A New Approach to ICSE Physics Part 2 Class 10 Solutions.<\/a> Here we have given A New Approach to ICSE Physics Part 2 Class 10 Solutions Work, Power And Energy.<\/p>\nQuestion 1.<\/span><\/strong>
\n(a) Define work.
\n(b) What are the conditions for doing work
\n(c) State the mathematical expression for work.
\nAnswer:<\/strong><\/span>
\n<\/span><\/span>(a)<\/strong> \u201cWhen force is applied on the body and body moves (covers , some distance) in the direction of force, work is said to be done.\u201d<\/p>\nOr<\/strong><\/p>\n\u201cWork is said to be done, when a force or its component causes a displacement in its own direction.\u201d
\n(b)<\/strong>
\n(1) Force should be applied.
\n(2) Displacement of body should be there.
\n(3) Work = Force x displacement
\nW = F x S<\/p>\nQuestion 2.<\/span><\/strong>
\nIn which case work is done and why<\/strong> ?
\n<\/strong>(a) A man pushing a wall.
\n(b) A girl climbing a stair case
\n(c) A boy swimming in a tank.
\n(d) A man standing at a place and holding a suitcase in hand.
\n(e) A lady cooking food.
\n(f) A porter carrying a load on his head walking along a level road.
\n(g) A porter carrying a load and climbing upstairs.
\nAnswer:<\/strong><\/span><\/p>\n(a)<\/strong> 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.
\n(b)<\/strong> A girl climbing a staircase does work as component of force is in the direction of displacement.
\n(c)<\/strong> A boy swimming in a tank is doing work as force is applied in a direction opposite to the direction of displacement.
\n(d)<\/strong> The man is not doing work as displacement is zero.
\n(e)\u00a0<\/strong> A lady cooking food is doing no work as displacement of lady is zero.
\n(f)<\/strong> A porter is doing no work as the direction of his displacement is at right angle (90\u00b0) and force of gravity is downward (vertically)
\n(g)<\/strong> Porter does work as component of force is in direction of displacement.<\/p>\nQuestion 3<\/span><\/strong>
\nA man climbs a slope and another walks the same distance on a level road. Who does more work and why ?
\nAnswer:<\/strong><\/span>
\nThe 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.<\/p>\nQuestion 4.<\/span><\/strong>
\n(a) State the CGS and SI units of work.
\n(b) How is joule related to erg ?
\nAnswer:<\/strong><\/span>
\n(a)<\/strong> CGS unit of work is erg or gcm2<\/sup>s2
\n<\/sup>S.I. unit of work is joule or kgm2<\/sup>s2
\n<\/sup>(b)<\/strong> 1 J = 107<\/sup> ergs or 1 erg = 10-7<\/sup> g<\/p>\nQuestion 5.<\/span><\/strong>
\nDefine power. State two mathematical expressions for power.
\nAnswer:<\/strong><\/span>
\nWork :<\/strong> \u201cRate of doing work is called power.\u201d
\nTwo mathematical expressions for power are :
\n<\/strong>\\(\\mathbf{P}=\\frac{w}{t}\\)
\n\\(\\mathrm{P}=\\frac{\\mathrm{F} \\times \\mathrm{S}}{t}=\\mathrm{F} \\times \\frac{s}{t}=\\mathrm{F} \\times v\\)<\/p>\nQuestion 6.<\/span><\/strong>
\n(a) State the absolute unit of power in SI system.
\n(b) What is horsepower ? What is its magnitude in SI unit ?
\nAnswer:<\/strong><\/span>
\n(a) Absolute unit of power is watt.
\n(b) Horse power is unit of power used in engineering 1 H.P = 746 w .<\/p>\nQuestion 7.<\/span><\/strong>
\n(a) What is energy ? State and define SI unit of energy.
\n(b) Define potential energy. Give two examples of potential energy.
\n(c) Define kinetic energy. Give four examples of kinetic energy.
