Non-contact forces – the objects are physically separated.

The idea

A non-contact force acts between objects that are physically separated; the objects do not need to touch for the force to be exerted. Gravity, electrostatic force and magnetic force are the main non-contact forces met at GCSE. For example, a magnet can pull a steel paper clip across a gap, and the Earth pulls a falling apple without touching it. The strength of a non-contact force usually decreases as the separation increases, although you do not need to calculate that relationship here. To identify a non-contact force, check whether the effect still happens when the objects are apart. In free-body diagrams, non-contact forces are drawn acting on the object even when no other object is in contact with it.

What earns the marks

• States that a non-contact force acts between objects that are physically separated.
• Gives a correct example such as gravity, electrostatic force or magnetic force.
• Explains that the force can still act across a gap without the objects touching.
• Distinguishes non-contact forces from contact forces by referring to physical separation rather than touching.
• Applies the idea to a scenario, for example a magnet attracting a paper clip from a distance or the Earth pulling on a satellite.

Common mistakes and how to put them right

• Calling magnetic attraction a contact force because a magnet must be near the object.

  Put it right by noting that the objects are still separated and no surfaces touch.

• Thinking gravity only acts when objects touch the ground.

  Put it right by explaining that gravity acts between any two masses at a distance.

• Assuming non-contact forces are always stronger than contact forces.

  Put it right by stating that strength depends on the situation, not on the contact/non-contact classification.

Exam tips

• Ask yourself whether the effect would still happen if the objects were moved apart; if yes, the force is non-contact.
• Name the two objects involved and state that they are physically separated in your answer.
• Use examples from the specification, such as gravity, electrostatic and magnetic forces, to support your classification.