Contact and Non-Contact ForcesAQA GCSE Physics: Revision notes

A contact force only acts when two objects are physically touching each other.

Common examples of contact forces:

• Friction — opposes motion between two touching surfaces.
• Air resistance — a type of friction acting on an object moving through air.
• Tension — the pulling force in a stretched rope, string or cable.
• Normal contact force — the support force a surface exerts at right angles to itself on an object resting on it.

A non-contact force acts between objects that are physically separated — they do not need to be touching.

The three non-contact forces in this specification:

• Gravitational force — an attractive force between any objects with mass.
• Electrostatic force — a force between electrically charged objects.
• Magnetic force — a force between magnets or between a magnet and a magnetic material.

Weight is the force acting on an object due to gravity: W = mg, where m is mass in kg and g is the gravitational field strength in N/kg.

• Weight is a non-contact force because gravity acts even when objects are not touching (e.g. the Moon orbiting the Earth).
• Weight acts at a single point called the object's centre of mass.
• Weight and mass are directly proportional — but they are not the same thing: mass is a fixed amount of matter (kg), while weight depends on the local gravitational field strength (N) and would be different on the Moon.
• Weight is measured using a calibrated spring balance, sometimes called a newtonmeter.

When two objects interact, each one exerts a force on the other — these forces form a pair acting on the two different objects, whether the interaction is through contact (e.g. pushing) or at a distance (e.g. two magnets repelling). This is true for both contact and non-contact interactions and can be represented using vector arrows on a diagram.