Newton's Law of Gravitation
Earth and Space

The strength of the gravitational force

Physics Narrative for 11-14 Supporting Physics Teaching

The gravitational force depends on mass

It is truly mind-bending that the gravitational force acts between any two objects with mass in the universe.

For example, if you meet a pupil walking down the corridor, there is a gravitational force acting between the pair of you.

You exert a gravitational force on the pupil and the pupil exerts an equal gravitational force on you. However, it is obviously not the case that as the pupil approaches, you can feel the pull of the pupil and are drawn towards them.

The size of the gravitational force depends on the mass of the objects involved. The greater the mass, the greater the gravitational force. Furthermore, if the gravitational force is to be detectable on a human scale, then one of the objects must be as massive as a planet.

For example, at the surface of the Earth:

  • The pull of the Earth on a 1 kilogram mass is approximately 10 newton.
  • The pull of the Earth on a 2 kilogram mass is approximately 20 newton.

Here the gravitational force acts in a direction towards the centre of the Earth. In other words a 1 kilogram mass at the surface of the Earth experiences a gravitational force of about 10 newton pulling it towards the centre of the Earth.

Newton's Law of Gravitation
is expressed by the relation F=G(m_1)(m_2)/r^2
can be used to derive Kepler's First Law

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