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# Measurement

Measurement is the comparison of unknown physical quantity with the known or standard quantity.
For e.g.: When we measure length of a house, we use scale, which already has the known length.

Physical Quantities
Physical quantities are the one that can be measured directly or indirectly regarding their quantity and quality.
It is of two types – fundamental and derived quantities.

Fundamental Quantities
Fundamental quantities are independent physical quantities that do not depend on other quantities. Seven fundamental quantities are:

1. Length
2. Mass
3. Time
4. Temperature
5. Current
6. Luminous Intensity
7. Amount of Substance

Derived Quantities
Derived quantities are the dependent physical quantities that depend upon fundamental quantities.
For e.g.: velocity, force, power etc.

Units
Units are the standard reference quantity with reference to other physical quantities of the same kind and measured with.
For e.g: length of a pen is 15cm means that the measured length is 15 times the standard reference quantity which is 1 cm.
Units are of two types: Fundamental and Derived Units.

Difference between fundamental and derived units.

 Fundamental Units Derived Units They do not depend upon other units. They depend upon other units. They are also called basic units or independent units. They are also called dependent units. There are seven fundamental units. They are numerous derived units.

Concepts
Force is a derived physical quantity and its unit is derived unit because it depends upon mass and acceleration.
Force = mass $\times$ acceleration

Questions

1. Define measurement. What is its importance?
2. Mention two differences in fundamental and derived quantities.
3. Unit of volume is derived unit. Why?

Measurement of Mass
Mass can be defined as total quantity of matter contained in a body. Its SI unit is Kilogram (Kg).
It is measured using beam balance or pan balance. Some of the units of mass are milligram, centigram, gram, decagram, hectogram etc.

Sub multiples of kilogram:

1 Kilogram = 10 Hectogram
1 Kilogram = 100 Decagram
1 Kilogram = 1000 gram
1 Kilogram = 1,00,000 centigram
1 Kilogram = 10,00,000 milligram

Multiples of Kilogram:
100 Kg = 1 qunital
1000 Kg = 1 tannes

Measurement of Weight
Weight of body is defined as the force of gravity by which an object is pulled towards the centre of the earth or any planet. Weight and mass are two different terms. However, they are related to each other. Weight depends upon mass of the object and acceleration due to gravity.
It is measured in a spring balance. It's SI unit is Newton (N).
Weight = mass $\times$ acceleration due to gravity
i.e. W = mg

Difference between Mass and Weight

 Mass Weight Total quantity of matter contained in a body is called its mass. The force of gravity by which an object is pulled towards the centre of the earth is called weight. It is measured in kilogram (kg). It is measured in Newton (N). It is measured using beam balance or a pan balance. It is measured using a spring balance. It is a constant quantity. Weight differs from place to place.

Measurement of Length
Length is defined as the distance between any two points.
It is measured by using a scale, measuring tape etc.
The SI unit of length is metre.

 Sub- multiples of meter Multiples of meter 1 m = 10 decimeter 1 m = 100 centimeter 1 m = 1000 millimeter 10 m = 1 decameter 100 m = 10 hectometer 1000 m = 1 kilometer

Some larger units used to measure length between heavenly bodies are:

• Light year
• Parsec
• Astronomical Unit

Questions regarding Conversion of Units

Convert 60 kilometer into SI units.
SI unit of length is meter.
1 km = 1000 m
$\therefore$ 60 km = 60 $\times$ 1000 m = 6 $\times$ 104 m

Practice Yourself

Convert the following:
a) 20 meter into millimeter
b) 2 quintal into kilogram

Questions:

1. Weight of a substance is different from place to place? (Hint: Weight depends upon force of gravity and mass of substance i.e. W = mg)
2. Give any 3 differences between mass and weight.
3. Mention any 2 units used to measure distance between stars.
4. Mention uses of pan balance, spring balance and metric ruler.

Measurement of Time
Duration between any two events is called as time.
Watches, Pendulum clock, quartz clock etc. are used to measure time. Units of time are second, hour, minute, day, year. Smaller units are millisecond, microsecond.

One Second
One Solar Day = 24 hours
One Solar Day = 24 $\times$ 60 $\times$ 60 sec
One Solar Day = 86400 sec
$\therefore$ $\frac{1}{86400}$th part of one solar day is called one second.

Zenith
The point exactly above the observer's head is called as Zenith.

Devices used to measure time

• Mechanical (Pendulum) watch that works on the oscillation of simple pendulum. It gives less accurate time.
• Quartz watch: Works on the principle of vibration of quartz crystals. It is more accurate.
• Atomic Watch: It works due to emission of radiation of Cs-133 isotopes. It is very accurate to give time.

Questions:

1. How is 1 second defined?
2. Give any two differences between:
a) Mechanical and Quartz watch.
b) Larger units and Smaller units of time.
3. A pendulum clock cannot give accurate time. Why?
4. Why is atomic clock not used in daily life?

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