Gravity and the Inverse Square Law

- Demonstrate an understanding that gravity is the force responsible for maintaining orbits and its inverse square law nature.
- Demonstrate an understanding of the inverse square law nature of the intensity of light (Stars Section)

Isaac Newton is known for formulating his thinking of gravity when struck by an apple falling from a tree. The force that made the apple fall was the same force that kept the Moon going around the Earth. The Moon doesn't fall to Earth because it is moving around Earth really fast. If the Earth disappeared it would move in a straight line until it was disturbed by another body.

Newton thought that if you fired a cannonball off a cliff it would eventually fall to Earth but if it was fast enough it would continue on and on travelling through the sky and orbit the Earth.

The two things that control gravity are mass and size. The further away you are from the centre of Earth, the weaker the gravity of Earth. You would weigh less on the Moon than the Earth because it has a lot less mass than Earth.

Newton proposed the Inverse Square Law. The effect of gravity (and also on forces such as sunlight) works like this. If say we have a half-mass Earth, it would produce a gravity of not half but a quarter (the square of 2). If it was three times as close to the Sun as Earth it would get not 3 times as much light but 9 times as much (the square of 3 is 9).




Questions involving the Inverse Square Law will ask you to compare values such as light or gravity and ask you to make a calculation based on the Inverse Square Law.

Question 1

Planet A lies at 2 AU from the Sun. Planet B lies at 4 AU from the Sun.
How much more light does Planet A receive than Planet B?

4 divide 2 = 2
22 = 4
Answer = 4 times


Question 2

Venus orbits the Sun at 108 million km.
Mars orbits the Sun at 226 million km.
How much more light does Venus receive from the Sun than Mars?

1082 = 11664
2262 = 51076
51076 divide 11664 = 4.37 times


Question 3

Two asteroids revolve around a common centre of gravity. They are of Masses 4M and 7M. They are 3 distances apart.
What is their gravitational pull?

4M x 7M
3d x 3d
= Gravitational Pull of 3.1