Physical quantities

A physical quantity is meaningful only when its numerical magnitude is paired with a recognised unit. The magnitude expresses the size, while the unit fixes the standard against which that size is measured.

Writing a value without its unit is incomplete and loses marks in Cambridge 9702 examinations, because a number alone has no defined physical meaning. Every answer in physics, from a single calculation step to a final result, must therefore carry both parts together.

Estimation is a complementary skill that develops physical intuition for the typical magnitudes of everyday quantities. By memorising a small bank of reference values, such as the mass of a person, the speed of sound, or the density of water, you can quickly check whether a calculated answer is plausible.

Reference values Useful estimates for plausibility checks include: mass of an adult ≈ 70 kg, speed of sound in air ≈ 330 m s⁻¹, density of water ≈ 1000 kg m⁻³, acceleration of free fall g ≈ 9.81 m s⁻², and atmospheric pressure ≈ 1.0 × 10⁵ Pa.

Examiners reward order-of-magnitude reasoning and clearly stated assumptions, so estimation is a high-yield skill that supports every later topic in the syllabus.