Weight and mass · Year 5

Mass is the amount of matter in an object, measured in kilograms; it is the same everywhere. Weight is the force of gravity on that mass, measured in newtons, and it depends on where you are. On Earth each kilogram is pulled with about 9.8 N.

Key facts

  • W = mg: weight (N) = mass (kg) × gravitational field strength (N/kg).
  • On Earth g = 9.8 N/kg; on the Moon about 1.6 N/kg; on Mars about 3.7 N/kg.
  • Mass is measured with a balance; weight with a newton meter (spring balance).
  • Mass does not change from place to place; weight does.
  • Weight acts from the centre of mass, straight down towards the centre of the planet.

Worked examples

Example 1

Which has the bigger mass: a 1.2 kg bag of flour or a 900 g bag of sugar?

Answer: the flour: 1,200 g is more than 900 g

Example 2

A suitcase has a mass of 23 kg. What is its weight on Earth? (g = 9.8 N/kg)

Answer: 225.4 N

Example 3

An astronaut weighs 784 N on Earth (g = 9.8 N/kg). What does she weigh on the Moon (g = 1.6 N/kg)?

Answer: 128 N

Example 4

A 5.0 kg test mass weighs 18.5 N on a planet. What would a 72 kg astronaut weigh there?

Answer: 266.4 N

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