CHP # 6 ORK AND ENERGY Q.1 Define work and is uni? A. ORK I can be define as when we applied a force on a body and he body covers a disance in he direcion of force, hen we say ha work is done. I is a scalar quaniy. I is denoed by. I can also be defined as he produc of force and displacemen in he direcion of force is called work. Mahemaically displaceme n ork Force F S hen here is an angle beween he force and displacemen, hen FS cos The SI uni of work is joule. Joule I is defined as he work done is said o be one joule if a force of one Newon is applied o displace an objec hrough one meer in he direcion of force. Mahemaically 1J 1N 1m J N m Q. ha is energy? rie is differen ypes. A. Energy The abiliy or capaciy of a body o do work is called energy. I is a scalar quaniy. I is denoed by E. is SI uni is joule. Some common ypes of energy are; (1) Mechanical energy The energy due o which he differen pars of he machines are running is called mechanical energy. () Hea energy The energy which flows from a hoer body owards a colder body due o emperaure difference is called hea energy. (3) Elecrical energy The energy associaed wih he charges in a conducor is called elecrical energy. (4) Chemical energy The energy presen in molecules or compounds o hold heir bond formaion is called chemical energy. hen he bond breaks his energy is released in he form of hea energy.
(5) Solar energy The energy obained from he sun is called solar energy. I is a big source of energy. Q.3 ha is kineic energy? Derive is mahemaical expression. A. Kineic energy The energy posses by a body due o virue of is moion are called kineic energy. I is denoed by. Mahemaically 1 mv Proof Consider a body of mass m is moving wih an iniial velociy of v on a horizonal surface. I comes o res due o he fricion force f afer Covering a disance of s. The work done agains F on he objec is given as; F S ----------- (1) As we know from Newon s second law, F ma Also from hird equaion of moion, as v f v i By puing values we ge ( a) S 0 v v S a Now pu he value of F and s in equaion (1) v ma a 1 mv This work done is sored in he body as kineic energy, so 1 mv
Q.4 ha is poenial energy? Derive is mahemaical expression. A. Poenial energy The energy posses by a body due o virue of is posiion or Configuraion is called poenial energy. I is denoed by E. Mahemaically E mgh Proof Consider a body of mass m is lying a a disance h from he ground. The force acing on he body from he earh is; F w mg Le he body is lifed from heigh h. The work done by he force in lifing his body hrough he heigh h is F h Bu F mg mgh This work done is sored in he body as poenial energy, so E mgh Q.5 Sae he law of conservaion of energy wih examples? A. Law of conservaion of energy This law sae ha Energy can neiher be creaed nor desroyed in any process, bu only can be change from one form o anoher form. For examples (1) The chemical energy sored in he food is convered ino hea energy as a resul of digesion in he body. This hea energy keeps our body warm and enables us o do work. () In dams he sored poenial energy of waer due o heigh is convered ino kineic energy o run he urbine for producing elecrical energy. Q.6 How elecrical energy can be produced? A. In he elecric power saion he energy of a fuel is used o boil waer. The ligh pressure seam is used o urn a urbine, which urns a dynamo o generae elecriciy.
Q.7 Define power and is uni? A. Power The ime rae of doing work is called power. I is a scalar quaniy. I is denoed by P. Mahemaically work power ime E Uni of power The SI uni of power is wa and can be define as he power of he body which does a work of one joule in one second ime. Mahemaically 1J 1wa 1sec Horse power is also a uni of he power which is saed as; 1hp 746was Concepual Quesions Q.1 The mahemaical relaion for work and power are given by FS using he above wo relaions derive a relaion beween power and force. Soluion Given ha ---------- (1) FS By puing he value of in equaion (1) we ge FS As we know ha S So F This is he required relaion beween power and force.
Q. ha ype of energy conversion can be observed in waer dams? A. In dams he sored poenial energy of waer due o heigh is convered ino kineic energy o run he urbine for producing elecrical energy. Q.3 ha happen o he kineic energy of a bulle when i peneraes ino he sand bags? A. hen he bulle hi he sand bags, is kineic energy is used in doing work o penerae cerain disance in sand bag before coming o res. Q.4 A meeor eners ino earh s amosphere and burns. ha happen o is kineic energy? A. The meeor eners ino earh s amosphere wih very high velociy. i burns due o he fricion force of he air on i. As a resul he kineic energy of he meeor is convered ino hea energy. Q.5 ha do you undersand by he erm energy ransformaion suppor your answer by giving wo examples? A. Energy can be ransferred from one form o anoher. For examples (1) The chemical energy sored in he food is convered ino hea energy as a resul of digesion in he body. This hea energy keeps our body warm and enables us o do work. () In dams he sored poenial energy of waer due o heigh is convered ino kineic energy o run he urbine for producing elecrical energy. Q.6 hy he efficiency of an engine can no be 100%? A. Efficiency I can be as he raio of he work done by he machine o he work done on he machine is called efficiency. Or The raio of he oupu o he inpu of a machine is called efficiency. I is always aken in percenage. Mahemaically oupu Efficiency 100% inpu The efficiency of a machine can no be 100% because due o fricion force some of he work done on he machine is losses. Q.7 Sae he energy changes aking place in each of he following cases. a) The bulb of a orch glows, when i is swiched on b) The burning of he fuels. A. The energy changes aking place as; a) hen he orch is swiched on, he chemical energy of he cell is convered ino elecrical energy, hen he bulb of he orch changes his elecrical energy ino ligh and hea energy.
