Units and Measurements in Physics

Basic Units

  • The basic units of measurement in this topic include metres (m) for length, kilograms (kg) for mass, and seconds (s) for time.
  • Consider the International System of Units (SI units) as the standard for measurements.

Derived Units

  • Units such as metres per second (m/s) for velocity, and kilograms per metre cubed (kg/m³) for density are termed as derived units, because they are produced by combining basic units.
  • Other derived units include newtons (N) for force, pascals (Pa) for pressure, and joules (J) for work done or energy.


  • Volume is a physical quantity expressing the three-dimensional space enclosed by a object’s surface.
  • It is measured in cubic metres (m³) in the SI system, but more commonly in litres (L) or millilitres (ml).


  • Pressure is measured in pascals in the SI system, but is often expressed in atmospheres (atm) or millimetres of mercury (mmHg) as well.
  • 1 atmosphere is equivalent to 101325 pascals (Pa) or 760 mmHg.


  • The SI unit for temperature is the kelvin (K), although degrees Celsius (°C) is also commonly used.
  • Temperature is critical in determining state changes in matter i.e. whether a substance will be a solid, liquid or gas.


  • Density is the mass of a substance per unit volume, measured in kilograms per cubic metre (kg/m³) in the SI system.
  • Density affects the buoyancy of objects in a fluid, and is useful in understanding state changes in matter.


  • Frequency, used when discussing wave properties of particles in gases, is measured in hertz (Hz).
  • It refers to the number of times that a complete motion cycle occurs during the period of one second.

Remember, consistency in units during calculations is critical for accuracy in physics, as misaligned units can lead to incorrect answers and misconceptions. It is therefore crucial to have a clear understanding of the different units and their meanings in the context of solids, liquids and gases.