Work, Power and Energy

 Important question answer (Chapter Revision) 

Q1. A man climbs a slope and another walks the same distance on a level road. Who does more work and why ?

Answer: The man who walk on a level road does no work as he is walking at right angle to the direction of gravitational force. Hence the man climbs a slope is doing more work.

Q2. State the CGS and SI units of work.

Answer : CGS unit of work is erg or gcm2s2

S.I. unit of work is joule or kgm2s2

1 J = 107 ergs or 1 erg = 10-7 g

Q3. What is horsepower ? What is its magnitude in SI unit ?

Answer : Horse power is unit of power used in engineering 1 H.P = 746 w. 

Q4. Prove mathematically the law of conservation of energy.

Answer : Law of conservation of energy : “Energy can neither be created nor destroyed. Though it can be transformed from one form to other.”

At A Consider a body of mass m at A at a height h above the ground level.

P.E. = mgh K.E. = 0 at rest.

P.E. + K.E. = mgh + O = mgh …………….. (i)

Let it fall from A to B covering a distance x and still at (h-x) above ground.

In doing so

v2 – u2 = 2 gx

v2 – 0 = 2gx

v2 = 2gx …(a)

At B P.E = mg (h – x)

AT C P.E + K.E = 0 + mgh

P.E. + K.E. = mgh

Thus we find sum of P.E. + K.E. at A, B, C remains same mgh

Hence varifies the law of can servation of energy.

Q5. Give one example in each case

(a) when heat energy changes into kinetic energy.

(b) when kinetic energy changes into heat energy.

(c) when sound energy changes into electric energy.

(d) when electric energy changes into sound energy.

(e) when light energy changes into chemical energy.

(f) when chemical energy changes into light energy.

(g) when electric changes into magnetic energy.

(h) when magnetic energy changes into electric energy.

(i) when potential energy changes into electric energy.

(j) when electric energy changes into potential energy.

Answer :

(a) In steam engine heat energy moves the wheels of engine and changes into kinetic energy.

(b) When we rub the palms of our hands fast they become warm hence K.E changes into heat energy.

(c) Sound energy of microphone changes into electrical energy.

(d) Electrical energy changes into sound energy while flowing through the speaker.

(e) Exposure of photographic film in Sun light. Or During photosynthesis light energy changes into chemical energy.

(f) Burning of match stick by friction.

(g) The electrical energy in an electromagnet changes into magnetic energy.

(h) Motion of magnet in the coil i.e. in generaters.

(i) When water stored in dam rotates the turbine to produce electricity.

(j) During the pumping of water in an overhead tank by an electric motor pump, the electrical energy changes into kinetic energy of water. The kinetic energy of water then changes into potential energy.

Q6. A spring is kept compressed by a toy car of mass 100 g. On releasing the pressure the car moves out with a speed of 0.5 ms-1. Calculate the potential energy of the compressed spring.

Answer : 

Kinetic Energy = P. E. 

1/2 mv2 = 1/2 × 100/1000kg × 0.5 × 0.5

1/2 × 25/100 = 12.5/1000 = 0.0125 J

P. E. = 0.0125J

P. E. of compressed spring = K. E. 

Q7. An accelerated electron has energy of 9.6 x 10-18 J. Express the energy in electron volts (eV)

Answer : 

Q8. A body P has KE energy E. Another body Q, whose mass is 9 times than P, also has kinetic energy E. Calculate the ratio of velocities of P and Q.

Answer : 

Q9. In a hydroelectric power station, 1000 kg of water is allowed to drop a height of 100 in in ¡ s. If the conversion of potential energy to electric energy is 60%, calculate the power output.

[Take g = 10 ms-2]

Answer : 

Q10. The bob of a simple pendulum is imparted a velocity of 5m s-1 when it is at its mean position. To what maximum vertical height will it rise on reaching at its extreme position if 60% of its energy is lost in overcoming the friction of air?

(Take g = 10 m s-2).

Answer : 

Total kinetic energy at mean position = 1 / 2 mv2

= 1 / 2 m × 52

Energy lost = 1 / 2 m × 52 × 60 / 100

Energy available = 1 / 2 m × 52 × 40 / 100

According to energy conservation

1 / 2 m × 52 × 40 / 100 = mgh

5 = 10 × h

h = 0.5m

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