\nAnswer:<\/strong><\/span>
\n(a) Energy :<\/strong> \u201cCapacity of doing work\u201d
\nS.I. unit of energy is Joule
\n(b)Potential energy:<\/strong> \u201cEnergy posserred by a body by virtue of it position or configuration is called potential energy.\u201d
\nExample :<\/strong><\/p>\n\n- 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.<\/li>\n
- A stone kept at a height, when drop can break a plate of glass because of P.E. possessed by it<\/li>\n<\/ol>\n
(c) Kinetic energy:<\/strong> \u201cEnergy possessed by a body by virtue of its motion.\u201d<\/p>\nExamples :<\/strong><\/p>\n\n- A bullet though of very small mass but moving with high speed and hence kinetic energy can peneterate a body.<\/li>\n
- Running water of the river due its kinetic energy can rotate a turbine to produce electricity.<\/li>\n
- A trunk running at high speed possesses kinetic energy and when hits a body can damage it.<\/li>\n
- A shooting arrow possesses kinetic energy.<\/li>\n
- Blowing wind possesses K.E.<\/li>\n<\/ol>\n
Question 8.<\/span><\/strong>
\nWhat kind of energy is possessed by a body in the following cases ?
\n<\/strong>(a) A cocked-up spring and an air gun.
\n(b) A shooting arrow.
\n(c) A stone lying on the top of a housi.
\n(d) Water stored in the dam.
\n(e) An electron spining around the nucleus.
\n(f) A fish moving in water.
\nAnswer:<\/strong><\/span><\/p>\n(a)<\/strong> Potential energy, (stretched spring)
\n(b)<\/strong> Kinetic energy as arrow is in motion
\n(c)<\/strong> A stone lying on the top of a house has potential energy due to its position above the ground level.
\n(d)<\/strong> Water stored in dam has potential energy.
\n(e)<\/strong> An electron spining around the nucleus has kinetic energy.
\n(f)<\/strong> A fish moving in water has kinetic energy.<\/p>\nQuestion 9.<\/span>
\n<\/strong>(a) State the law of conservation of energy.
\n(b) Prove mathematically the law of conservation of energy.
\n(c) Explain how a freely swinging pendulum obeys the law of conservation of energy.
\n(d) Name six kinds of energy familiar to you.
\nAnswer:<\/strong><\/span>
\n(a) Law of conservation of energy :<\/strong> \u201cEnergy can neither be created nor destroyed.
\nThough it can be transformed from one form to other.\u201d
\n(b) Mathematical proof of law of conservation of energy :
\n<\/strong>At A Consider a body of mass m at A at a height h above the ground level.
\nP.E. = mgh K.E. = 0 at rest.
\n<\/p>\nP.E. + K.E. = mgh + O = mgh\u00a0 \u00a0 \u00a0…………….. (i)
\nLet it fall from A to B covering a distance x and still at (h-x) above ground.
\nIn doing so
\nv2<\/sup> – u2<\/sup> = 2 gx
\nv2<\/sup> – 0 = 2gx
\nv2<\/sup> = 2gx\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 …(a)
\nAt B P.E = mg (h – x)
\n
\nAT C P.E + K.E = 0 + mgh
\nP.E. + K.E. = mgh
\nThus we find sum of P.E. + K.E. at A, B, C remains same mgh
\nHence varifies the law of can servation of energy.
\n(c)<\/strong> Energy changes at B K.E. = O
\nat highest pt. vel. = O
\nP.E. = mgh
\n\u2234 the bob is at height h<\/p>\n
\nK.E. + P.E. = O + mgh = mgh\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 …(i)
\nAt 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.
\n\u2234 At A, P.E. = 0 and K.E. = mgh
\n\u2234 P.E. + K.E. = 0 + mgh = mgh\u00a0 \u00a0 ….(ii)
\nAt C : As the bob continues to move forward from B to C, its
\nvelocity = 0 but height increases to h
\n\u2234 At C K.E. = 0 and P.E. = mgh
\n\u2234 K.E..+ P.E. = 0 + mgh
\n= mgh\u00a0 …(iii)
\nThus we find that sum total of K.E. and P.E remains constant which is in accordance with conservation of energy.
