Elementary Physics.ppt

8/19/2019 Elementary Physics.ppt

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Lecture-3

Elementary Physics:Elementary Physics:

Measurements & MechanicsMeasurements & Mechanics

Text:

“Physics” by A. F. Abbott

5th ed, 2002


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‘Measurement’ and ‘Unit’

Measurement is the

process or the result of

determining the

magnitude of a quantity,

such as length or mass

relative to a unit of

measurement, such as

a meter or a kilogram.

A unit of

measurement is

a definite magnitude of

a physical quantity. For

example, ‘length’ is a

physical quantity.

The ‘meter’ is a unit of

length.

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!undamental and "erived #uantities

!undamental

quantities a!e those

which do not de$end on

othe! %uantities, !athe!

othe! %uantities de$endon the". These a!e

"ass, length, ti"e,

elect!ic cu!!ent,

te"$e!atu!e, lu"inousintensity, and a"ount o

substance.

"erived quantities a!e

those which de$end on

unda"ental %uantities

and a!e de!i*ed !o"

the". The!e can benu"e!ous de!i*ed

%uantities+ a!ea,

*olu"e, o!ce, *elocity,

density etc. a!e ew othe de!i*ed %uantities.

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$ase and "erived %nits$ase and "erived %nits

$ase %nits: The units o

which a!e not de$endent

on each othe! o! on units

o othe! %uantities, a!e

called base units. -ikese*en unda"ental

%uantities, se*en base

units a!e "ete!, kilog!a",

second, /el*in, a"$e!e,candela and "ole.

"erived %nits: The units

which a!e based on the

othe! base units a!e

called de!i*ed units. Fo!

exa"$le, s%ua!e "ete!,ewton, watt, etc.

1nit o A!ea, A: "2

1nit o Fo!ce, F: /g."s2

3 ewton

1nit o Powe! is 4att

1 Watt = 1 Joule/1 Second 1 Joule = 1 Newton ×1 meter

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Physical quantities had many different types of

units, lie

yste" -ength 6ass Ti"e

i' !. P. (. Foot

Pound

econd

ii' ). *. (. 7enti"ete! 8!a" econd

iii' M. +. (. 6ete! /ilog!a" econd

Nowadays, scientists have agreed to adopt one set of

units – the SI units (International System of Units).



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Sci ntific Notation:

Us of Pow r of 10

cientists oten use such nu"e!ical *alues which can

be *e!y la!ge o! *e!y s"all.

Fo! exa"$le, the s$eed o light is a$$!oxi"ately

)0,00,00,000 "s−'

. The elect!ical cha!ge in anelect!on is 0.000000000000000000'9 coulo"b.

o!"ally it is *e!y diicult to count, !e"e"be!, !ead

and w!ite all the ;e!oes in these exa"$les.

4e can use the $owe! o '0 to eli"inate this$!oble". Fo! exa"$le, we can si"$ly ex$!ess the

*elocity o light as ) × '0< "s−'.

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The law of indices

The law o indices can be a$$lied in ex$!essing

scientiic notation. The laws a!e as ollows:

'0" × '0n 3 '0 " = n

>e!e ", n can be eithe! $ositi*e o! negati*e e.g.

'09 × '0: 3 '09=: 3 '0')

'0: × '0−20 3 '0: = ?−20@ 3 '0 −')

Fo! di*ision, 3 '0n × '0−" 3 '0n−"

e.g. '09 ÷ '0, 3 '09−, 3 '02

'0) ÷ '0−: 3 '0)−?−:@ 3 '0'0 etc.

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Mechanics

Mechanics is the science of motion.

It deals with the speed, velocit, acceleration, force and

ener! of lar!e o"#ects such as cars, aeroplanes and tin

o"#ects such as the electrons.

Mechanics also helps to investi!ate the motion of athletes or

the motion of a "all "owled in cric$et or hit in !olf.

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Mechanics contd!"

# an obBects "o*es, we a!e inte!ested in how

ast it "o*es, that is its s$eed.

# the s$eed o the obBect changes with ti"e,

we a!e also inte!ested in how "uch it

changes with ti"e, that is, the accele!ation.

Ceo!e we can ully unde!stand why an obBect

"o*es, we "ust be able to desc!ibe its"otion $!ecisely.

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#istance and #is$lacement

"istance is the total length covered by a moving

ob8ect irrespective of the direction of motion,

that is, only the magnitude is of importance.

The ( unit of distance is the unit of length i.e.

meter 9m'.

"isplacement is the distance measured in a

straight line 9linear distance' and in a specified

direction so both magnitude and directions are

important.

( unit of displacement is also meter 9m'.



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%&am$le

Suppose a car travels 5 m to the east and maes a U!

turn "ac to travel a further distance of # m. $ind (a)

the distance covered, (") the displacement.

