Measurement
Measurement is the process of finding size, length
or amount of any quantity. Those quantities that can be measured and expressed
in mathematical form are called physical quantities e.g. area, mass, time,
length, etc.
There are two types of physical
quantities:
i) Fundamental physical quantities
ii) Derived physical quantities
Fundamental
physical quantities
Those physical quantities that
do not depend on other quantities are called fundamental quantities. Mass,
length and time have no interrelation of each other.
Derived physical
quantities
Those physical quantities which
depend on other physical quantities are called derived quantities. Some
examples of derived quantities are area, volume, density, etc.
A standard quantity used to
express unknown physical quantity of same kind is called Unit.
Measurement is the process of
comparing an unknown quantity with a known standard quantity. The measurement
of a physical quantity is expressed in number and its unit. For example, the
mass of a body is 5 kg. Here 5 is quantity and 'kg' is the unit of the mass. It
means that the given
mass is 5 times greater than 1
kg mass. So, it is necessary to write
unit in measurement.
There are mainly two types of
units:
i) Fundamental unit:
Those units which do not depend
on other units are called fundamental units. Meter (m), kilogram (kg) and
second(s) are fundamental units. The followings are the fundamental units used
in physics.
|
S N |
Physical quantity |
Fundamental unit |
Symbol |
|
1 |
length |
meter |
m |
|
2 |
mass |
kilogram |
kg |
|
3 |
time |
second |
s |
|
4 |
temperature |
Kelvin |
K |
|
5 |
electricity |
Ampere |
A |
|
6 |
Intensity of light |
Candela |
Cd |
|
7 |
Amount of matter |
Mole |
mol |
ii) Derived unit
The units which are formed by
the combination of fundamental units are called derived units. For example: The
unit of speed depends on the units of length and time. So, the unit of speed
'm/s' is derived unit since it has two fundamental units meter and second.
Some derived units are given
below:
|
S N |
Physical quantity |
Derived unit |
Symbol |
|
1 |
Area |
square meter |
m2 |
|
2 |
Volume |
cubic meter |
m3 |
|
3. |
Density |
kilogram/cubic meter |
kg/m3 |
|
4 |
Speed/ velocity |
meter per second |
m/s |
|
5 |
Acceleration |
meter per second2 |
m/s2 |
|
6 |
force |
Newton |
N |
|
7 |
work/energy |
Joule |
J |
|
8 |
Power |
watt |
W |
|
9 |
Pressure |
Pascal |
Pa |
The quantity of matter
contained in a body/object is called its mass. The mass of a body/object
depends on the number of atoms and their atomic mass. The mass of a body is
always constant. We use beam balance (or physical balance) to measure mass beam
balance consists of two pans.
The standard unit of mass is kilogram.
The smaller units of mass are gram (g), milligram (mg), etc. The mass of large
bodies are measured in quintal and tone.
1000 mg = 1 g
1000 g = 1 kg
100 kg = 1 quintal
1000 kg = 1 ton
What is 1 kg mass?
The mass of standard weight made
up of platinum and iridium, kept in International Bureau of Weights and
Measures (IBWM) in France, is considered as 1 kilogram in SI system. The
standard weight of 1 kg in other countries is made by comparing with this
standard weight. In Nepal, the standard mass of 1 kg should be equal to that of
1 kg weight kept in the department of metrology.
Why does an object thrown upward falls back?
The earth attracts any object
towards its center. So any object thrown up always returns to earth's surface. The
force of attraction of the earth is called ‘gravity’. The gravity acting upon a
body is
called its weight. The force by
which an object is attracted towards the center of the earth is called its
weight.
The SI unit of weight is Newton
(N). Weight is a kind of force. So it is measured by spring balance. The
acceleration produced on a body when it falls down from a certain height is
called
acceleration due to gravity.
The weight is equal to the product of mass of a body and acceleration due to gravity
on that place.
Weight (W) = mass (m) ×
acceleration due to gravity (g)
Although we use weight and mass
in a similar way, these two quantities are quite different. When we measure
mass of a body with a physical balance, the effect of gravity on both the pan
is equal in balanced condition.
The interval between two events is called time. The
unit of time is determined on the basis of the time taken by the earth to
rotate about its axis. The time taken by the earth to complete one rotation
about its axis is called one solar day.
Therefore, 1 solar day = 1 ×24 hours
= 24×60 minutes = 1440 minutes
= 24 × 60 × 60 second
= 86400 seconds
1 day has 86400 seconds.
1 second is defined as (1/86400) part of one solar
day.
That is 1 second = (1/86400) day
Different types of watches are used to measure the
time. For example: simple watch, digital watch, atomic watch, etc.
In our simple watch, few seconds may fluctuate from
the exact time. So, scientists use atomic watch to measure the accurate time.
1) Fill in the
gaps with appropriate words.
a) The unit which does not depend on
other units is called …………unit.
b) The standard unit of mass is ………..
c) The force of gravity acting on a
body is called ……….
d) 1 second is 1 part of ………….parts of
a day.
e) Mass of a body is measured by …………..
2) Choose the
correct answer.
a) Which is the standard unit of mass?
i) gram ii) kilogram iii) quintal iv)
ton
b) Which device is used to measure the
weight of a body?
i) physical balance ii) spring balance
iii) beam balance iv) top- pan balance
c) Which watch is used by scientists
for correct measurement of time?
i) simple watch ii) digital watch
iii) atomic watch iv) quartz watch
d) Which for the following is derived
unit?
i) meter ii) kilogram
iii) second iv) joule
3. Differentiate
between.
a) Fundamental unit and derived unit
b) Mass and weight
4. Answer in brief:
a) Define physical quantity.
b) What is mass?
c) What is 1kg mass?
d) What is unit of speed?
5) Convert the
following units as:
a) 4.5 hours into second b) 2.5 kilograms into gram
c) 1 day into second [Ans: a) 16200 sec., b) 2500 gm, c) 86400
sec.]
6) Manita carries
a box of 40 kg. What is the weight of the box?
(g= 9.8 m/s2) [Ans: 392 N]
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