\n(d) Six kinds of energy :<\/strong><\/p>\n\n- E. (wind energy)<\/li>\n
- Heat energy<\/li>\n
- Sound energy<\/li>\n
- Solar energy<\/li>\n
- Electrical energy<\/li>\n
- Nuclear energy<\/li>\n<\/ol>\n
Question 10.<\/span><\/strong>
\nState the energy changes taking place in the following cases:
\n<\/strong><\/p>\n(a) Glowing of a torch bulb
\n(b) A toy car is wound and then allowed to move on the floor
\n(c) A truck climbing up a hill
\n(d) Water in a dam rotates a turbine coupled to a generator
\n(e) An air gun is loaded and then fired
\n(f) A piece of magnesium burns in air
\n(g) Water freezes in the freezing chamber of a fridge
\n(h) A stone dropped from a cliff
\n(i) Food eaten by humans
\n(j) Exposure of photographic film in sunlight<\/p>\n
Answer:<\/strong><\/span><\/p>\n(a)<\/strong> Electric energy into heat light energy.
\n(b)<\/strong> Potential energy into mechanical energy or kinetic energy.
\n(c)<\/strong> Heat energy into mechanical (P.E.) energy
\n(d)<\/strong> E. of water into electrical energy.
\n(e)<\/strong> Potential energy of spring into K.E. of bullet (pallel)
\n(f)<\/strong> Chemical energy into heat energy.
\n(g)<\/strong> Electrical energy into mechanical energy (to run compresor)
\n(h)\u00a0<\/strong>Chemical energy into heat energy.
\n(i)<\/strong> Potential energy into kinetic energy.
\n(j)<\/strong> Light energy of Sun into chemical energy.<\/p>\nQuestion 11.<\/span><\/strong>
\nGive one example in each case<\/strong><\/p>\n(a)<\/strong> when heat energy changes into kinetic energy.
\n(b)<\/strong> when kinetic energy changes into heat energy.
\n(c)<\/strong> when sound energy changes into electric energy.
\n(d)<\/strong> when electric energy changes into sound energy.
\n(e)<\/strong> when light energy changes into chemical energy.
\n(f)<\/strong> when chemical energy changes into light energy.
\n(g)<\/strong> when electric changes into magnetic energy.
\n(h)<\/strong> when magnetic energy changes into electric energy.
\n(i)<\/strong> when potential energy changes into electric energy.
\n(j)<\/strong> when electric energy changes into potential energy.<\/p>\nAnswer:<\/strong><\/span><\/p>\n(a)<\/strong> In steam engine heat energy moves the wheels of engine and changes into kinetic energy.
\n(b)<\/strong> When we rub the palms of our hands fast they become warm hence K.E changes into heat energy.
\n(c)<\/strong> Sound energy of microphone changes into electrical energy.
\n(d)<\/strong> Electrical energy changes into sound energy while flowing through the speaker.
\n(e)<\/strong> Exposure of photographic film in Sun light.<\/p>\nOr<\/strong><\/p>\n\u00a0During photosynthesis light energy changes into chemical energy.
\n(f)<\/strong> Burning of match stick by friction.
\n(g)<\/strong> The electrical energy in an electromagnet changes into magnetic energy.
\n(h)<\/strong> Motion of magnet in the coil i.e. in generaters.
\n(i)<\/strong> When water stored in dam rotates the turbine to produce electricity.
\n(j)<\/strong> 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.<\/p>\nQuestion 12.<\/span><\/strong>
\nDefine kilowatt hour and convert it into joules
\nAnswer:<\/strong><\/span>
\nKilowatt hour :<\/strong> \u201cWhen 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.\u201d
\n1 KWH = 1000 W x 1h
\n1 KWH = 1000 W x (60 x 60) sec
\n= 1000 J\/S x 3600 s
\n= 100000J
\n1 KWH = 105<\/sup> J<\/p>\nQuestion 13.<\/span><\/strong>
\nDefine electron volt and express it in joule.
