PHYSICAL QUANTITY :
Physics is the branch of science that deals with the study of matter, energy, and their interactions. To describe any physical phenomenon scientifically, measurement is necessary. Anything that can be measured and expressed by a numerical value along with a unit is called a Physical Quantity. A physical quantity is a quantity that can be measured and represented by a number and a unit.
PHYSICAL QUANTITY = MAGNITUDE ( Numerical value ) x UNITS
Length = 5 m , Mass = 10 kg , Time = 20 s
TYPES OF PHYSICAL QUANTITIES:
Physical quantities are mainly classified into two categories:
1. Fundamental (Base) Quantities
2. Derived Quantities
1. FUNDAMENTAL PHYSICAL QUANTITIES
The quantities which are independent and cannot be expressed in terms of other physical quantities are called Fundamental Quantities. Example - Mass, Length , Time
2. DERIVED PHYSICAL QUANTITIES
The quantities that are obtained from fundamental quantities by mathematical operations such as multiplication or division are called Derived Quantities. Example - Velocity , Speed , Charge , Momentum , Etc.
Why Did We Need a Unit System?
Imagine Hansu sir asks a simple question:
A car covers a distance of 100 km in 1000 seconds. Find its speed."**
The students calculate the speed, but since no specific unit system is mentioned, each student expresses the answer in a different unit:
One student writes 100 m/s
Another writes 0.1 km/s
Another writes 10,000 cm/s
Another writes 360 km/h
Another writes 6,000 m/min
Another writes 36,000,000 cm/h
Another writes 100,000 mm/s
Interestingly, all of these answers are mathematically correct because they represent the same speed in different units.
However, a serious problem arises:
The numerical values are different.Direct comparison becomes difficult.
It becomes confusing to determine whether two answers represent the same physical quantity.
Scientific communication becomes complicated.
Now imagine the same situation in scientific experiments. If scientists in different laboratories use different units for their measurements, comparing results would become extremely difficult. A result reported in cm/s may appear completely different from one reported in km/h, even though both may represent the same physical quantity. To eliminate this confusion, scientists developed Unit Systems
A unit system is a standardized set of units in which fundamental quantities such as length, mass, and time are fixed. Once these base units are fixed, all other derived units are automatically determined.
For example:
In the CGS System, length is measured in centimetres, mass in grams, and time in seconds.
In the FPS System length is measured in feet, mass in pounds, and time in seconds.
In the MKS System, length is measured in metres, mass in kilograms, and time in seconds.
By adopting a common unit system, scientists, engineers, and students can express measurements in a uniform manner. This makes calculations easier, comparisons meaningful, and scientific communication reliable throughout the world. Therefore, the need for a unit system arose to remove confusion caused by multiple units, to standardize measurements, and to make scientific comparison and communication possible.
UNIT SYSTEMS :
A complete, standard set of units or group of units containing both fundamental and derived units that is used to measure all kinds of physical quantities uniformly. The three important classical systems are:
1.MKS System
2.CGS System
3.FPS System
1.MKS UNIT SYSTEM :
MKS System is a coherent, decimal-based metric system of physical units. It treats Metre, Kilogram, and Second as its absolute base units for the mechanical dimensions of Length (L), Mass (M), and Time (T). It was formally proposed by Giovanni Giorgi in 1901 to eliminate the impracticality of the CGS system, which used units that were far too small for engineers, and the FPS system, which relied on non-decimal conversion factors.
2.CGS UNIT SYSTEM:
The CGS system is a metric framework established formally by the British Association for the Advancement of Science (led by James Clerk Maxwell and William Thomson) in 1874. It treats the Centimetre, Gram, and Second as its foundational mechanical units
3.FPS UNIT SYSTEM:
FPS system is a unit system in which Foot (ft) is taken as the fundamental unit of length, Pound (lb) as the fundamental unit of mass, and Second (s) as the fundamental unit of time. All derived units are obtained from these fundamental units according to the laws and equations of physics."
COMPLETE VIDEO LECTURE ON PHYSICAL QUANTITY AND UNIT SYSTEM:
