form of energy is known as transformation of energy, e.g. in a cell chemical energy is transformed into

electrical energy

b) a. When a ball is thrown upwards its kinetic energy gradually converts into potential energy and potential energy

becomes maximum at the maximum height attained by the ball

b. When a stone is dropped from the roof of the building its potential energy gradually converts into kinetic energy and kinetic energy becomes maximum when the stone is just above the ground**Solution 32**

a) Law of conservation of energy states that whenever energy changes from one form to another form, the total amount of energy remains constant. Energy can

never be created nor destroyed, it transforms from one form to another. For example, when electrical energy is converted into light energy in an electric

bulb, then some energy is wasted as heat during conversion but the total energy remains the same.

b) Initially the pendulum is at rest. The bob is pulled to one side to position B to give it potential energy due to higher position of B and then released, the bob

starts swinging

i. When the bob is at position B, it has potential energy but no kinetic energy

ii. As the bob starts moving down from position B to A, its potential energy starts decreasing and kinetic energy starts increasing

iii. When the bob is at position A, it has maximum kinetic energy and zero potential energy

iv. As the bob starts moving down from position A to C, its kinetic energy starts decreasing and potential energy starts increasing

v. On reaching the extreme position C, the bob stops for a very small instant of time and bob maximum potential energy and zero kinetic energy.

Therefore at extreme positions B and C bob has only potential energy and at A it has only kinetic energy and at other intermediate positions bob has both kinetic

and potential energy. Thus the total energy of the pendulum is same for any instant of time (conserved)

**Solution 33**

a) The unit kWh stands for kilowatt-hour. One kilowatt-hour is the amount of electrical energy consumed when an electrical appliance having power of 1 kilowatt is used for 1 hour. It represents the amount of electrical energy consumed in 1 hour.

b) Power = 1000 W = 1 kW

Time = 60 minutes = 1 hour

Energy(kWh) = 1 x 1 = 1 kWh**Solution 34**

a) 1 kilowatt-hour = 1 kW for 1 hour = 1000 W for 1 hour

**Solution 35**

a) Power is the rate of doing work.

Si unit of power is watt(w)

(b) Mass of the body = 40 kg

Mass of the box = 20 kg

total Mass = 60 kg

Height h = 15m

g= 10m/s^{2}

time taken =25s

woek done = m x g x h = 60 x 15 x 10 = 9000 J

__Page 164__**Solution 46**

Mass of body = 1 kg

g = 10 m/s^{2}

I. At h= 5m; v= 0 m/s

PE = 10 x 5 = 50 J

KE = 0 J

at total Energy = PE+KE = 50 J

ii. at h = 3.2 m; v =10 m/s

PE = 0 J

total Energy = PE+KE = 50 J

The total energy in all three cases is constant . this prove the law of conservation of energy

**Solution 47**

Since from point A total energy = PE + KE = 80 J

And according to the law of conservation of energy Total energy remains constant

a) PE = 0

Total energy = PE + KE = 80 J

KE = 80 J PE = 80 J

b) At point B KE = 48 J

Total energy = PE + KE = 80 J

PE = 80 J KE = 80 – 48 = 32 J

c) Law of conservation of energy

__Page 165__**Solution 48**

No. of steps = 28

Height of each step = 28 cm

Total height = 20 x 28 = 560 cm = 5.6 m

Mass of student = 55 kg

g = 9.8 m/s^{2}

time = 5.4 s

Work done = m x g x h = 55 x 9.8 x 5.6 = 3018.4 J

**Solution 49**

Weight of box = 100 N

Height = 1.5 m

work done = m x g x h = 100 x 1.5 = 150 J

potential energy = m x g x h= 100 x 1.5 = 150 J

iii. weight of 4 boxes = 400 N

time = 1 minute = 60 s

work done = 400 x 1.5 = 600 J

**Solution 50**

a) Electrical energy to sound energy

b) Sound energy to electrical energy

c) Electrical energy to light (and heat) energy

d) Chemical energy to electrical energy to light

energy(and heat energy)

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