eet
ffi"
64,
Build your understanding!
Attempt the following questions on your own or in a group setting.
1.
Complete Table 6.1 on some major types of energyand their sources.
vTable6.1
,i#it,l,[l:i,ffiii:r#tftktA
Chemical potential
energy
Fuelssuchas oil, wood and coal, electric
cells,food and explosives
1
Nuclear
energy
Atomic bombs,nuclearreactors
3
Light
energy
Electrical
energy
lnternal
energy
6
The electromagnetic{EM) spectrumsuch
as visiblelight, radio waves,infra-red
(lR) radiation,ultraviolet([IV) radiation,
X-raysand Gammarays
The energyassociatedwith the current
in electricdrills, power tools and
immersionheaters
The energypossessed
by the atoms
or moleculesof matter in the form of
kinetic energyand potentialenergy
Mechanical energy
( a ) Kinetic
energy (a) All objectsin motion
(b)Cravitationalpotential
energy (b) \X/aterfall.raisedobjects
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PhysicsMattersWorkbookVolurne1
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2,
(a) Statethe Principleof Conservationof Energy.
Energy can neither be creatednpr destroyedin any process.It can be convertedfrom one form to
another or transferredfrom one body to another,but the total amount remains constant.
(b) Figure6.1 showsa simplependulumsetinto oscillationin a vacuum.Note that P
and S are the two extremepositionsof the oscillation,R is the lowest or equilibrium
position and Q is an intermediateposition betweenP and R. Describethe energy
changesthat occur asthe pendulumoscillatesfrom P to Q to R to S.\7hich principle
is illustratedhere?
<Figure6.1
O'-P
o
Taking position R as the reference level where the gravitational potential energy of the pendulum
is zero. the pendulum has the maximum gravitational potential energy at either of the two extreme
positions P and S where it is momentarily at rest (i.e. zero kinetic energy). As it swings down
acrossfrom P to O. it loses part of its gravitational potential energv but gains kinetic energv as its
speedincreases.At Q, the pendulum possessesboth kinetic energy and potential energy.When it
reachesR. the kinetic energy of the oendulum is a maximum. The principle illustrated here is the
Principle of Conservationof Energy.
3.
Describequalitatively the term efficiencyas applied to a power station which converts
input energyfrom fuels to useful output electricalenergy.
The efficieilcy of a power station measuresthe fraction of ase,r?/electrical energy output we can obtain
from the total energyinput in the form
"ff
Between the input and output, there are a series of energy transfers which waste energy in the form of
heat to the surroundings.
tr
Haveyoulearntthatenergycanneitherbecreated
nordestroyed,
andthatit canonlybe
transformed?
Canyoustatethe Principle
of Conservation
of Energy?
p
,#
50
Energy,Workand Power
@2007MarshallCavendish
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4.
An obiect of mass L kg at rest falls through a height of 5 m before hitting the ground.
(Takeg=10ms-2)
(a) \[hat is its initial gravitationalpotentialenergy?
Eo:mgh:lx10x5:50J
(b)
'Without
calculation, what is the gain in kinetic energy when the object hits the
ground (neglectingair resistance)?
Explain how you arrivedat your answer.
The gain in kinetic energy would be 50 J. This answer is derived using the Principle of
the loss in initial gravitational potential energy equalsthe gain in kinetic energy.
(c) Find the maximum speedof the object just before it hits the ground.
By thePrincipleof Conservation
of Energy,
lossin initialgravitational
potentialenergy
: gain in kinetic energy
t_
mgh: ,mrP.* - v.* -- ,pA .: Jzxtoxs
: 10m s-twhichis themaximumspeedof theobject
J.
Define each of the following with an equation (using symbols that are clearly
explained).
