This is an important topic for RRB NTPC, SSC CGL, and other competitive exams because questions from this topic are direct and easy to score.

📚 Units and Measurements (RRB NTPC Physics)

📚 What is Physics?

👉 Physics is the branch of science. It deals with the study of the nature and properties of matter and energy. In simple words — Physics explains how the world around us works.

1️⃣ What is Measurement?

👉 Measurement is the comparison of a physical quantity with a known standard (called unit).
👉 Example: Measuring the length of a table with a scale.

2️⃣ Physical Quantities

A physical quantity is anything that can be measured and expressed in numbers.
Example: Length, Mass, Time, Temperature.

3️⃣ Units

Unit is the standard quantity used to measure a physical quantity

Types of Units

Fundamental Units
→ Independent units that do not depend on other units.

Physical Quantity	SI Unit	Symbol of Unit
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Thermodynamic temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

Derived Units
→ Units obtained from fundamental units. Physical Quantity Derived Unit Symbol Speed metre/second m/s Force Newton N Pressure Pascal Pa Energy Joule J Power Watt W

4️⃣ Systems of Units

Common Systems:

System	Length	Mass	Time
CGS	cm	g	s
MKS	m	kg	s
FPS	foot	pound	s
SI	m	kg	s

👉 The SI System (International System of Units) is used worldwide.

5️⃣ Prefixes used with SI units

Sometimes very large or very small numbers are used, so we use prefixes:

Prefix	Symbol	Multiple
Kilo	k	10³
Mega	M	10⁶
Giga	G	10⁹
Milli	m	10⁻³
Micro	μ	10⁻⁶
Nano	n	10⁻⁹

6️⃣ Dimensions

👉 Each physical quantity has a dimension, which tells how it depends on basic physical quantities.

Example:

Force = Mass × Acceleration
→ Dimensions of Force = [M¹ L¹ T⁻²]

Common dimensions:

Quantity	Dimensions
Speed	[L T⁻¹]
Acceleration	[L T⁻²]
Force	[M L T⁻²]
Work/Energy	[M L² T⁻²]
Power	[M L² T⁻³]

Quick Revision Points:

✅ Physical quantities → Measured with units.
✅ Fundamental units → 7 basic units in SI.
✅ Derived units → Formed from fundamental units.
✅ Conversion → Practice common unit conversions.
✅ Dimensions → Help in checking formula correctness.
✅ Significant figures → Related to precision of measurement.