Measurements

Correct Measurement of quantities is the first and the most important step in all sciences especially Physics.Quantities must be described after careful observation, repeated experiments and accurate measurements. No fact in science is accepted and no law can be formed till it is bad on exact measurements.

Quantities that can be measured: 

Mass, Weight, Time, Length, Volume and many more.( Physical Quantities)

How they can be measured:

Through various measuring devices such as

• Mass by Beam Balance, Electronic Balance or Spring Balance,
• Weight by calibration of Spring Balance,
• Time by Clocks, Watches or Pendulums,
•  Length by Measuring Tape, Meter rule, Vernier Caliper or Screw Gauge,
• Volume by Measuring cylinders, calibrated test tubes etc.

 and so on.

Physical Quantities may be Basic or Derived.

Basic Quantities are Length, Mass, Time, Force, Current, and Temperature. These have units of their own origin, not from anything. (metre, kg, second, Newton, Ampere, Kelvin).

Derived  Quantities are those whose units are derived from some basic quantity, such as Moment and Speed, whose units are Newton-metre and metre per second, respectively.

 

A Physical Quantity has two parts, numerical magnitude and unit

If for example we talk of a length of 85 m, 

85 is the numerical magnitude while metre is the unit, whose symbol is m.

You tell

Physical Quantity	Numerical Magnitude	Unit	Symbol
Length:  7 km
Mass:  400 mg
Time:      22 seconds
Length: 8 cm
Mass: 5 kg
Time: 3 hours

In this you must have noticed that there are different units for the same physical quantity. This is a problem! There should be a standard. So the scientists have devised many standards over time, calling them system of units. In past there were FPS (foot, pound, second) , CGS (centimeter, gram, second) and MKS (metre, kilogram, second) systems.

 The MKS system is further modified into SI or System International, which is what we follow today.  The units of this are called SI units.

SI units of length mass and time are meter, kilogram and second. SI units of various basic and derived quantities are standardized (and found in various books, not here)

 

Branches of Physics

Physics is basically divided as physicists went on exploring nature at deeper levels,  into the following different branches. However it must be kept in mind that these divisions are done to facilitate our own understanding, the Nature has not compartmentalized itself.

Mechanics  deals with the behavior of objects and systems in response to various forces.

(Classical Mechanics describes motion of objects on large scale (macroscopic level) while Quantum Mechanics deals with phenomena on sub-atomic (microscopic) level.
 
Thermodynamics is the study of heat and other forms of energy, and of the conversion of energy from one form to another. 

Optics is the study of the nature and behavior of light. 

Electromagnetism analyses the relationship between electrical and magnetic forces. 

Geophysics is the study of earth and its atmosphere and waters by means of the principles of physics. 

Nuclear physics is concerned with the structure and properties of the atomic nucleus, and with nuclear reactions and their applications. 

Solid-state physics examines the physical properties of solid materials. 

Acoustics studies the production and properties of sound. 

Molecular physics examines the structure, properties, and behavior of molecules. 

Particle physics also called high energy physics, analyses the behavior and properties of elementary particles.  

and then there are many others, including the inter disciplinary ones, such as Astrophysics, Bio Physics,  etc.