Physics is the study of nature and its laws. The word physics has come from the Greek word fusis which means nature. Physics can be explained with the help of following theories.

1. Mechanics
2. Thermodynamics
3. Electromagnetism
4. Theory of relativity
5. Quantum mechanics
6. Electronics

Measurement

• Measurement is a process of comparison of an unknown quantity with a standard quantity of the same kind.
• Here metre, kilograms and minute are units.
• We have learnt that many physical quantities have both magnitude and unit.
• Measurement of a quantity by different people should give the same value.
• This is called standard measurement.
• Hence cubit, hand span are not standard units.
• Metre, Kilogram and second are the standard units.
• SI system (System International d’Units)
• In early days people in different parts of the world used different system of units for measuring length, mass and time.
• A few systems of units are

FPS(Foot, Pound, Second)System.

CGS(Centimetre, Gram, Second) System.

MKS(Metre, Kilogram, Second) system. 

• In order to overcome the difficulties of using different systems of units, an International system was adopted in 1971.

• In the above activity, larger distances such as the distance between two place are expressed in kilometer.
• This is called multiple of length.
• We express smaller lengths such as length of a pencil, pen nib etc. in centimeter and millimeter.
• These are called submultiples.

1. The mass of a body is the amount of matter contained in it.

• The SI unit of mass is kilogram.
• We use beam balance, physical balance and electronic balance for measuring mass.
• The mass of sugarcane, cotton etc. larger than 1 kilogram are measured in quintal and metric tone.
• These are called multiples of mass.
• Similarly, the mass less than 1 kilogram is measured in gram and the mass than 1 gram is measured in milligram.
• These are called submultiples of mass.

QUANTITIES AND THEIR SI UNITS

• Radian is the SI unit of plan angle.
• Steradian is the SI unit of solid angle.
• The intensity of sound is measured in a logarithmic unit called decibel (dB).
• Intensity of earthquakes are measured in Ricter scale.
• Very long distances are measured in Astronomical Unit.
• The measure of a surface is known as area.
• Area is the extent of plane surface occupied.
• Area=length x length
• 1Acre=4000m²=100 cent
• 1hectare =2.47 acre
• 1mwtre=3.28 feet
• The space occupied by a body is called its volume.
• Volume=base x height
• 1 litre=1000cm²
• One cubic centimeter is otherwise known as 1 millilitre written as m1
• Density is the mass of unit volume of the substances
• Density =
• The unit of density is kg/m³
• The time taken to complete one oscillation is called time period.
• Amplitude is the distance up to which the bob is pulled from he position of rest.
• First successful pendulum clock was constructed by the Dutch scientist Christian Huygens only in 1657.
• Astronomical Unit is the average distance between the earth and the sun.
• 1 Astronomical Unit=150 million kilometer
• Light year is the distance traveled by light in vacuum in one year.
• 1 light year=9046 x 10¹²km(9,4,000 crore kilometers)
• Light travels distance of 3 lakh km in one second.