Gravitational field strength

Weight and mass

Weight is not the same as mass. Mass is a measure of how much stuff is in an object. Weight is force acting on that stuff.

In physics, the term weight has a specific meaning, and is measured in newtons. Mass is measured in kilograms. The mass of a given object is the same everywhere, but its weight can change.

Gravitational field strengths on the Earth and Moon

Weight is the result of gravity. The gravitational field strength of the Earth is 10 N/kg (10 newtons per kilogram). This means an object with a mass of 1 kg would be attracted towards the centre of the Earth by a force of 10 N. We feel forces like this as weight.

You would weigh less on the Moon because the gravitational field strength of the Moon is one-sixth of that of the Earth (1.6 N/kg). Other planets have different gravitational field strengths. On Mars it is 3.8 N/kg. But note that your mass would stay the same on all planets regardless of their gravitational field strengths.

On Earth, if you drop an object it accelerates constantly towards the centre of the planet. You can calculate the weight of an object using this equation:

weight (N) = mass (kg) × gravitational field strength (N/kg)

Question

A person has a mass of 60kg. How much do they weigh on Earth, if the gravitational field strength is 10 N/kg?

weight = mass × gravitational field strength

weight = 60 kg × 10 N/kg

weight = 600 N

Question

How much would the same person weigh on the Moon if the gravitational field strength is 1.6 N/kg?

weight = mass × gravitational field strength

weight = 60 kg × 1.6 N/kg

weight = 96 N

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