A force causes an object to undergo a specific change. Unbalanced forces cause changes in speed, shape or direction. Gravitational forces act between masses. Friction opposes motion.

Weight is not the same as mass. Mass is a measure of how much matter is in an object. Weight is a force acting on that matter. Mass resists any change in the motion of objects.

In physics, the term weight has a specific meaning - which is the force that acts on a mass due to gravity. **Weight is measured in newtons. Mass is measured in kilograms**.

The mass of a given object is the same everywhere, but its weight can change. We use balances to measure weights and masses.

Weight is the result of gravity. The gravitational field strength of Earth is 10 N/kg (ten newtons per kilogram). This means an object with a mass of 1 kg would be attracted towards the centre of 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 Earth (**1.6 N/kg**). But note that your mass would stay the same.

On Earth, if you drop an object it accelerates towards the centre of the planet. The weight of an object is calculated using this equation:

- Question
A person has a mass of 60 kg. 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**