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.
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