(i)
Kinetic energy
Eu=
)mfwhere
{
= kinetic energy(inJ), m: massof the body (in kg), and
v: speedof the body (in m rt)
(ii) Gravitational potential energy
Er: mgh where Eo = gravitational potential energy (inI), m: mass of the body (in kg),
g : accelerationdue to gravity (in m sr), and h: height raised (in m)
\
\
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( b ) Figure6.2 showsa box of mass2 kg, initially at rest, being pulled up a smooth
slopeby a 25 N appliedforce.If the distancemovedis 4 m alongthe slopeand the
speedof the box at that instantis 8 m s-1,calculate
surface
<Figure6.2
(at rest)
(i) the work done by the 26 N force,
Work done= 26 x 4
=104J
(ii) the gain in kinetic energy of the box,
= ! *v'= I
2x82=64J
Kineticenergygained
zz
"
(iii) the gain in gravitational potential energy of the box (Take g = L0 m s-2).
Gravitationalpotentialenergygained= mgh
=2xl0x4sin30o=40J
(iv) tUfhatdo you noticeabout the answersin (i), (ii) and (iii)?
Sincethe answersin (ii) and (iii) total up to give the answerin (i), it showsthat the work done
energyofthe box.
tr
n
52
potentialenergy?
for kineticenergyand gravitational
Haveyou learntthe relationships
Do you know what the quantitiesm, v, g and h represent?
The aboveequationsare used to calculatepotentialenergychangesnearthe Earth's
surface.Do you know how to use them to calculatethe speedof a 100 kg sackof gold
nuggetsjust beforeit hlts the groundif it toppleddown a hill?
Energy,
WorkandPower
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yourself!
#"hallenge
Attempt the following questions on your own. You are advised to spend no more than the
time indicated.
1,. Figure 6.3 below shows the side view of a roller coastertrack. The car is releasedfrom
position P.
Which of the following statementsis/are true?
I The total energy is the same at each of the three positions shown.
il The kinetic energy of the car is greater at Q than at R.
III The gravitational potential energy of the car is greater at R than at P.
A
I only
B
I and II
C
I and III
D I,II and III
(B)
A boy drops a ball from a height, as shown in Figure 6.4.Ignoring the effects of air
---4.
resistance,the total energYis
^$,|
A
B
C
D
greatestat point L.
greatestat point 2.
greatestat point 3.
the sameat all points.
ball
/
o2
<Figure6.4
(
D
)
o3
3,
Figure 6.5 shows a car, initially at rest, rolling down a hill with its engineturned off.
<Figure
6.5
A student uses ideas about energy to produce a spreadsheetto model the motion of
the car. The student assumesthat there is no air resistanceor friction. The student's
spreadsheetis shown inTable 6.2.
(Nov02/P21Q1,2)
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The mass of the car used in the calculationsis 860 kg. Take g, the gravitational field
strength,as 10 N kgr.
(a) (i)
Explain how the valuesfor the gravitational potential energyin column B
are calculatedfrom the valuesof the height of the car in column A.
The gravitational potential energy in column B is given by the product of the mass of the
car, the acceleration due to gravity and the height ofthe car.
(ii) Calculatethe height of the top of the hill.
Heightof the car: (gravitationalpotentialenergy)/ (massof car x gravitationalacceleration)
: (l I 18000J) / (860kg x l0 N/kg): 130m
(b)
(i)
State the Principle of Conservation of Energy.
ThePrincipleof Conservation
of Energystatesthat energyof any systemcanbe converted
from one form to anotherandno energyis lost from the system.
(ii)
Use this principle to explain how the values of the kinetic energy in column C
were calculated from the values in column B.
The total energyfor column B and C is 1 ll8 000 J. According to the Principle of
Conservation
of Energy,the lossin gravitationalpotentialenergyis convertedinto a gain in
kinetic energy.
(c) Calculatethe velocity of the car when the height of the car is zero.
I
kineticenergy: i *t':
I
t
,#
54
i
l:
I 118000J
(s6oke)(l): I 118000J
J2ffi'm s-r
50.9901
m s-r: 51.0m s-t
O
l l 1 80 0 0J / 4 3 0k g
Energy,Workand Power
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