# Work Power Energy Class-10 Goyal Brothers ICSE Physics Solutions Ch-2

**Work Power Energy ICSE Class-10 Goyal Brothers** Physics Solutions Chapter-2 **Work Power Energy** . We Provide Step by Step Answer of Exercise, MCQs, Numericals Practice Problem Questions of Exercise-1** Work Power Energy ICSE Class-10**** **. Visit official Website **CISCE ** for detail information about ICSE Board Class-10 Physics.

## Work Power Energy Class-10 Goyal Brothers ICSE Physics Solutions Ch-2

**-: Select Exercise :-**

**Exe-1 Work Power Energy Class-10 Goyal Brothers ICSE Physics Solutions Ch-2**

**Page-24,25**

**Question 1.**

(a) Define work.

(b) What are the conditions for doing work

(c) State the mathematical expression for work.

**Answer 1:**

**(a)** “When force is applied on the body and body moves (covers , some distance) in the direction of force, work is said to be done.”

**Or**

“Work is said to be done, when a force or its component causes a displacement in its own direction.”

**(b)**

(1) Force should be applied.

(2) Displacement of body should be there.

(3) Work = Force x displacement

W = F x S

**Question 2.**

**In which case work is done and why** **?
**(a) A man pushing a wall.

(b) A girl climbing a stair case

(c) A boy swimming in a tank.

(d) A man standing at a place and holding a suitcase in hand.

(e) A lady cooking food.

(f) A porter carrying a load on his head walking along a level road.

(g) A porter carrying a load and climbing upstairs.

**Answer 2:**

**(a)** A man pushing a wall does no work as wall does not move from its place and there is no displacement of wall in the direction of force.

**(b)** A girl climbing a staircase does work as component of force is in the direction of displacement.

**(c)** A boy swimming in a tank is doing work as force is applied in a direction opposite to the direction of displacement.

**(d)** The man is not doing work as displacement is zero.

**(e) ** A lady cooking food is doing no work as displacement of lady is zero.

**(f)** A porter is doing no work as the direction of his displacement is at right angle (90°) and force of gravity is downward (vertically)

**(g)** Porter does work as component of force is in direction of displacement.

**Question 3 Work Power Energy : Goyal Brothers ICSE Physics Class-10 **

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

**Answer 3:**

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.

**Question 4.**

(a) State the CGS and SI units of work.

(b) How is joule related to erg ?

**Answer 4:**

**(a)** CGS unit of work is erg or gcm^{2}s^{2
}S.I. unit of work is joule or kgm^{2}s^{2
}**(b)** 1 J = 10^{7} ergs or 1 erg = 10^{-7} g

**Question 5.**

Define power. State two mathematical expressions for power.

**Answer 5:**

**Work :** “Rate of doing work is called power.”

**Two mathematical expressions for power are :**

**Question 6.**

(a) State the absolute unit of power in SI system.

(b) What is horsepower ? What is its magnitude in SI unit ?

**Answer 6:**

(a) Absolute unit of power is watt.

(b) Horse power is unit of power used in engineering 1 H.P = 746 w .

**Question 7.**

(a) What is energy ? State and define SI unit of energy.

(b) Define potential energy. Give two examples of potential energy.

(c) Define kinetic energy. Give four examples of kinetic energy.

**Answer 7:**

**(a) Energy :** “Capacity of doing work”

S.I. unit of energy is Joule

**(b)Potential energy:** “Energy posserred by a body by virtue of it position or configuration is called potential energy.”

**Example :**

- A key to works on P.E. when we wind the key its shape changes and on unwinding this energy is used by to do work.
- A stone kept at a height, when drop can break a plate of glass because of P.E. possessed by it

**(c) Kinetic energy:** “Energy possessed by a body by virtue of its motion.”

**Examples :**

- A bullet though of very small mass but moving with high speed and hence kinetic energy can peneterate a body.
- Running water of the river due its kinetic energy can rotate a turbine to produce electricity.
- A trunk running at high speed possesses kinetic energy and when hits a body can damage it.
- A shooting arrow possesses kinetic energy.
- Blowing wind possesses K.E.

**Question 8. Work Power Energy : Goyal Brothers ICSE Physics Class-10 **

**What kind of energy is possessed by a body in the following cases ?
**(a) A cocked-up spring and an air gun.

(b) A shooting arrow.