b) The fuels are basically coal, oil, gas or wood. These maerials are made from he aoms bonded ogeher. hen i is burn he sored chemical energy is convered ino hea and ligh energy. Q.8 ha kind of energy is possessed in he following siuaion? a) A sone lying on he op of a roof. b) A flying aero plane. c) A wound up spring of a oy car. d) A speeding rain. A. The energy possessed can be saed as; (a) The sone lying on he op of he roof has graviaion poenial energy. (b) A flying aero plane has kineic energy due o moion and poenial energy due o is posiion. (c) A wound up spring of a oy has elasic poenial energy. (d) A speeding rain has kineic energy. Q.9 In each of he following siuaion he sysem consis of a ball and earh. Describe he work done and changes in energy forms. a) You hrough a ball horizonally b) The horizonally hrown by a fielder c) A ball is hrown verical and i comes o res a op of is fligh. d) The ball falls back o earh, where you cach. A. The work done and energy changes can be saed as; (a) hen we hrow a ball horizonally, i possesses kineic energy due o is moion. This kineic energy of he ball is used in doing work. (b) The horizonally hrown ball by he fielder possessed kineic and graviaion poenial energy. The kineic energy is used o do work agains air resisance while he graviaion poenial energy remains consan. (c) hen a ball is hrown verically, work is done agains graviy. This work done is sored in he ball as graviaion poenial energy. (d) hen he ball falls back o earh he graviaion poenial energy change o kineic energy o do work. Q.10 Sae weaher he following objecs possess KE, PE or boh. a) A man climbing a hill b) A saionary sone lying a he op of a hill. c) A flying bird A. a) A man climbing a hill possessed boh kineic and poenial energy. b) A saionary sone lying a he op of a hill possess poenial energy. c) A flying bird possessed boh kineic and poenial energy. Q.11 hich would have a greaer effec on he K.E of an objec, doubling he mass or doubling he velociy? A. Kineic energy can be defined as; 1 mv This equaion shows ha kineic energy can be effec more by doubling he velociy.
Numerical Quesions (1) A Kg objec is released from res from a heigh of 10m above he ground. Calculae (i) he iniial poenial energy a he momen of release ii) he kineic energy a he momen i reaches 4m above he ground iii) he speed of he objec jus before impac wih he ground Afer he impac he objec finally comes o res. Explain wha happens o he los kineic energy. [Take g=10m/s ] Given daa i) m Kg h 10m E? E mgh E 1010 E 00j ii) m Kg S 10 4 6m? v i 0m f gs 10 6 10m 1 m 1 10 10 j iii) m Kg S 10m gs 1010 00 141m
The kineic energy is used in doing work o penerae he objec in ground. () In he following siuaions, calculae he power involved. (i) A force of 50N moves hrough a disance of 10m in 5s. Given daa F 50N S 10m 5s P? FS 5010 5 100wa ii) An objec of mass 1Kg is lifed up verically hrough 5m in 10s. Given daa m 1Kg h 5m 10s? mgh 1105 10 5wa
(3) A rubber ball of mass 0.1Kg is held a a heigh of.5m above he ground, and hen released. (i) Calculae he kineic energy of he ball jus before i his he ground. Given daa m 0.1Kg h.5m? By law of conservaion of energy we can wrie ha; E mgh 3 j 0.110.5 (ii) Calculae he velociy of he ball jus before i his he ground. Given daa m 0.1Kg S.5m? gs gs 10.5 50 7.1m (4) Two bodies of equal masses move wih uniform velociy and 3 respecively. Find he raio of heir kineic energies. Given daa Mass of firs body = m elociy of firs body = 1 K-E of firs body = m Mass of second body = m elociy of second body = 3 1 K-E of second body = m (3 ) Raios of he kineic energies are
1 m 1 : m (3 ) 1 : 9 (5) A man whose mass is 75Kg walks up a fligh of 1 seps each 0cm high in 5seconds. Find he power he develops in was. Given daa m 75Kg No of seps = n = 1 Heigh of each sep = 0cm = 0.0m Toal heigh =h = 1 0.0m =.4m 5s P? mgh 7510.4 5 360was (6) A mason of 600N weigh is climbing a ladder 10m high. Find his poenial energy a he middle of he ladder. Given daa mg 600N h 10m E? Poenial energy a he middle of he ladder can be saed as; mgh E 60010 E E 3000j
(7) A ball of weigh 100N is moving on a fricionless surface wih he velociy of 10m/s. Compue is kineic energy. Given daa mg 100N m g 100 m 10Kg 10 10m? 1 m 1 10 (10) 5100 500 j (8) A car of mass 800Kg acceleraes uniformly from res o a speed of 5m/s in 10 seconds. ha is is power? Given daa m 800Kg 5m 10s? Energy ime 1 m 800 (5) 10 800 65 5000was 5kwas 0
(9) A girl does a work of 800j, in lifing a 40Kg bucke from a well. If he acceleraion due o graviy is 10m/s. Calculae he deph of he well. Given daa 800j m h h h h h 40Kg? mgh mg 800 4010 800 400 m