Quantities and Units
Mechanics is built on SI units. Knowing the base quantities, their units, and the difference between scalars and vectors keeps your equations consistent and your answers correctly labelled.
What you'll be able to do
- Know the SI base units used in mechanics
- Distinguish scalar and vector quantities
- Use consistent units in calculations
- Recognise derived units like the newton
SI units
Mechanics uses SI units: length in metres (m), time in seconds (s), and mass in kilograms (kg). Quantities derived from these include velocity (m/s), acceleration (m/s²) and force in newtons (N).
Scalars and vectors
A has magnitude only (distance, speed, mass, time). A has magnitude and direction (displacement, velocity, acceleration, force). Pairing them up — distance/displacement, speed/velocity — is a common exam point.
Tip — Speed is a scalar; velocity is its vector partner. Same number, but velocity has direction.
Consistent units
Always convert to consistent SI units before calculating (e.g. km/h to m/s, grams to kg). Mixing units is one of the most common sources of wrong answers.
Formula recap
Common mistakes to avoid
Key takeaways
- SI base units: metres, seconds, kilograms.
- Force is in newtons (N = kg·m/s²).
- Scalars have size only; vectors also have direction.
- Always use consistent units before calculating.
Test yourself
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