(c) A stone lying on the top of a housi.

(d) Water stored in the dam.

(e) An electron spining around the nucleus.

(f) A fish moving in water.

**Answer 8:**

**(a)** Potential energy, (stretched spring)

**(b)** Kinetic energy as arrow is in motion

**(c)** A stone lying on the top of a house has potential energy due to its position above the ground level.

**(d)** Water stored in dam has potential energy.

**(e)** An electron spining around the nucleus has kinetic energy.

**(f)** A fish moving in water has kinetic energy.

**Question 9.
**(a) State the law of conservation of energy.

(b) Prove mathematically the law of conservation of energy.

(c) Explain how a freely swinging pendulum obeys the law of conservation of energy.

(d) Name six kinds of energy familiar to you.

**Answer 9:**

**(a) Law of conservation of energy :**“Energy can neither be created nor destroyed.

Though it can be transformed from one form to other.”

**(b) Mathematical proof of law of conservation of energy :**

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

v^{2} – u^{2} = 2 gx

v^{2} – 0 = 2gx

v^{2} = 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.

**(c)** Energy changes at B K.E. = O

at highest pt. vel. = O

P.E. = mgh

∴ the bob is at height h

K.E. + P.E. = O + mgh = mgh …(i)

At A: As the bob moves towards A from B its height goes on decreasing and h at A is zero, but its vol. goes on increasing.

∴ At A, P.E. = 0 and K.E. = mgh

∴ P.E. + K.E. = 0 + mgh = mgh ….(ii)

At C : As the bob continues to move forward from B to C, its

velocity = 0 but height increases to h

∴ At C K.E. = 0 and P.E. = mgh

∴ K.E..+ P.E. = 0 + mgh

= mgh …(iii)

Thus we find that sum total of K.E. and P.E remains constant which is in accordance with conservation of energy.

**(d) Six kinds of energy :**

- E. (wind energy)
- Heat energy
- Sound energy
- Solar energy
- Electrical energy
- Nuclear energy

**Question 10. Work Power Energy : Goyal Brothers ICSE Physics Class-10 **

**State the energy changes taking place in the following cases:
**

(a) Glowing of a torch bulb

(b) A toy car is wound and then allowed to move on the floor

(c) A truck climbing up a hill

(d) Water in a dam rotates a turbine coupled to a generator

(e) An air gun is loaded and then fired

(f) A piece of magnesium burns in air

(g) Water freezes in the freezing chamber of a fridge

(h) A stone dropped from a cliff

(i) Food eaten by humans

(j) Exposure of photographic film in sunlight

**Answer 10: **

**(a)** Electric energy into heat light energy.

**(b)** Potential energy into mechanical energy or kinetic energy.

**(c)** Heat energy into mechanical (P.E.) energy

**(d)** E. of water into electrical energy.

**(e)** Potential energy of spring into K.E. of bullet (pallel)

**(f)** Chemical energy into heat energy.

**(g)** Electrical energy into mechanical energy (to run compresor)

**(h) **Chemical energy into heat energy.

**(i)** Potential energy into kinetic energy.

**(j)** Light energy of Sun into chemical energy.

**Question 11.**

**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 11:**

**(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.

**Question 12. Work Power Energy : Goyal Brothers ICSE Physics Class-10 **

Define kilowatt hour and convert it into joules

**Answer 12:**

**Kilowatt hour :** “When an electric power of one kilowatt flows through a conductor for one hour, then electrical energy which flows through the conductor is one kilowatt hour.”

1 KWH = 1000 W x 1h

1 KWH = 1000 W x (60 x 60) sec

= 1000 J/S x 3600 s

= 100000J

1 KWH = 10^{5} J

**Question 13.**

Define electron volt and express it in joule.

**Answer 13:**

**Electron volt :** “The electric work done when an electron moves through an electric field at a potential difference of 1 volt.”

