OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Describe how such examples involve interactions between pairs of objects…
Full specification wording
Newton's laws: describe how such examples involve interactions between pairs of objects which produce a force on each object
Forces never come alone: they arise from an interaction between two objects, and each object in the pair experiences a force.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 6 of 20
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Forces never come alone: they arise from an interaction between two objects, and each object in the pair experiences a force. If you push a wall, your hand pushes the wall and the wall pushes back on your hand with an equal-sized force in the opposite direction. The same pattern appears when a swimmer pushes water backwards and the water pushes the swimmer forwards, and when the Earth pulls a falling ball downwards while the ball pulls the Earth upwards. To describe an example, name both objects, state the direction of the force each one exerts on the other, and note that the two forces are equal in size and opposite in direction. The two forces act on different objects, so they never cancel each other out.
What earns the marks
- Names both objects in the interaction and states the direction of the force each exerts on the other.
- States that the two forces in the pair are equal in size and opposite in direction.
- Identifies that each force in the pair acts on a different object, so the pair does not cancel.
- Applies the pair description to a named example such as a swimmer pushing water backwards while the water pushes the swimmer forwards.
Common mistakes and how to put them right
Treats a single object as producing a force on its own.
Put it right: Identify the second object and the force it exerts back.
Claims the two forces in the pair cancel because they are equal and opposite.
Put it right: State that they act on different objects.
Reverses the direction of one force in the pair.
Put it right: Check each force points from one object towards the other.
Exam tips
- Name both objects explicitly in every answer, for example 'the hand' and 'the wall', rather than saying 'it pushes back'.
- State the size relationship and the direction relationship separately so both are visible to the examiner.
- Explain that the forces act on different objects whenever a question asks why equal and opposite forces do not cancel.
Try the lesson
The start of the Newton's laws: describe how such examples involve interactions between pairs of objects which produce a force on each object lesson, made from checked teaching notes for this specification point.
Have a go first
A swimmer pushes backwards against the water with her hands. What does the water do to the swimmer?
Listen
Forces never come alone: they arise from an interaction between two objects, and each object in the pair experiences a force. If you push a wall, your hand pushes the wall and the wall pushes back on your hand with an equal-sized force in the opposite direction. The same pattern appears when a swimmer pushes water backwards and the water pushes the swimmer forwards, and when the Earth pulls a falling ball downwards while the ball pulls the Earth upwards. To describe an example, name both objects, state the direction of the force each one exerts on the other, and note that the two forces are equal in size and opposite in direction. The two forces act on different objects, so they never cancel each other out. A strong answer earns marks for these points. Names both objects in the interaction and states the direction of the force each exerts on the other. States that the two forces in the pair are equal in size and opposite in direction. Identifies that each force in the pair acts on a different object, so the pair does not cancel. Applies the pair description to a named example such as a swimmer pushing water backwards while the water pushes the swimmer forwards. Exam tip: Name both objects explicitly in every answer, for example 'the hand' and 'the wall', rather than saying 'it pushes back'.
Key words
Say what each word means before you turn the card.
interaction between pairs of objects
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paired forces
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direction of the force
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do not cancel
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See every definition
- interaction between pairs of objects
- A situation in which forces arise between two objects, and each object in the pair experiences a force.
- paired forces
- The two forces in an interaction, which are equal in size and opposite in direction.
- direction of the force
- The way each object in the interaction exerts a force on the other, which must be stated when describing an example.
- do not cancel
- The two forces act on different objects, so they never cancel each other out.
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