Physical quantities

Physical quantities are properties of objects or phenomena that can be measured. Every physical quantity consists of two parts:

• A numerical magnitude (how much)
• A unit (what it is measured in)

For example, if you measure the length of a table to be 1.5 metres, "1.5" is the numerical magnitude and "metres" (m) is the unit.

Physical quantities can be classified as either scalar or vector:

• Scalar quantities have only magnitude (e.g. mass, temperature, energy)
• Vector quantities have both magnitude and direction (e.g. velocity, force, displacement)

Making reasonable estimates

In physics, it is important to be able to estimate the values of common physical quantities. This helps you check if your answers are sensible.

Some typical estimates:

• Mass of a textbook: about 1 kg
• Height of a door: about 2 m
• Speed of a walking person: about 1.5 m/s
• Volume of a cup: about 250 cm³ (0.25 L)

When making estimates, use your everyday experience and round to the nearest sensible value.

Summary

• All physical quantities have a magnitude and a unit.
• Scalars have magnitude only; vectors have magnitude and direction.
• Being able to estimate physical quantities is a key skill in physics.