AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Students should be able to describe the interaction between pairs of…
Full specification wording
Contact and non-contact forces: Students should be able to describe the interaction between pairs of objects which produce a force on each object. The forces to be represented as vectors.
Forces always come in interaction pairs.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 5 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Forces always come in interaction pairs. When two objects interact, each exerts a force on the other: the forces are equal in magnitude, opposite in direction, of the same type, and crucially they act on different objects. The Earth pulls a satellite towards its centre and the satellite pulls the Earth with an equal force the other way. A charged rod held near an earthed conductor attracts it while being attracted itself. A hammer pushing a nail is pushed back by the nail. Drawn as vectors, the pair is two arrows of equal length pointing in opposite directions, each drawn on the object it acts on, which is why they never cancel each other out on any single object.
What earns the marks
- Naming both interacting objects, for example the Earth and the satellite.
- Each object exerting a force on the other, equal in magnitude and opposite in direction.
- The two forces acting on different objects, so they never cancel on one object.
- Two arrows of equal length drawn in opposite directions, each on the object it acts on.
Common mistakes and how to put them right
Drawing both arrows of the pair on the same object.
Saying the two forces cancel out so nothing ever moves.
Naming the weight of a resting book and the normal contact force on it as an interaction pair, when both act on the book.
Drawing the two arrows with different lengths.
Describing only the force on one object and never naming the partner force.
Exam tips
- Write each force as 'the force of A on B', which makes it obvious the pair acts on different objects.
- Check that both forces in your pair are the same type, both gravitational or both contact.
- Draw the two arrows separately, one on each object, rather than head to head in the gap between them.
Try the lesson
The start of the Contact and non-contact forces: Students should be able to describe the interaction between pairs of objects which produce a force on each object. The forces to be represented as vectors. lesson, made from checked teaching notes for this specification point.
Have a go first
A book rests on a table. Which statement describes the interaction between the book and the table?
Listen
Forces always come in interaction pairs. When two objects interact, each exerts a force on the other: the forces are equal in magnitude, opposite in direction, of the same type, and crucially they act on different objects. The Earth pulls a satellite towards its centre and the satellite pulls the Earth with an equal force the other way. A charged rod held near an earthed conductor attracts it while being attracted itself. A hammer pushing a nail is pushed back by the nail. Drawn as vectors, the pair is two arrows of equal length pointing in opposite directions, each drawn on the object it acts on, which is why they never cancel each other out on any single object. A strong answer earns marks for these points. naming both interacting objects, for example the Earth and the satellite. each object exerting a force on the other, equal in magnitude and opposite in direction. the two forces acting on different objects, so they never cancel on one object. two arrows of equal length drawn in opposite directions, each on the object it acts on. Exam tip: Write each force as 'the force of A on B', which makes it obvious the pair acts on different objects.
Key words
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interaction pairs
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acting on different objects
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equal and opposite
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representing pairs as vectors
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See every definition
- interaction pairs
- When two objects interact, each exerts a force on the other, and these forces are equal in magnitude, opposite in direction and of the same type.
- acting on different objects
- The two forces of an interaction pair each act on a different object, which is why they never cancel each other out on any single object.
- equal and opposite
- The forces in an interaction pair are equal in magnitude and opposite in direction.
- representing pairs as vectors
- Drawn as vectors, the pair is two arrows of equal length pointing in opposite directions, each drawn on the object it acts on.
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