University of Edinburgh

BSL Physics Glossary - Earth's pull - example

The moon orbits round the Earth. This is a model of the Earth and the moon. The Sun sends its light out from here.

The UK is here on the Earth. In this position at night we can see what we call the new moon. When the moon is in this position what we see from Earth is a half moon. And from this position we see a full moon. The Sun's light is reflected from the moon back to us on Earth. When the moon is in this position again we see a half moon. And when the moon gets back to its starting point we see the new moon again. The moon takes 28 days to do the orbit round the earth. 

On Earth there is a gravitational field strength of 10 N/kg. But the moon has a smaller mass and so there is a smaller gravitational force there. On the moon the gravitational field strength is 1.6 N/kg. So when astronauts walk on the moon they are able to jump six times higher than here on Earth.

The gravitational pull from the Earth keeps the moon going in orbit round the Earth. In a similar way, the gravitational pull from the Sun keeps the Earth going in orbit round the Sun.