📌 Physical Quantities
A physical quantity is any property of matter or phenomenon that can be measured.
| Fundamental Quantities | Derived Quantities |
|---|---|
| Cannot be expressed in terms of other physical quantities | Derived by combining fundamental quantities |
| Length, Mass, Time, Temperature, Electric current, Luminous intensity, Amount of substance | Area, Volume, Speed, Velocity, Acceleration, Force, Pressure, Density, Energy |
📏 SI System of Units (International System)
The SI system is the internationally accepted system of measurement, adopted in 1960.
| Fundamental Quantity | SI Unit | Symbol |
|---|---|---|
| Length | Metre | m |
| Mass | Kilogram | kg |
| Time | Second | s |
| Temperature | Kelvin | K |
| Electric current | Ampere | A |
| Luminous intensity | Candela | cd |
| Amount of substance | Mole | mol |
📐 Measurement of Length
- Metre rule: Measures lengths from 1 mm to 1 m. Least count = 1 mm.
- Vernier calipers: Measures small lengths accurately (e.g. diameter of a coin). Least count = 0.1 mm.
- Screw gauge (Micrometer): Measures very small lengths (e.g. thickness of a wire). Least count = 0.01 mm.
Least Count: The smallest measurement that an instrument can reliably measure. The lower the least count, the more precise the instrument.
Prefixes for Length
| Prefix | Symbol | Value |
|---|---|---|
| Kilometre | km | 1000 m = 10³ m |
| Centimetre | cm | 0.01 m = 10⁻² m |
| Millimetre | mm | 0.001 m = 10⁻³ m |
| Micrometre | μm | 10⁻⁶ m |
| Nanometre | nm | 10⁻⁹ m |
⚖️ Measurement of Mass and Time
Mass
- Mass is the amount of matter in a body. It is constant everywhere (unlike weight, which changes with gravity).
- SI unit: kilogram (kg)
- Measured using: common balance, physical balance, electronic balance.
Time
- SI unit: second (s)
- Measured using: stopwatch, stopclocks.
- Simple pendulum: A simple pendulum of length 1 m has a time period of approximately 2 seconds. Time period T = 2π√(L/g).
- The time period of a simple pendulum depends only on its length and g — not on mass or amplitude (for small amplitudes).
📦 Area, Volume and Density
Area
- SI unit: m². Common units: cm², mm².
- Area of rectangle = length × breadth
- Area of circle = π r²
Volume
- SI unit: m³. Common units: cm³, litre (L). 1 L = 1000 cm³ = 10⁻³ m³.
- Volume of cuboid = l × b × h
- Volume of irregular solid: measured by displacement method using a measuring cylinder.
Density
Density (D) = Mass (M) ÷ Volume (V)
- SI unit of density: kg/m³. Also expressed as g/cm³.
- Density of water = 1 g/cm³ = 1000 kg/m³ (at 4 °C).
- An object floats in a liquid if its density is less than the density of the liquid; it sinks if its density is greater.
🏷️ Key Terms
Physical quantity
Fundamental quantity
Derived quantity
SI unit
Least count
Metre rule
Vernier calipers
Screw gauge
Mass
Time period
Simple pendulum
Volume
Density
Displacement method