Introduction
Measurement is an important part of our day-to-day life. We see and use the two types of quantities in our daily life. They are measurable and non-measurable quantities. For example quantities like mass, length, time, temperature, volume, etc which are measurable are called the physical quantities. And there are also other quantities that cannot be measured (so far) such as love, anger, kindness, hatred. Quantities like these are called non-physical quantities.
Measurement is the process of finding the value of any unknown physical quantity with a known and standard quantity of the same kind. For example, when the shopkeeper measures the mass of an unknown quantity of sugar he uses standard one of half kg pre-defined mass and tries to figure out the value of sugar in that unit using beam balance. This process is measurement. In this example, the mass of sugar is the physical quantity that needs to be determined (measured).
There are two types of physical quantities (measurable quantities). They are:
- Fundamental physical quantities
- Derived physical quantities.
1. Fundamental Physical Quantities
Those physical quantities which do not depend upon other physical quantities and have their own identity are called the fundamental physical quantities. These quantities have independent. For example when you ask how far the bus has traveled? You are talking about the only physical property distance (length) whose answer could be say 1 kilometer. But if you were to ask how fast the bus is moving then you would be talking about the distance and the time. Which involved two physical quantities!
Hence the first case is about the distance only making distance (length) the fundamental quantity whereas the second case is derived quantity (made up of length and time).
Why time is called fundamental quantity?
→ Time is called fundamental quantity because time is just the measure of the gap of duration between two events that took place and doesn't require the involvement of other physical quantities to be measured.
2. Derived Physical Quantities
Those physical quantities which depend upon the fundamental quantities and do not have their own identity are called the derived physical quantities. There are many derived quantities in nature. For example, velocity, pressure, area, force, density, volume, etc.
Let's consider the volume of an object. The volume is the product of three dimensions (length, breadth, and the height of the object). Hence volume is derived from the length.
Units
If we measure the mass of any substance, we get a certain numerical value. To express this numerical value, we use a unit. For example the mass of sugar is5 kg. It means the mass of sugar is 5 times more than the standard reference quantity i.e., 1 kg. Therefore, the standard reference quantity, with reference to which other physical quantities of the same kind are measured is called the unit. For example, the unit of the mass is the kilogram (kg), the unit of length is meter (m), etc. Depending upon the fundamental and derived quantities, the units are broadly divided into two types. They are
1. Fundamental unit
Those units of measurement which do not depend upon any other units are called the fundamental units. These are also called independent units or basic units. There are seven fundamental quantities. To measure these seven fundamental quantities, we have seven fundamental units. The fundamental quantities and their SI units and the symbols of the units are given in the table below:
S.N | Fundamental quantities | SI Unit | Symbol |
---|---|---|---|
1. | Length | meter | m |
2. | Mass | kilogram | kg |
3. | Time | second | s |
4. | Temperature | kelvin | K |
5. | Current | ampere | A |
6. | Luminous intensity | candela | cd |
7. | Amount of substance | mole | mole |
2. Derived unit
Those units of measurement which are expressed in terms of fundamental units are called derived units. These are also called the dependent units. For example, the units of speed, velocity, acceleration, work, power, pressure, density, etc. are some derived units.
These units are expressed in terms of seven fundamental units mentioned above. For example, the unit of velocity is a meter per second (m/s). It is also called a derived unit as it depends upon the unit of length and time.
Some of the derived quantities and their SI units are as given below:
SN | Derived quantities | SI unit | Involved basic quantities |
---|---|---|---|
1. | Area | m2 | length (twice) |
2. | Volume | m3 | length (thrice) |
3. | Density | Kg/m3 | mass. length |
4. | Force | N(Kg m s-2) | mass, length, time |
5. | Velocity (Speed) | m/s | length, time |
6. | Acceleration | Pa (Kg m-1 s-2) | mass, length, time |
7. | Pressure | w (Kg m2 s-3) | mass, length, time |
Check out the difference between Fundamental units and derived units
Measurement of Mass
Mass is defined as the total amount of matter contained in a body. It is measured by a beam balance and its SI unit is kilogram (kg). There are other units of measurement of mass such as gram (g), milligram (mg), ton, quintal, and so on.
Below are the sub-multiple of 1 kg (kilogram)
- 1 Kg = 1,000,000 mg (milli gram)
- 1 kg = 100,000 cg (centi gram)
- 1 kg = 10,000 dg (deci gram)
- 1 kg = 1000 g (gram)
- 1 kg = 100 dag (deckgram)
- 1 kg = 10 hg (hectogram)
Below are the multiples of 1 kg (kilogram)
- 100 kg = 1 quintal
- 1000 kg = 1 ton
Beam Balance
A beam balance is an instrument used to measure the mass of an unknown quantity by check and balancing with the known quantity. In the figure, a beam balance is shown. It has two pans. Each of which consists of known and unknown quantities (mass). Once the pointer is shown straight upward the unknown mass will be equal to the quantity of known mass.
The following points should be kept in mind while using a beam balance:
- The pan balance should be horizontal to the ground.
- Standard mass having an official stamp of the meteorology department should be used.
- During measurement, the pointer must coincide with the mark at its center.
- Before the beginning measurement, the pointer must mark at its center.
Definition of one Kg: One kg is defined as the mass of a platinum-iridium rod kept at the International Bureau of Weights and Measures in Paris of France.
Measurement of Weight
For our general concept, weight is supposed to be the mass of the body. This concept is totally wrong from the point of view of physics. For example, when we are measuring our mass, we simply say that weight is 55kg. This is not true as weight is totally different from the mass of the body. Weight is force whereas mass is the quantity of matter. But, the mass of the body directly influences its weight. Thus,
The amount of force with which a body is pulled towards the center of the earth due to the force of gravity is called weight. In other words, it is defined as the product of the mass of any body and acceleration due to gravity. If 'm' is the mass of any body and 'g' is the acceleration due to gravity then weight of the body(w) = mass (m) x acceleration (g)
Since, weight is a type of force i.e., force of gravity, its SI unit if (Newton N). It's a derived quantity as it depends upon the mass of the body and acceleration due to gravity. Weight is measured by using a spring balance. It is a vector quantity as its direction is always towards the center of the earth.
Measurement of Time
The duration of the gap between any two events is called the time. Its both SI and CGS units are seconds (s or sec). Time is measured using a clock or a watch. It has other different units as well. The relationship of different units can be written as:
60 seconds | 1 minute |
60 minutes | 1 hour |
24 hours | 1 day |
7 days | 1 week |
365 days | 1 year |
52 weeks | 1 year |
10 years | 1 decade |
100 years | 1 century |
In the ancient time, different types of clocks such as a sundial clock, a sand clock, a water clock were used to measure time. These devices were not so convenient and easy to use. So, these devices are not used any more.
In this modern age, different types of clocks such as a pendulum clock, a quartz clock, an electronic clock are used to measure time.