Measurement

Published by: Nuru

Published Date: 29 Oct 2021

Measurement in Grade 8 Science Notes

Measurement

The process of comparison of an unknown quantity with a known or standard quantity is called measurement. The branch of science to study measurement is called metrology.

Importance of measurement:

  1. It is essential for selling and buying goods.
  2. It is essential for preparing foods.
  3. It is essential for performing scientific experiments.
  4. It is essential for preparing medicines and treating patients.
  5. It is essential to construct buildings and bridges.
  6. It is essential to sew cloths of particular size.

Physical Quantity

There are differnt things around us, some are measurable and others are not measureble.  

The quantites which can be measured are called physical quantities. Mass, volume, length, time, temperature, etc can be measured in terms of number so they belongs to physical quantity cateogory.

The qunatities which cannot be measured are called non-physical quantities. Sadness, happyness, love, etc cannot be expressed in numbers so, they belong to non physical quantity catoegory.

Types of Physical quantites:

They are of two types:

  1. Fundamental physical quantities
  2. Derived physical quantities 

Fundamental physical quantities:

Those physical quantities which do not depend on other physical quantities are called fundamental physical quantities. Examples are length, mass, temperature, current, luminous intensity, amount of substance.

Derived physical quantities:

Those physical qunatities which are derived from fundamental physical quantites or depends on fundamental physical qunatities are called derived physical quantities. Examples are volume, area, speed, etc.

Area = length*breadth

Here, area depends on length and breadth respectively. So, it is derived physical quantities.

Speed = distance travelled / time taken 

Here, speed = length / time, hence speed deends upon two fundamental physical quantities. So, it is derived physical quantities. 

Unit:

For measuring any physical quantity, we need known quantity or standard quantity. The measurement of any unknown quantity can be done by comparing it with a known or standard quantity. 

Example; a meter road is used to measure length, 1 kg known quantity is used to measure mass.

Unit is a known or standard quantity in terms of which other physical quantities are measured. Kilogram, meter, second, etc are units of mass, length, and time respectively.

Meter is a unit because it is used to measure length and value of one meter is known.

Second is a unit because it is used to measure time and value of one second is known.

Kilogram is a unit because it is used to measure mass and value of one kilogram is known.

Fundamental units:

Those units which are used to measure fundamental physical quantites are called fundamental units.

OR

The units which are independent of other units are called fundamental units.

Fundamental quantites with their respective funamental units:

Length - meter (m)

Mass - kilogram (kg)

Time - second (s)

Temperature - kelvin (K)

Current - ampere (A)

Luminous intensity - candela (cd)

Amount of substance - mole (mol)

Derived Units:

Those units whcih are used to measure physical quantities are called derived units. 

OR

The units which are derived from fundamental units are called derived units. Example; volume(m³).

Difference between fundamental units and derived units:

Fundamental units Derived units
They are the units of fundamental physical quantities. They are the units of derived quantities.
They are independent of other units. They depend on fundamental units.
They are less in numbers. They are more in numbers.

Fundamental units involved on the following derived quantities:

Area:

A = l*b = m*m = m²

Unit of area is dervide from fundamental units meter and meter. So, it derived units.

Volume:

V = l*b*h = m*m*m = m³ 

Unit of volume is derived from fundamental units meter, meter, and meter. So, it is derived unit.

Density:

d = mass / volume = kg / m³ 

Unit of density is derived from fundamental units kg and meter. So, it is derived unit.

Speed:

Speed = distance travelled / time taken = m/s 

Unit of speed is derived from fundamental units meter and second. So, it is derived unit.

Standard units:

The units which are used all over the world are called standard units. They are used to make common units all over the world. Meter, second, kilogram are the standard units of length, time, and mass respectively.

Local units:

The units which are used by certain people of certain locality are called local units. They may vary from person to person, and place to place. For example; of our country, we still use hand span, arm length for the measurement of length, mana, pathi to measure mass, etc.

System of measurement:

FPS system

The system of measurement in which length is measured in foot, mass in pound, and time in second is called FPS system.

CGS system

The system of measurement in which length is measured in centimeter, mass in gram and time in second is called CGS system.

MKS system

The system of measurement in which length is measure in meter, mass in kilogram and time in second is called MKS system.

SI system (System international de'units)

It is internationally adopted or agreed system of units for all scientific works. It is the extended form of MKS system. It was introduced in 1960 AD. It is based on 7 fundamental quantities including three quantities of MKS system. So, it is the extended form of MKS system.

Importance of SI system brings the uniformity in measurement in all over the world.

Measurement of length:

Length is defined as the distance etween any two points. It's SI units is meter.

One meter:

One standard meter is defined as the distance between two fine gold lines marked on the platinum indium alloy rod kept at 0°C and 760 mm of Hg pressure in the international bureau of weight and measurement in France.

10 decagram = 1 hectogram,

10 hectogram = 1 kg,

100 kg= 1 quintal,

1000 kg = 1 metric ton

Measurement of time:

The duration between any two events if called time. It's SI unit is second. 

1 second is defined as the one by 86400 part of a mean solar day.

Time is measured by differnet types of watches such as:

  • Mechanical watch
  • Quartz watch
  • Atomic watch

Mechanical watch works on the basis of oscillation of a simple pendulum.

Quartz watch works due to the vibration of quartz crystals.

Atomic watch works on the basis of vibration of cesium-133 atom. It measures time more accurately.

Why is unit of velocity a derived unit?

We know, Velocity = displacement / time taken = meter / second

Thus, unit of velocity depends on two fundmental units i.e. meter and second so, it is derived unit. 

Why unit of mass is called funamental unit?

Because it is independent of any other units.

Weight:

Weight if a body is defined as the force with which the body is attracted towards the center of the earth. It's SI unit is Newton.

Mass Weight
Total amount of matter contained in a body is called mass. Total force of gravity acting on body is called weight.  
It's SI unit is kg. It's SI unit is Newton.
Mass of a body does not vary from place to place. Weightness of a body varies from place to place.
Beam balance is used to measure mass. Spring balance is used to measure weight.

 

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