Weight - a force downwards due to gravity

Large bodies such as the Sun, the Moon and the Earth all have very large masses.

Due to the large mass each body has a gravitational field that attracts smaller masses to it.

Mass ( m) is a measure of the quantity of matter that makes up an object. It is measured in kilograms ( kg). The mass of an object can only change if some mass is added or taken away.

Weight ( W) is a force, measured in newtons ( N). Weight is caused by the pull of gravity acting on a mass, usually near the surface of a planet. The weight of an object can change if it is on a different planet, with a different gravitational field strength, g.

The bigger the body, the greater the gravitational field strength ( g) measured in Newtons per kilogram ( Nkg^{-1} ).

For example, we are attracted to the Earth.

The Moon is attracted to the Earth, which is why it stays in orbit around the Earth.

The Earth is attracted to the Sun and so the Earth stays in orbit around the Sun.

The gravitational field strengths of different objects in our solar system are shown in the table.

Object in our solar systemGravitational field strength on the surface in Nkg^{-1}
Earth9∙8
Jupiter 23
Mars3∙7
Mercury 3∙7
Moon 1∙6
Neptune 11
Saturn9∙0
Sun 270
Uranus8∙7
Venus 8∙9

The gravitational field strength for a planet (or satellite – like the Moon) indicates the force on a 1 kg mass on the planet or moon’s surface.

Earth, Mars and the Moon in space with labels showing that a mass of 1 kg on Earth has a weight of 9.8 N; 1 kg on Mars has a weight of 3.8 N; 1 kg on the Moon has a weight of 1.6 N.Gravity on Earth, Mars and the Moon

The force that holds you on the Earth – on the seat you are sitting on now – is known as your weight.

Weight can be calculated as:

Weight = mass \times gravitational\,field\,strength

W = m \times g

Weight and gravitational field strength

Example

Question

The lunar rover used by the Apollo 15 astronauts has a mass of 120kg.

Q1: What is the weight of the rover on the Earth?

Q2: What is the weight of the rover on the Moon?

Remember that wherever you go, your mass will remain the same.

Answer one

W = m \times g

= 120 \times 9.8

= 1176N

Answer two

W = m \times g

= 120 \times 1.6

= 192N