# Newton's Law of Gravitation

Unit 4 - AQA A

- Created by: Amy
- Created on: 07-01-13 13:25

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PHY4A

Newton's Law of universal gravitation

Statement:

Every particle in the universe attracts every other with a force which is directly proportional

to the product of their masses, and inversely proportional to the square of their separation.

The law is usually summarized using this diagram:

Equation

The law gives this equation:

F = GMm

r²

Where M and m are the two masses, r is the separation, F is the force and G is a constant of

proportionality, known as the universal constant of gravitation.

Note that:

The force is always attractive and the range is infinite

Strictly, the equation applies only to point masses.

r is the distance between the centres of the bodies

G is very small hence gravitational forces are very small, unless one of the masses is huge,

as with a planet.

Inverse square laws

Newton's law of universal gravitation is an example of an inverse square law. This simply means that

the size of the force is inversel proportional to the square of the separation of the objects:

If the separation is doubled, the force is quatered

If the separation is made 10 times bigger, the force is made 10² times smaller

Inverse-square laws generate graphs which have a characteristic shape

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