Newton's law of gravitation

  2. Circular motion and gravity
  3. Gravitation
  4. Newton's law of gravitation
    5. 20'

The force between two 1 kg masses is very small. It was a big step for Newton to realise that it exists without being able to measure it.

Newton´s universal law of gravity applies to the whole universe and, while on your data booklet, is certainly worth remembering.

The field concepts here are also used in Electric fields. 

Key Concepts

Force between point masses

Gravitational forces form Newton's third law pairs, with the forces in opposite directions, the same magnitude and acting on different bodies.

 

Inverse square law

The force experienced by a mass in the gravitational field of the earth is inversely proportional to its distance from the centre squared.

 

Newton's universal law of gravitation

Every particle of mass attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to their separation squared.

\(F=G{Mm\over r^2}\)

The constant of proportionality is very small so the force between two 1 kg masses 1 m apart is only 6.67 x 10-11 N.

 

 

 

Essentials

Field strength (g)

The field strength tells us how much force would be experienced per kg if a body was placed at that point. So the force experienced by an body of mass m would be mg.

For a radial field:

\(g=G{Mm\over r^2} \div m\)

\(\Rightarrow g=G{M\over r^2}\)

 

Field lines around a spherical mass

Field lines are drawn to show the direction and strength of the field. The field lines around the Earth are radial, this shows that the field gets weaker the further away we go, as is indicated by the \(1\over r^2\) term in the equation.

 

 

 

Test Yourself

Use quizzes to practise application of theory.  


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Exam-style Questions

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