IGCSE Physics / Forces and movement
Mass and weight
Edexcel International GCSE Physics (4PH1), chapter 3: Forces and movement, specification point 1.18. Free lesson.
Read the revision notes · IGCSE Physics past papers
Mass is the amount of matter in an object, while weight is the gravitational force acting on it. The equation W = m × g links them and lets you calculate either quantity, using the correct units.
How mass and weight differ
Mass describes how much matter an object contains. Weight is the gravitational force pulling on it. This difference matters because an object's mass stays the same in different places, but its weight changes if the gravitational pull changes. Mass is measured in kilograms (kg) with a balance. It is a scalar, so it has size but no direction. Weight is measured in newtons (N) with a newtonmeter. It is a vector, so it acts downwards, towards the centre of the planet. On the Moon, mass stays the same but weight is less because the gravitational pull is weaker. Weight is always given…
How to calculate weight from mass
Use W = m × g. The equation is on the equation list. Here, g is gravitational field strength: the force on each kilogram of mass. Its unit is N/kg. On Earth, g = 10 N/kg. On the Moon, g ≈ 1.6 N/kg.
How a newtonmeter measures weight
Hang an object from a newtonmeter to measure its weight. The object's weight pulls down, while the stretched spring pulls up. If the object hangs still, the forces are balanced, so the spring's pull has the same size as the weight. The newtonmeter reading is therefore the weight in newtons. For a bag of rice hanging still, the weight is 15 N downwards and the spring pulls upwards with 15 N.
How to calculate mass from weight
Rearrange W = m × g to make m the subject: m = W ÷ g. Divide the weight by the gravitational field strength. The answer is a mass, so give it in kilograms.