Work, Energy and Efficiency

7/23/2019 Work, Energy and Efficiency

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UNDERSTANDING

WORK, ENERGY, POWERAND EFFICIENCY



WORK

• Work is done, when a force that acts on anobject through a distance in the directionof the force.

• Work is the product of a force and thedistance traveled in the direction of theforce.

WORK = FORCE X DISPLACEMENT

W = F x s

W( Joule/J)F(force in Newton/N)

S(displacement in meter / m)



Example

!orce, !

s

W " !s

#f, ! " $% & and s " 'm(ence, W " $% x '

" )% *



Example '

Example +



E&ER-

• Energy is the potential to do wor• Energy cannot !e created nor !e

destroy• Exist in "arious forms such as

potential energy# electrical energy#

sound energy# nuclear energy# heatand chemical energy$



• Example of energy transformation%

when we are running up a staircase thewor done consists of energy change from&hemical Energy 'inetic Energy otential Energy$ he energy *uantity isconsumed is e*ual to the wor done$

• Example% if +,, J of wor is done# itmeans +,, - of energy is consumed$



WORK O&E /& 0(E 1(/&E

!orce, !

s

. Kinetic energ2 is energ2 of an object dueto its motion.

'. Refer to the 3gure above, Work " !s ! " ma

W " mas 44456



• Through, v 2 = u2 +2as

•

u = 0• and, s = v 2 /

2a…….(2)

7ubstitute 5'6 in56W"ma5v 2 / 2a)

W" 8 mv 2Ek " 8 mv 2



Example

/ small car of mass %% kg is moving alonga 9at road. 0he resultant force on the caris '%% &.

• a6 What is its kinetic energ2 of the carafter moving through % m:

• b6 What is its velocit2 after movingthrough % m:Solution : Given : m = 100 g , ! = 200 "a. #ineti$ energ%, & = !s= 200 ' 10= 2000 . *elo$it%, v mv 2 = 2000

v = .-2 m s1



Work done and gravitational otential energ!

h".;m

. ravitational potential energ2 is energ2 of anobject due to its position.

'. Refer to the 3gure above, W"!s"mgh where,!"mg

so, gravitational energ2,Ep " mgh



Example

• #f m " % kg h".;m g"%ms<'

= 10 (10) 1.

= 10 Thereore or done = 10

nd, &3 = 10



Prin"ile o# "on$ervation o# energ!

0o show the principle of conservationof energ2.

. Energ2 cannot be created ordestro2ed but can be changed fromone to another form.

'. example=



Example=

/ coconut falls from a tree from aheight of '% m. What is the velocit2of coconut just before hitting the

earth:

• Given : h = 20 m, u = 0 , g = 4.5 ms2 , v = 6

• &3 = &

• mgh = mv 2

• m(4.5)(20) = mv 2

• v 2 = -42 v = 14.5 m s1



power

• >ower is the rate of doing work

therefore, power, >" work done so>" W

time takent

where > = power in watt?W

W= work in joule?*

t = time to do work inseconds?s



Example

• / weightlifter lifts )% kg of weightsfrom the 9oor to a height of ' mabove his head in a time of %.) s.

What is the power generated b2 theweightlifter during this time: g " @.)ms<'6

• 7olution= Given : m = 150 g, h = 2m, t = 0.5 s and g = 4.5 ms2. 7 = 6

7 = = mgh t t = 150 ' 4.5 ' 2 = 8 810 0.5



E%"ien"!

• e3ne as the percentage of the energ2 inputthat is transformed into useful energ2.

• !ormula of eAcienc2=

EAcienc2" useful energ2 output x %% B Energ2 #nput

• analog2 of eAcienc2=

unwanted energ2

energ2 input, Einput useful energ2, Eoutput

Energ2 transformation

EC#1EDE1(#&E



Example

An electric motor in a toy crane can lift a 0.12kg weight through a height of 0.4

m in 5 s. During this time, the batteries supply 0.8 of energy to the motor.!alculate

"a# $he useful of output of the motor.

"b# $he efficiency of the motor

Solution : Given : m = 0.12 kg, s= 0.4 m, t = 5 s, Einput = 0.8 J (a) Eoutput =

Eoutput = ! " s = (0.12 " 10) " 0.4= 0.48 J

(#) E$%ien%& =

E$%ien%&= &$e#&l energ! o&t&t ' ()) *

Energ! In&t

= 0.48 " 100'

0.80

= 0'



IPORTANCE OFA-IISING T+E

EFFICIENCY OF DE.ICES• uring the process of transformationthe input energ2 to the useful outputenerg2, some of the energ2

transformed into unwanted forms ofenerg2.

• 0he eAcienc2 of energ2 converters is

alwa2s less than %%B.• 0he unwanted energ2 produced in the

device goes to waste.



E/D>FE O! W/70#& 0(E E&ER-



W/-7 O! #&1RE/7#& 0(E E!!#1#E&1-O! EC#1E7

• /#R<1O&#0#O&ER7 =<choose a model with high eAcienc2

<accommodate the power of air<conditioner and siGe of

the room<ensure that the room totall2 close so that thetemperature in the room can be maintained

• RE!R#ERE0OR7<choose the capacit2 according to the siGe of the famil2

<installed awa2 from source of heat and direct sunlight

<the door must alwa2s be shut tight

<more economical use a large capacit2 refrigerator



• http=??cikguwong.blogspot.com?'%?%;?ph2sics<form<$<chapter<'<hookes<law.html

• http=??3Giknota.blogspot.com?'%%H?%@?elasticit2.html

http://cikguwong.blogspot.com/2011/05/physics-form-4-chapter-2-hookes-law.html



http://fiziknota.blogspot.com/2007/09/elasticity.html









'.' InderstandingElasticit2



• Elasticit2 is the abilit2 of a material toreturn to its original shape and siGe whenthe external force acting on it is removed.

• #t is due to the strong intermolecularforces between the molecules of thesolid.

&o external force /tractive intermolecularforce

Repulsive intermolecular

force

( k J F 7t t th t th t i f i



• (ookeJs Faw 7tates that the extension of a springis directl2 proportional to the applied forceprovided that the elastic limit is not exceeded.

= ! 5 &m< " $m1 or " mm1 )

x

!

El ti li it f i i th i f



• Elastic limit of a spring is the maximum forcethat can be applied to a spring such that thespring will be able to be restored to its original

length when the force is removed.

• #f the elastic limit is exceeded, the length ofthe spring is longer than the original lengtheven though the force no longer acts on it. #t

is said to have permanent extension



+ooke/$ la0 Gra1

k " force constant of the spring 5eual of the gradientof the graph6

x " extension

!orce CsExtension



!orce constant is the force that is reuired to produce one unit ofextension of the spring. #t is the measure of the stiLness of the

spring.

! = ' S3ringoe%ing9ooes;a<

S3ring not oe%ing9ooes la<

(e'$eeded the elasti$limit)



Example

• / spring has an original length of ;cm. With a load of mass '%% gattached, the length of the spring is

extend to '% cm.

a. 1alculate the spring constant.

b. What is the length of the spring

when the load is in increased b2 ;%g:

Massume that g = 10 " g1



Elastic >otential Energ2, I

• Elastic potential energ2 is the energ2transferred to the spring when workis done on the spring.

k " force constant

x " spring extension



!actors in9uencing the elasticit2 of

a spring=

a. 02pe of spring material

b. diameter of the coil of springc. diameter of the wire of springd. length of the spring.

Documents

Work, Energy and Efficiency