Measurements
Physical quantities and measurement
Physics begins with measurement. All measurable things in physics are called physical quantities. They fall into two categories:
Fundamental quantities are the base quantities that cannot be broken down further. The seven fundamental quantities in the SI system are: length (metre, m), mass (kilogram, kg), time (second, s), temperature (kelvin, K), electric current (ampere, A), amount of substance (mole, mol), and luminous intensity (candela, cd).
Derived quantities are built from fundamental ones. Speed = distance/time (m/s), area = length × width (m²), and force = mass × acceleration (Newton).
The vernier caliper measures lengths to 0.1 mm precision. It has a main scale and a sliding vernier scale. The micrometer screw gauge measures even smaller lengths — to 0.01 mm. It has a thimble that rotates and a sleeve with a scale.
Significant figures tell us how precise a measurement is. The number 4.53 has 3 significant figures. Zeros matter: 0.0045 has 2 significant figures, but 4.500 has 4. When calculating, your answer should have the same number of significant figures as the least precise measurement used.
Key Points to Remember
- 1Physical quantities
- 2SI units
- 3Measuring instruments
- 4Significant figures
Pakistan Example
Measuring Cricket Stumps at Gaddafi Stadium
The height of a cricket stump must be exactly 71.1 cm — an international standard. When the PCB sets up wickets at Gaddafi Stadium, they use precise measuring instruments. Even a 2 mm error could affect an LBW decision.