\nAnswer:<\/strong><\/span>
\nElectron volt :<\/strong> \u201cThe electric work done when an electron moves through an electric field at a potential difference of 1 volt.\u201d
\n1 electron volt (eV) = charge on 1 electron x 1 volt
\n= 1.6 x 10-19<\/sup>C x IV
\n= 1.6 x 10-19<\/sup> J\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 [\u2235\u00a01C x IV = 1J)
\n1 [eV] = 1.6 x 1019<\/sup> J<\/p>\nMultiple Choice\u00a0 Questions<\/strong><\/span><\/p>\nTick (\u2713 ) the most appropriate option.<\/strong><\/p>\n1: A boy drags a load \u2018L\u2019 along horizontal plane AB by applying a force F. The boy does<\/strong><\/p>\n<\/strong>
\n(a) no work
\n(b) some positive work<\/strong>
\n(c.) negative work
\n(d) none of these<\/p>\n2. The SI unit of work is joule. It is expressed in terms of mass, length and time as<\/strong>
\n(a) kg m2<\/sup>s-3<\/sup><\/strong>
\n(b) kg m2<\/sup>s-2<\/sup>
\n(c) kg2 m2<\/sup>s-2<\/sup>
\n(d) kg m2<\/sup>s-2<\/sup><\/p>\n3. The SI unit of power is watt. It is expressed in terms of mass, length and time as:<\/strong>
\n(a) kg m2<\/sup>s-3<\/sup><\/strong>
\n(b) kg ms-3<\/sup>
\n(c) kg2\u00a0m2<\/sup>s-2<\/sup>
\n(d) kg ms-2<\/sup><\/p>\n4. A stone resting on the roof ofa building has<\/strong>
\n(a) potential energy<\/strong>
\n(b) gravitational energy
\n(c) kinetic energy
\n(d) none of these<\/p>\n5. A falling raindrop has :
\n<\/strong>(a) only kinetic energy
\n(b) only potential energy
\n(c) both kinetic and potential energy<\/strong>
\n(d) none of these<\/p>\n6.<\/strong>\u00a0One horse power is equal to :<\/strong>
\n(a) 764 W
\n(b) 746 W<\/strong>
\n(c) 700 W
\n(d) 1000 W<\/p>\n7.<\/strong>\u00a0One electron volt is equal to :
\n<\/strong>(a) 6 x 10-17<\/sup> J
\n(b) 6.1 x 10-19<\/sup>\u00a0J
\n(c) 6 x 10-19<\/sup> J\u00a0<\/strong>
\n(d) 1.6 x 10-10<\/sup> J<\/p>\n8. Kilowatt hour is the commercial unit of:
\n<\/strong>(a) electric power
\n(b) electric energy<\/strong>
\n(c) electric force
\n(d) none of these<\/p>\n9. Power is the product of:<\/strong>
\n(a) force and velocity<\/strong>
\n(b) force and displacement
\n(c) force and acceleration
\n(d) force and time<\/p>\n10.<\/strong>\u00a0An aeroplane is flying at an altitude of 10,000 m at a speed of 300 km\/hour. The aeroplane at this height has :
\n<\/strong>(a) only kinetic energy
\n(b) only potential energy
\n(c) both kinetic and potential energy<\/strong>
\n(d) zero kinetic and potential energy<\/p>\n11. Kilocalorie is the amount of heat required to raise the temperature of:<\/strong>
\n(a) one gram of water through 1\u00b0C
\n(b) 1 kg of water through 100\u00b0
\n(c)\u00a0 one kg of water through 1\u00b0C<\/strong>
\n(d) 1 kg of water through 10\u00b0C Arts.<\/p>\n12. When a flash light is switched on the electric energy
\n<\/strong>(a) directly changes to light energy
\n(b) first changes to light energy and then to heat energy<\/strong>
\n(c) first changes to heat energy and then to light energy
\n(d) none of above<\/p>\n13. A pendulum is swinging freely. The bob ofpendulum has:
\n<\/strong>(a) maximum K.E. at its extreme positions
\n(b) minimum K.E. at its mean position
\n(c) maximum K.E. at its mean position<\/strong>
\n(d) both (b) and (c)<\/p>\n14. A pendulum is oscillating freely. Its bob has :
\n<\/strong>(a) only kinetic energy
\n(b) maximum kinetic energy at extreme position
\n(c) maximum potential energy at its mean position
\n(d) a constant energy which is the sum of potential and kinetic energy<\/strong><\/p>\n15. A ball of mass m is dropped from height \u2018h \u2019.
\n<\/strong>(a) Potential energy of the ball at ground level is mgh.
\n(b) Potential energy of the ball at height h is mgh.<\/strong>
\n(c) kinetic energy of the ball at ground level is mgh
\n(d) both (b) and (c)<\/p>\n\u00a0Numerical Problems on Work, Power & Energy<\/strong><\/span><\/p>\nPractice Problems 1
\n<\/strong><\/p>\nQuestion 1.<\/span><\/strong>
\nA girl of mass 50 kg climbs a flight of 100 stairs each measuring 0.25 m in height, in 20s. Find<\/strong>