Physics Measurement and Units
Use SI units, dimensional analysis, significant figures, and order-of-magnitude reasoning to describe physical quantities.
A lesson of Physics, in Measurement, Motion, and Kinematics, about 10 minutes.
By the end of this lesson, you will be able to identify SI units for physical quantities, perform unit conversions using dimensional analysis, express measurement uncertainty, and distinguish scalar and vector quantities.
Why Measurement Matters in Physics
Physics is the science of measurement. Every physical law is ultimately a statement about measurable quantities. Without precise, reproducible measurement, physics can't predict, verify, or communicate.
SI Units for Physics
The same seven SI base units used in chemistry, with physical emphasis:
| Quantity | Unit | Symbol |
|---|---|---|
| Length | meter | m |
| Mass | kilogram | kg |
| Time | second | s |
| Temperature | kelvin | K |
| Electric current | ampere | A |
| Amount of substance | mole | mol |
Common derived units in physics:
- Velocity: m/s
- Acceleration: m/s²
- Force: Newton (N) = kg·m/s²
- Energy: Joule (J) = kg·m²/s²
- Power: Watt (W) = J/s
- Pressure: Pascal (Pa) = N/m²
Significant Figures and Uncertainty
Every measurement has uncertainty — no measurement is perfectly exact.
Expressing uncertainty: $g = 9.81 \pm 0.01$ m/s² — the last digit is uncertain.
Propagating uncertainty:
- Addition/subtraction: uncertainties add
- Multiplication/division: relative uncertainties add
Orders of Magnitude
Physics spans 40 orders of magnitude — from quarks ($10^{-18}$ m) to the observable universe ($10^{26}$ m).
Estimating order of magnitude is a critical skill: "Is this answer in the right ballpark?" A car traveling at 60 mph ≈ 27 m/s. If you get 2700 m/s, something's wrong.
Dimensional Analysis in Physics
Checking dimensions: equations must be dimensionally consistent.
Example: Is $v = \sqrt{2gh}$ dimensionally consistent?
Right side: $\sqrt{\text{m/s}^2 \times \text{m}} = \sqrt{\text{m}^2/\text{s}^2} = \text{m/s}$ ✓ (same as velocity)
Summary
SI units: m (length), kg (mass), s (time), and derived units for force, energy, power.