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Physics is a science of measurement. The
physicist always tries to reduce his physical concepts and conclusion to measurable
quantities. It is believed that when one can measure what one is speaking about and
express it in numbers one knows something about it but when you cannot measure it in
numbers one’s knowledge is limited and unsatisfactory. Therefore measurement is
necessary for expressing precise ideas about physical problems.
Physics deals with nature and natural phenomena. It can be considered as the systematic
study of properties of material bodies in nature under different physical conditions.
Measurement of physical quantities such as length, mass and time are involved in the
understanding of the properties of material bodies.
Man has always been curious about the world around him. The repetition of day and night,
of seasons, movements of stellar bodies have fascinated him. Science is an organized
attempt to know. The word ‘Science ‘ comes from the Latin word
‘Scientia’ meaning ‘ to know’. The Shastras of our early History are
also sciences in a sense. To acquire knowledge in science we have to use several steps.
They are systematic observations, reasoning, model making and theoretical prediction.
These taken together are said to constitute the scientific method. The progress in physics
is directly related to the advances in experimental observation and to the development of
concepts.
As civilization developed and life became more complex with man planting crops, trading
goods, purchasing property, it became essential for him to have some means of measuring
length, area, volume, mass and time. The daily movement of Earth with the resulting
apparent motion of the sun and the stars provided a ready made device for measuring time
but the measurement of the other dimensions could not be managed so naturally.
Man’s first systems for measuring these other dimensions were imprecise but very
convenient. Measurement of length was based upon the parts of the human body. Measurements
of weight were made by using crude scales in which the object to be weighed was balanced
against stones or seeds of grain. The convenience of these units is obvious. In measuring
a board a man did not have to hunt for a ruler. He simply had to use his arm or his hand.
However in one fundamental respect these early units were unsatisfactory because they were
not uniform or standard. Moreover with the beginning of science and the growth of
technology from the invention of first simple machines the need for definite and accurate
systems of measurements became necessary. Since the aim of science is to describe the
facts of experience as simple, as completely and as accurately as possible measurement
forms the very basis of science. Physics is that branch of science that is specially
concerned with the means of measurement- which is why Physics is called an
‘’exact science’’ and why the other divisions of science depend in
varying degrees on its special techniques.
We see a wide diversity in the physical world. Yet we wish to understand all these based
on a few concepts. Let us clarify how this is possible. You can see enormous ranges of
length and time in physical processes. We may deal with motions of tiny electrons or the
speeding away of a galaxy. We can understand the vibrations of an air column or movements
of a cloud. The dimensions and time scales vary over wide ranges. Astronomical phenomena
may cover lengths like 1026 m and time like 109 years. Microscopic
or atomic level processes may involve lengths of the order of 10-10m or less
and time anywhere from 10-15 s to 10-21 s. In the same manner masses
involved also show a wide variation. In the middle of this mind- boggling range we have
what we may call everyday phenomena at our human scale. Here the lengths may be in meters
and time in terms of hours and days.
In spite of such a wide range and complexity of
physical phenomena, man has succeeded in measuring directly or indirectly these quantities
and has attempted to analyze and understand physical phenomena on the basis of his
observations.
We try to express any natural phenomena in terms of relationships among the quantities
involved. For example, if I drop a body from a certain height, it falls. We would like to
know why it falls. Also we would like to know the answers to several other questions. What
is the speed with which it falls? Is the speed the same all the time? Does the speed
differ from body to body? If I drop the body into a tank of water does it fall in water
with the same speed?
In order to answer such questions we have to express the
characteristic features of the fall of the body through certain relations. These relations
may be stated in sentences or even better in mathematical equations. Here we may say that
the speed of the falling body increases with time or show the same more precisely and
quantitatively in a graph or in an equation.
Whenever we are involved in situations like this it becomes necessary to make observations
and to make measurements. We may have to measure the speed i.e., the distance covered by the body in
unit time. To measure distance and time we should have references. In all measurements we
compare the quantity to be measured with a reference quantity. When we say a body has
fallen through 10 meters we mean that it has fallen by a distance, which is 10 times the
chosen unit of length, namely meter. If the time taken for this is three seconds, this
time is measured by our watch.
Thus in almost every physical situation we see the need for measurement- to measure our
body temperature, to test the purity of milk we buy and so on. Therefore measurement of
various physical quantities becomes important.
Measurement involves comparison of the quantity to be measured with the reference
standard. We need as many reference standards, as there are independent quantities to be
measured.
The reference standard of measurement is what we call a unit.
Measurement of length: Historically several different choices of length have been made.
Examples are the meter, the foot and the cubit etc. They of course differ from one
another. But it is however necessary that different copies of the same unit e.g. all meter
scales be of identical length. Alternatively we must take one-meter scale to be of
standard length and calibrate all scales with respect to it. However the unit of length
chosen must be appropriate. For example to use meter consistently for all measurements is
good but if you want to indicate the distance between any two cities in India
you would choose
Kilometer as the unit of length as it is more appropriate being a larger unit of length.
The thickness of a sheet of paper will be measured in mm (1mm = 10-3 m). The
choice of the unit of measurement as well as the mode of measurement depends upon the
magnitude of the physical quantity to be measured.
Suppose you wish to measure the length of a book. You would use an ordinary scale and use cm
as the standard unit of length, to measure the dimensions of a room you would use an inch
tape and use meter as the standard unit of length. To measure the distance of a star,
you would use light year as the unit of distance and use indirect methods to find the
distance. Large distances such as height of a mountain or distance of a planet cannot be
obviously measured directly. We have to use other methods like angular methods,
trigonometric methods or reflection methods such as radar and Sonar. In the lab
measurement of small lengths is done using Vernier calipers and screw gauge. Length is an
elementary physical quantity. The device generally used in the lab to measure length is
the meter rod. The least count of the meter rod is 1mm. Therefore it can measure length
accurately up to 0.1 cm. When we wish to measure length more accurately up to 1/10 th
or 1/100th of a mm we use three important instruments – the vernier
calipers, the screw gauge and the spherometer.
Mass: is a basic property of matter. It is the measure of the quantity of matter contained
in a body. The masses of the objects we come across in this universe have a wide range of
values. At the lower end of the masses is the mass of the electron (10-30 kg)
and at the upper end the mass of the universe, which is estimated to be of the order of 1055
Kg!
Time: The concept of time is very familiar to us. We talk about yesterday, today and
tomorrow. Day and night repeat regularly due to the rotation of the earth about its axis.
We divide one day into 24 hours, one hour into 60 minutes and one minute into 60 seconds.
A natural standard for the measurement of time can therefore be derived from the rotation
of the earth. The second is the standard unit of time in the SI system.
Thus we see how man managed to assemble and organize facts and figures and devised systems
of units and measurements to verify and validate all that he discovered and comprehended.
Thus we see why measurement forms an important aspect of a foundation course in Physics,
which is largely experimental in nature.
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