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Information
• Mars Climate Orbiter send by NASA
to study the Martian
climate, atmosphere, and surface
changes , on December 11, 1998 was
lost as the spacecraft went into
orbital insertion, due to ground-
based computer software which
produced output in non-SI units of
pound-seconds instead of the metric
units of newton-seconds (N×s)
specified in the contract between
NASA and Lockheed. $125 million
Rs 787 Crore

So Why this happened ?
• Different countries use different
measurement
• Can any of you name some
measurements?

Ancient Tamil
Measures
• 10 koaN= 1 nuNNaNu
• 10 nuNNaNU= 1 aNu(atom)
• 8 aNu= 1 kathirtthugaL
• 8 kathirtthugaL= 1 thusumbu
• 8 thusumbu= 1 mayirnuni
• 8 mayirnuni= 1 nuNmaNal
• 8 nuNmaNal= 1 siRu-kadugu
• 8 siRu-kadugu= 1 eL
• 8 eL= 1 nel
• 8 nel= 1 viral= 8^8 aNu(atom)= 1.9444 centimetre
• 12 viral= 1 saaN= 100 immi= 23.3333 centimetre
• 2 saaN= 1 muzham= 46.6666 centimetre
• 2 muzham= 1 kajam
• 4 muzham= 1 paagam
• 625 paagam= 1 kaadham= 5000 saaN= 1166.66 metres= 1.167
kilometre

Ancient Greek Measures
• Distance Measurements
• 1 digit or daktylos - plural : daktyloi (19.3 mm)
• 2 digits = 1 condylos
• 4 digits = 1 palaiste
• 8 digits = 1 dichas
• 12 digits = 1 spithame
• 16 digits = 1 pous or foot - plural podes (309 mm) (Variations such
as Ionic foot 296 mm to the Doric foot 326 mm)
• 20 digits = 1 pygon (called “Remen” by the Egyptians)
• 24 digits = 1 pechya or pechys (πεχυα) ( small cubit)
• 40 digits = 1 bema
• 72 digits = 4.5 feet = 1 xylon

• So are you not confused ??
• Its no wonder our satellite fell
• So scientists world over thought
of finding a solution to overcome
these problems.
• In 1960 an International
Committee established set of
standards for this basic quantities
which is called the SI system. SI
system stands for the french
name (Systeme International d’
Unites)

Learning Objectives
• Fundamental Units !
• Defining Length !
• Measuring Length !
• Instruments Used !

FUNDAMENTAL UNITS
• All the laws of physics are
expressed in terms of basic
quantities that require clear
definition. The three basic
quantities of physics are length
mass and time. All the other
quantities are expressed in term
in these three.
• The other four basics
units are:
oTemperature measured in

What is length?
• Definition [SI]
Length is defined as the distance
travelled by light in vaccum in 1/299792458
seconds.
• Basic unit:
The basic unit of length is metre

How to measure length?
Can you name some length measuring instrument ?
Instruments used to measure height of a person ?
Distance from earth to sun and nearest star ?
Diameter of an atom ?

Instruments used to
find Length
• Lengths can be measured with a
ruler to an accuracy of about
1mm.
• Some investigations may need a
more accurate measurement of
length, which can be achieved
by using Vernier Calipers or a
micrometer screw gauge.

Vernier Caliper
• The calipers shown in the last slide use a
vernier scale. The simplest type enables a
length to be measured to 0.01 cm. It is a small
sliding scale which is 9 mm long but divided
into ten equal divisions so
1 vernier division = 9/10 mm
= 0.9 mm
= 0.09cm

Vernier Caliper
• One end of the length to be measured is made
to coincide with the zero of the millimetre
scale and the other end with the zero of the
vernier scale.
• The length of the object in Figure 1.11b is
between 1.3cm and 1.4cm.
• The reading to the second place is obtained by
finding the vernier mark which is exactly
opposite (or nearest to) a mark on millimetre
scale.

Vernier Caliper
• In this case it is the 6th mark and the length is
1.36 cm, since
OA = OB – AB
OA = (1.90 cm) – (6 vernier divisions)
= (1.90 – 0.54)cm
= 1.36 cm

Vernier Caliper
Vernier scales are also used
on barometers, travelling
microscopes and
spectrometers.

Micrometer Screw Gauge
• This measures very small objects to 0.001 cm.
One revolution of the drum opens the accurately
flat, parallel jaws by one divisions on the shaft of
the gauge; this is usually ½ mm, i.e. 0.05 cm.
• If the drum has a scale of 50 divisions round it,
then the rotation of the drum by one division
opens the jaws by 0.05/50 = 0.001 cm. A friction
clutch ensures that the jaws exert the same force
when the object is gripped.

Micrometer Screw
Gauge
The object shown in the next
slide has a length of
2.5mm on the shaft scale + 33
divisions on the drum scale
= 0.25 cm + 33(0.001) cm
= 0.283 cm

Micrometer Screw Gauge
Before making a measurement, check to
ensure that the reading is zero when the jaws
are closed. Otherwise the zero error must be
allowed for when the reading is taken.

Amazing Facts on Length
• Distance of Earth to the boundary of universe is
= 1.4 x 1026 metres
• Distance from sun to the nearest star is = 4 x
1016 metres
• Distance of light travels in a year = 9.46 x 1016
metres
• Radius of Earth = 6.37 x 106 metre
• Length of a housefly = 5 x 10-3 metre
• Size of a cell = 1 x 10-5 metre
• Diameter of hydrogen = 10-10
• Diameter of proton = 10 -15

Question
1. How many millimetres are there in
a) 1 cm =
b) 4 cm =
c) 0.5 cm =
d) 6.7 cm =
e) 1 m =
Duration for taking down the questions – 1 minutes
Duration for writing the answers – 3 minutes
Duration for decorating the paper – 2 minutes
Marks
Answers – 10 marks
Decoration - 5 marks
Discipline – 5 marks
Co-operation – 5 marks
10
40
5
67
1000

Question
1. What are these lengths in metres:
a) 300 cm = 3.00
b) 550 cm = 5.50
c) 870 cm = 8.70
d) 430 cm = 0.43
e) 100 mm = 0.1
Marks

Questions
1. What are the readings in these vernier scales:
Marks

Question
2. Write the following as powers of ten with
one figure before the decimal point:
a) 100 000
b) 3500
c) 428 000 000
d) 504 27 056
Marks
Duration for taking down the questions – 10 seconds
Duration for writing the answers – 1 minute
Duration for decorating the paper – 15 seconds

Question
2. What are the readings in the micrometer
screw gauge in the picture present in the book
page 8: Marks

Question
1. Write out the following in full:
a) 103 =
b) 2x106 =
c) 6.92x104 =
d) 1.34x102 =
e) 109 =
Marks

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Length

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