1 electron volt (eV) = charge on 1 electron x 1 volt

= 1.6 x 10-^{19}C x IV

= 1.6 x 10^{-19} J [∵ 1C x IV = 1J)

1 [eV] = 1.6 x 10^{19} J

** ****Multiple Choice Questions**

**Work Power Energy**

** Class-10 Goyal Brothers ICSE Physics Solutions Ch-2 **

**1: A boy drags a load ‘L’ along horizontal plane AB by applying a force F. The boy does
**

(a) no work

**(b) some positive work**

(c.) negative work

(d) none of these

**2. The SI unit of work is joule. It is expressed in terms of mass, length and time as**

**(a) kg m ^{2}s^{-3}**

(b) kg m

^{2}s

^{-2}

(c) kg2 m

^{2}s

^{-2}

(d) kg m

^{2}s

^{-2}

**3. The SI unit of power is watt. It is expressed in terms of mass, length and time as:**

**(a) kg m ^{2}s^{-3}**

(b) kg ms

^{-3}

(c) kg2 m

^{2}s

^{-2}

(d) kg ms

^{-2}

**4. A stone resting on the roof ofa building has**

**(a) potential energy**

(b) gravitational energy

(c) kinetic energy

(d) none of these

**5. A falling raindrop has :
**(a) only kinetic energy

(b) only potential energy

**(c) both kinetic and potential energy**

(d) none of these

**6.** **One horse power is equal to :**

(a) 764 W

**(b) 746 W**

(c) 700 W

(d) 1000 W

**7.** **One electron volt is equal to :
**(a) 6 x 10-

^{17}J

(b) 6.1 x 10-

^{19}J

**(c) 6 x 10-**

^{19}J(d) 1.6 x 10-

^{10}J

**8. Kilowatt hour is the commercial unit of:
**(a) electric power

**(b) electric energy**

(c) electric force

(d) none of these

**9. Power is the product of:**

**(a) force and velocity**

(b) force and displacement

(c) force and acceleration

(d) force and time

**10.** **An aeroplane is flying at an altitude of 10,000 m at a speed of 300 km/hour. The aeroplane at this height has :
**(a) only kinetic energy

(b) only potential energy

**(c) both kinetic and potential energy**

(d) zero kinetic and potential energy

**11. Kilocalorie is the amount of heat required to raise the temperature of:**

(a) one gram of water through 1°C

(b) 1 kg of water through 100°

**(c) one kg of water through 1°C**

(d) 1 kg of water through 10°C Arts.

**12. When a flash light is switched on the electric energy
**(a) directly changes to light energy

**(b) first changes to light energy and then to heat energy**

(c) first changes to heat energy and then to light energy

(d) none of above

**13. A pendulum is swinging freely. The bob ofpendulum has:
**(a) maximum K.E. at its extreme positions

(b) minimum K.E. at its mean position

**(c) maximum K.E. at its mean position**

(d) both (b) and (c)

**14. A pendulum is oscillating freely. Its bob has :
**(a) only kinetic energy

(b) maximum kinetic energy at extreme position

(c) maximum potential energy at its mean position

**(d) a constant energy which is the sum of potential and kinetic energy**

**15. A ball of mass m is dropped from height ‘h ’.
**(a) Potential energy of the ball at ground level is mgh.

**(b) Potential energy of the ball at height h is mgh.**

(c) kinetic energy of the ball at ground level is mgh

(d) both (b) and (c)

**Numerical Problems on Moment of Force**

**Work Power Energy**

**Class-10 Goyal Brothers ICSE Physics Solutions Ch-**

**Page-27,28,29.30,31**

**Practice Problems 1
**

**Question 1. Work Power Energy : Goyal Brothers ICSE Physics Class-10 **

**A girl of mass 50 kg climbs a flight of 100 stairs each measuring 0.25 m in height, in 20s. Find**

(a) force acting on the girl

(b) work done by the girl

(c) gain in potential energy

(d) power in (1) watts (2) Horse

[Taking g = 10 ms-^{2}, 1 HP = 750 W]

**Answer 1:**

mass = 50 kg g = 10 ms^{-1
}**(a)** Force acting on the girl F = mg

F = 50 x 10 = 500 N

**(b)** Work done by girl W = F x S

W = 500 x 25 = 12500 J

**(c)** Gain in pot. energy = mgh

50 x 10 x 25 = 12500 J

**Or**

Gain in P.E. = Workdone = 12500 J

**Question 2.
**

**A load of 220 kg is vertically pulled up by a crane through a vertical height of 16 m in 40 s. Calculate**

(1) Force acting in the upward direction

(2) Total work done

(3) Horse power of the engine pulling the rope [Take g = 9.8 ms-

^{2}; 1 HP = 750 w]

