Newton’s law (AQA A-Level Physics): Revision Notes

Gravity is a force that acts on any objects that have mass and is always attractive. Unlike electric forces, which can be either attractive or repulsive depending on the charges involved, gravitational forces only pull objects together. Gravity is the fundamental force that governs the motion of planets, stars, and galaxies, and it plays a central role in the behaviour of objects on Earth as well.

Explanation with Example Imagine two objects with masses $m_1$ and $m_2$ placed a distance $r$ apart. According to Newton's law of gravitation, each object will exert an equal but opposite gravitational force on the other, directed towards the centre of the other object.

For example, if Earth and the Moon are considered:

• The mass of Earth $( m_1)$ and the mass of the Moon $( m_2 )$ contribute to the gravitational force between them.
• The force $F$ can be calculated by substituting their masses and the distance $r$ (average distance between the centres of Earth and the Moon) into the gravitational formula. This gravitational attraction is what keeps the Moon in orbit around Earth.

Key Points

• Gravitational forces are always attractive – they never repel.
• The force is mutual: the force that $m_1$ exerts on $m_2$ is equal in magnitude and opposite in direction to the force that $m_2$ exerts on $m_1$.
• Inverse-square law: The gravitational force diminishes rapidly with distance. If you triple the distance between two objects, the gravitational force becomes one-ninth as strong.