Section 1 of 5Mass and Weight
Mass and weight are often confused, but they measure different things.
Mass
- The amount of matter in an object, measured in kilograms and the same everywhere.
Weight
- The gravitational force acting on an object, measured in newtons and changing with location.
Gravitational field strength
- The force per kilogram on an object at a point, which is 10 N/kg on Earth in exams.
Weight direction
- Weight always acts vertically downwards, towards the centre of the planet.
Section 2 of 5Calculating Weight
The equation W = mg is used for any object near the surface of a planet.
1
Write down the mass of the object in kilograms, converting from grams if needed.
2
Write down the gravitational field strength g, which is 10 N/kg on Earth in exams.
3
Multiply the mass by g to give the weight in newtons.
4
Give the answer in newtons and state that weight is a vector acting downwards.
Section 3 of 5Measuring and Location
Measuring and Location
Measurement
- Weight is measured with a newton meter, while mass is measured using a balance.
Location
- On the Moon, g is about 1.6 N/kg, so the same object weighs about one sixth as much.
Centre of gravity
- Weight acts through the centre of gravity of an object, the point where its weight is considered to act.
Constant mass
- An object keeps the same mass wherever it is, even though its weight changes.
Section 4 of 5Worked Example
Question
- A person has a mass of 65 kg.
- Calculate their weight on Earth (g = 10 N/kg) and on the Moon (g = 1.6 N/kg).
- EarthW = m × g = 65 kg × 10 N/kg = 650 N.What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg−1 on Earth)
- MoonW = 65 kg × 1.6 N/kg = 104 N.
- The mass stays 65 kg, but the weight is much smaller on the Moon.
Answer The weight is 650 N on Earth and 104 N on the Moon.
Section 5 of 5Exam Checklist & Tips
1.2.5 – Key Points for the Exam
- Weight The gravitational force on an object, equal to mass multiplied by gravitational field strength.
- Mass The quantity of matter in an object, measured in kilograms.
- Gravitational field strength The gravitational force per unit mass at a location, measured in N/kg.
- Newton The SI unit of force and weight.
- Centre of gravity The point through which the weight of an object appears to act.
Common Mistakes
- Mixing up kg and N; mass is measured in kilograms and weight in newtons.
- Stating that weight is the same everywhere; weight changes with the gravitational field strength.
- Using mass in grams directly in W = mg without converting to kilograms.
- Writing that weight is the amount of matter in an object, which describes mass instead.