**Answer 2:**

Mass m = 220 kg g = 9.8 ms

^{-2}h= 16 m t = 40 s

**(1)**Force F = mg

= 220 x 9.8 = 2156 N

**(2)** Total work done = W = F x S

W = 2156 x 16 = 3496 J

**(3)
**

**Practice Problems 2**

**Question 1.**

A work of 1000 J is done on a body in 4 s, such that a displacement of 20 m is caused. Calculate

(a) force (b) power

**Answer 1:
**

**Question 2.**

What force must be applied to a body through a distance of 10 m, such that it does a work of 4000 J. If the mass of the body is 20 kg, what is the acceleration of the body ?

**Answer 2:**

**Question 3.**

An engine of power 200 W, operates for 4 s. Find the work done by the engine. If the force developed by the engine is 100 N calculate the maximum displacement caused.

**Answer 3:**

p = 200 W t = 4 s

workdone = p x t

200 x 4 = 800 J

**Practice Problems 3**

**Question 1. Work Power Energy : Goyal Brothers ICSE Physics Class-10 **

Calculate the horse power of the motor of an elevator, which can carry 10 persons of average mass 60 kg

through a vertical height of 20 m in 30 s. [Take g = 10 N/ kg]

**Answer 1:**

Total mass of 10 persons = 60 x 10 = 600 kg

F = mg = 600 x 10 = 6000 N

displacement = h = 20 m t = 30 s

work done = w = F x h = 6000 x 20 J

**Question 2.
**Calculate the power of an electric pump in horse power, which can lift 2000 m

^{3}of water from a depth of 20 m in 25 minutes. [Take g = 10 ms

^{2}and 1 ml of water = 10

^{3}kg]

**Answer 2:**

1 m

^{3}of water = 10

^{3}kg

∴ 2000 m

^{3}of water has mass = m = 20 00 x 10

^{3}kg

wt. = force = mg = 2000 x 1o

^{3}x 10N

Displacement = depth = 20 m

work done = F x S t = 25 min = (25 x 60) S

**Question 3.**

Calculate the height through which a crane can lift a load of 4 t, when its motor of 4 HP operates for 10 s.[Take g = 10 ms^{-2}]

**Answer 3:**

h = 2 t= 10 s

mass = m = At = 4 x 1000 = 4000 kg

force F = mg = 4000 x 10 N

work done = F x h

= 4000 x 10 h

Power = 4HP = 4 x 750 w

**Question 4.**

For how long must an electric motor pump of2 HP operate, so as to pump 5 m^{3} of water from a depth of 15 m.[Take g = 10 N/kg, 1 m^{3} of water = 10^{3} kg]

**Answer 4:**

t = ?

power = 2HP = 2 x 750w

p x t = work done

2 x 750x; = (mass x g) x displacement

**Practice Problems 4**

**Question 1. Work Power Energy : Goyal Brothers ICSE Physics Class-10 **

An electric pump is 60% efficient and is rated 2 HP. Calculate the maximum amount of water it can lift through a height of 5 m in 40 s. [Take g = 10 ms^{-2} and 1 HP = 750 W]

**Answer 1:**

Power of lift pump = 2 HP

Question 2.

Calculate the time for which a motor pump of 10 HP and efficiency 80% must be switched on, so as to pump 20 m^{3 }of water through a vertical height of 20 m.[Density of water = 1000 kg m^{3}; g = 10 ms^{-2}; 1 HP = 750 W]

**Answer 2:**

Power of motor pump =10 HP

**Question 3.**

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 3:
**

**Practice Problems 5**

**Question 1.**

A compressed spring is held near a small toy car of mass 0.15 kg. On the release of the spring, the toy car moves forward with a velocity of 10 ms’. Find the potential energy of the spring.

**Answer 1:**

mass of car m = 0.15 kg

velocity y = 10 ms^{-1}

**Question 2.**

A catapult throws a stone of mass 0.10 kg with a velocity of 30 ms^{-1}. If 25% of the RE. of the elastic band is wasted during transmission, find the magnitude of the potential energy.

**Answer 2:**

Let pot. energy = RE.

