Experiencing zero gravity on a NASA plane.
Credit: Steve Jurvetson
Gravity is an attractive force. It extends beyond the surface of the Earth, throughout our Universe. In other words, it makes things move towards each other. Everything which has a mass has a gravitational pull - even you and me - but the bigger (more massive) the object the larger it's gravitational pull. That is why if we drop a ball on the Moon it falls more slowly than it does on the Earth - the Moon is less massive and has a smaller gravitational pull

Although the pull of the Earth is stronger than that of the Moon - we still feel the effect of the gravitational pull of the Moon here on Earth, especially through the rise and fall of the oceans.

It is gravity which holds us down on the Earth, and keeps the Earth in orbit around the Sun. As well as increasing mass leading to a stronger gravitational pull, it also gets stronger the closer the object (or objects) are, this is why planets closer to the Sun move faster than those further away. It's also why the Moon orbits around the Earth first (which is close) then the Sun which is much more massive.

The Gravitational Constant (G)

Isaac Newton established that the force of gravity experienced by an object depends on the masses of the objects involved, the distance between them, and a Gravitational Constant.

So we can work out the strength of a gravitational force using the equation below:

F=G  (m1m2) / r2

Where F is the Force, G is called the Gravitational constant, r is the distance between the two objects and m1 and m2 are the masses of the objects. Newton worked out that the Gravitational Constant (G) is 6⁠.⁠6⁠7⁠x⁠10⁠-⁠11⁠ Newton square metre kilogram-2.

The pull of gravity around the Earth.
Credit: Wikimedia user Sjlegg

Gravitational Field Strength (g)

"g" (or little g) is the measurement of gravity on the surface of a planet.

It is an acceleration (how speed changes over time), and on Earth this means that when something falls, its speed increases by g. On Earth g=9.8 m/s2 (metres per second per second).

g is usually used as the unit of gravity for the surfaces of the other planets. The size of g depends on the mass of the planet (or moon). The smaller the mass, the weaker the gravity. 

We calculate weight by multiplying mass by the gravity on the surface of the planet.

Weight = Mass x Surface Gravity

So, on a planet with a surface gravity of less than 1 g you would feel lighter and on a planet with a surface gravity of more than 1 g you would feel heavier. Have a go at calculating your weight on other planets.


Gravitational Force
Credit: NSO

You can see how the effect of gravity changes on different planets in the Solar System, using the following tools:

Gravity Simulator - use your mouse to play with the ball (see image to the left).

Gravity Workshop - A classroom based lesson where the students use the simulator.

Our Weight on Planets Workshop uses algebra and graphs to reinforce ideas about the effects of gravity and how these effects depend upon where in the Universe you are.