Log in
Module 2: Foundations in physics›Physical quantities (2.1.1)

Physical quantities (2.1.1)

Scalar and vector quantities, base and derived quantities, estimating physical quantities, and orders of magnitude in A-level Physics.
1 min

On this page

Physical quantities are properties of systems that can be measured and quantified. They have a numerical value which may or may not have an associated unit.

Examples of physical quantities without a unit are:

  • Refractive index
  • Efficiency
  • Strain
  • Magnification

Examples of physical quantities with a unit are:

  • Length ()
  • Mass ()
  • Energy ()
  • Electric field strength ()
Add to favourites

Physical quantities can be estimated by using known values for everyday objects.

For example, the mass of an average human can be taken as and the average mass of a car is around

When making estimation calculations, approximations of different physical quantities can also be used. Examples of this include:

  • gravitational field strength at the Earth’s surface being taken as for estimations,
  • the diffraction of light; the refractive index of air can be approximated as while its true value is
Add to favourites

Calculations can be made using estimated values. These calculations do not require exact precision, and one significant figure is often sufficient. Finding only the order of magnitude ( etc.) of a physical quantity is often enough in some contexts, such as finding the approximate value in engineering solutions or drafting an experiment.

Known constants can also be approximated in calculations to provide numbers which are easier to work with. This technique can be used to quickly validate an answer obtained from a full calculation is approximately the right order of magnitude.

A table displaying various variables with their true and approximated values. The variables include: Acceleration due to gravity on Earth (True value: 9.81 m s−2, Approximated value: 10 m s−2), Specific heat capacity of water (True value: 4180 J kg−1 K−1, Approximated value: 4200 J kg−1 K−1), One year in seconds (True value: 3.156 × 10⁷ s, Approximated value: 3.2 × 10⁷ s), and Atmospheric pressure (True value: 101 325 Pa, Approximated value: 1 × 10⁵ Pa).

An example of an estimation calculation is determining the average weight of a person on Earth:

Where:

  • is the person’s mass, and
  • is the gravitational field strength.

Using and leads to:

Add to favourites

Question walkthrough

Estimating Gravity on Mars from Force and Mass

Estimates the acceleration due to gravity on Mars by rounding an astronaut's mass and the force felt to convenient values before dividing.