E4

5 k"

) k"

Solution: (a) distance covered = 5+3 = 8 km

(b) Displacement = 5 – 3 = 2 km to the east of the

starting point !

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S$eed

(peed is defined as the rate of change ofdistance. n other -ords, speed is the distance

moved per unit time. t tells us ho- fast or slo-

an ob8ect is moving.

5$eeddistance "o*ed

ti"e taken3 #n sy"bol, * 3 d

t

%here v & speed, d & distance moved and t & time taen

a*e!age s$eedtotal distance "o*ed

total ti"e taken3 #n sy"bol, D*E 3

d

t

"ost of the ob#ects do not move at constant speed!

$he %& 'nit of speed is ms

&anua!y '', 20'5 '(le"enta!y Physics


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'ercise!

An athlete !an a distance o '00 "ete!s in .


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'elocit(

Gelocity is deined as the !ate o change o

dis$lace"ent. #t is s$eed in a s$eciied

di!ection. 4hen you a!e asked o! the *elocity

o an obBect, you ha*e to state the s$eed o

the obBect and the di!ection it is t!a*eling.*elocity

dis$lace"ent ?linea! distance@

ti"e taken3

total dis$lace"ent

total ti"e taken3a*e!age *elocity

$he 'nits of velocit* are the same as the 'nits of speed!



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)cceleration

4cceleration is defined as the rate of change ofvelocity. 4n ob8ect -hose velocity 9not speed' is

changing is said to be accelerating.

%here a & acceleration, u & initial velocity, v & final

velocity, v & change in velocity, and t & t & time taen

change in *elocity

ti"e taken3 Accele!ation

#n sy"bols, a 3*

t3

* . u

t



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ike velocit*, acceleration has a direction!-cceleration is regarded as positive if the velocit*

is increasing and negative if the velocit* is

decreasing!

-cceleration (contd!)

egati*e accele!ation is usually called a

deceleration o! retardation.

$he %& 'nit for acceleration is meter per s.'are of

second, i!e! m/s2!

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'ercise!+

A bus sta!ts !o" !est and achie*es a *elocityo 20 "s in '0 s while "o*ing westwa!ds !o"

a sta!ting $oint H. 7alculate its a*e!age

accele!ation.

'ercise!#

A ca! t!a*eling eastwa!ds at )0 "s suddenly

co"es to a halt in 5s. Find its a*e!ageaccele!ation ?!eta!dation@.



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'ercise!

4hen t!a*eling westwa!d along a highway, a

d!i*e! slows down !o" 2 "ete!s $e! second

to '5 "ete!s $e! second in '2 seconds. 4hat

is the d!i*e!Is accele!ation

'ercise!-Hne ca! accele!ates !o" !est to 50 k" h! in

s and a second ca! accele!ates !o" !est to


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Mass m"%he mass of a su"stance is the &uantit

of matter contained in the su"stance.

It depends on the num"er of atoms it

contains and the si'e of those atoms.

It is the "asic propert of the su"stance

and cannot "e chan!ed " the location,

shape and the speed of the "od

(speed less than the speed of li!ht).

The unit of mass is ilogram.

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*ei+ht *"The force applied on an obect due to gravity is kno!n as

weight "!#. This is oten ex$!essed as $ 3 mg ,whe!e $ is the weight, m the "ass o the obBect, and g

g!a*itational accele!ation. *ravitational acceleration is

the accele!ation on an obBect caused by g!a*ity.

An obBect ha*ing "ass ' /g ?"@ is weightless at the oute!

s$ace, but its "ass will !e"ain ' /g.

Hn ea!thIs su!ace the g!a*itational o!ce is a$$!oxi"ately

'0kg, i.e. a ' kg obBect on ea!thIs su!ace has o!ce o

'0 $ulling on it due to ea!th. Hn the "oon it is '.9 /g.

% & m g

0eight is meas'red in 1eton (1) hich is the 'nit of force!


http://en.wikipedia.org/wiki/Gravitational_accelerationhttp://en.wikipedia.org/wiki/Accelerationhttp://en.wikipedia.org/wiki/Force_of_gravityhttp://en.wikipedia.org/wiki/Force_of_gravityhttp://en.wikipedia.org/wiki/Accelerationhttp://en.wikipedia.org/wiki/Gravitational_acceleration


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'. 4hat is K"easu!e"entI and KunitI a"e se*enbasic %uantities with base units and sy"bols.

2. 7o"$a!e between unda"ental and de!i*ed

%uantities, base and de!i*ed units.

). Jiscuss the i"$o!tance o use o $owe! '0.. 4hat is "echanics Jeine distance and

dis$lace"ent with exa"$les.

5. 4hat is # unit 7o"$a!e s$eed with *elocity.

9. Jeine accele!ation with exa"$le.

. (x$lain "ass and weight.

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