P.E. wasted = 25%

P.E. used 100—25 = 75%

**Practice Problems 6
**

**Question 1. Work Power Energy : Goyal Brothers ICSE Physics Class-10**

A body of mass 20 kg is moving with a velocity of 1 ms

^{-1 }Another body B of mass 1 kg is moving wills a velocity of 20 ms

^{-1}. Find the ratio of kinetic energy of A and B.

**Answer 1:**

**Question 2.**

A bullet of mass 0.2 kg, moving with a velocity of 200 ms^{-1 }, strikes a stationary wooden target of mass 5 kg. If all the energy is transferred to the wooden target, calculate the velocity with which the target towards direction.

**Answer 2:**

Let the other body (B) moves with kinetic energy of bullet = K.E. of wooden target (B)

**Practice Problems 7**

**Question 1.**

A body of mass m has a velocity v. If the mass of the body increases 81 times, but the kinetic energy remains same, calculate the new velocity.

**Answer 1:**

Question 2.

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 2:**

**Practice Problems 8**

**Question 1.
**(a) Force of gravity acting on the barrel. (Take g — 10 ms

^{-2})

(b) Work done by the force in pulling body along the inclined plane

**(c)**Work done against the force of gravity.

**Answer 1:**

**(a)**Force of gravity F mg

F = 2.5 x 10 = 25

**(b)**Work done in pulling the body along the inclined plane = F x displacement

= 40x 7.5 J

W = 300 J

**(c)**Work done against force of gravity

**Question 2.**

**Adjacent diagram shows a body of mass 5 kg pulled up an inclined plane by a
**

**force of 30 N.**

(a) Calculate forced by gravity acting on body. (Take g = 10 ms

^{-2})

(b) Work done by the force in pulling body along the inclined plane.

(c) Work done against the force of gravity.

**Answer 2:**

Mass of body = 5 kg

**(a)**Force of gravity acting on the body

F = mg

F = 5 x 10 = 50 N

**(b)**Work done by force in pulling the body along the inclined plane = F x displacement

W = 30 N x 5 m

_{ }= 150 J

**(c)**Work done against force of gravity

= (mg) x BC

**Practice Problems 9**

**Question 1. Work Power Energy : Goyal Brothers ICSE Physics Class-10 **

A scooter develops a power of 1 HP while running at 36 km hr^{-1}. Calculate the force generated by its engine.

**Answer 1:**

**Question 2.**

The engine of a car develops a power of 5 HP and force 500 N while running a uniform speed S. Calculate the value of S.

**Answer 2:**

**Practice Problems 10.**

**Question 1.**

The heart of a normal person beats 72 times in a minute and does a work of 1 joule per beat. What is power of the heart ?

**Answer 1:**

Work done per beat = 1 J

work done for 72 beats = 72 J

**Question 2.**

The heart of a deer chased by a tiger beats 200 times in a minute and does a work of 1.4 joules per beat. What is the power of heart ?

**Answer 2:**

Heart beats per minute = 200

**Practice Problems 11**

**Question 1.**

A beam of electrons has an energy of 1 joule. How many electrons are in the beam ? [1 eV = 1.6 x 10^{-19}]

**Answer 1:**

Let there are n electrons in beam

Energy of n electrons in beam = 1 J

**Question 2.
**An accelerated electron has energy of 9.6 x 10

^{-18}J. Express the energy in electron volts (eV)

**Answer 2:**

**Practice Problems 12**

**Question 1. Work Power Energy : Goyal Brothers ICSE Physics Class-10 **

Calculate the kinetic energy of a body of mass 100 g and having a momentum of 20 kg ms^{-1}.

**Answer 1:**

Question 2.

Calculate the kinetic energy of a body of mass 5 kg momentum 50 kg ms^{-1
}**Answer 2:**

Mass m = 5 kg

move (Questions from ICSE Examination Papers) (Questions from ICSE Examination Papers) mentum p = mv = 50 kg ms^{-1}

**Practice Problems 13
**

**Question 1.**

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 1:**

Kinetic energy = P.E.

**Question 2.**

A lead pallet of mass 10 g leaves an air gun with a velocity of 40 ms^{-1 }. What is the magnitude of potential energy stored by its spring?

**Answer 2:**

**-: End of ****Work Power Energy ****: Goyal Brothers ICSE Physics Class-10 Solution :-**

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