AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Whenever two objects interact, the forces they exert on each other are…
Full specification wording
Newton's Third Law: Whenever two objects interact, the forces they exert on each other are equal and opposite.
This statement is the full form of Newton’s Third Law.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 2 of 3
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This statement is the full form of Newton’s Third Law. Whenever two objects interact, object A exerts a force on object B, and object B exerts a force on object A. The two forces are equal in magnitude and opposite in direction. They are always the same type of force: both gravitational, both contact, or both magnetic, for example. The forces act on different objects, so they do not cancel each other out. A reliable method is to write the pair as “force of A on B” and “force of B on A”, then check that the sizes are equal and the arrows point in opposite directions.
For example: When a rocket pushes exhaust gases downwards, the gases push the rocket upwards with an equal force.
What earns the marks
- State that two interacting objects exert forces on each other.
- State that the two forces are equal in magnitude.
- State that the two forces are opposite in direction.
- Identify the forces as acting on different objects, for example force of A on B and force of B on A.
- Recognise that the two forces are the same type, such as both gravitational or both contact forces.
- Apply the law to a named interaction, such as a rocket pushing gases downwards while the gases push the rocket upwards.
Common mistakes and how to put them right
Treating the two forces as acting on the same object and therefore balanced.
Put it right by identifying the two different objects and the force each exerts on the other.
Mixing up Newton’s Third Law pairs with balanced forces on one object.
Put it right by checking whether the two forces act on the same object or on different objects.
Stating that the forces are equal and opposite but forgetting to say they act on different objects.
Put it right by naming both objects in the pair.
Exam tips
- Write the force pair in the form “force of X on Y” and “force of Y on X” to make the two objects clear.
- Check that both forces are the same type, for example both gravitational, before describing them as a Newton’s Third Law pair.
- Use a labelled diagram with arrows of equal length pointing in opposite directions to support your explanation.
Try the lesson
The start of the Newton's Third Law: Whenever two objects interact, the forces they exert on each other are equal and opposite. lesson, made from checked teaching notes for this specification point.
Have a go first
A swimmer pushes backwards against the water with her hands. According to Newton's Third Law, what does the water do to the swimmer?
Listen
This statement is the full form of Newton’s Third Law. Whenever two objects interact, object A exerts a force on object B, and object B exerts a force on object A. The two forces are equal in magnitude and opposite in direction. They are always the same type of force: both gravitational, both contact, or both magnetic, for example. The forces act on different objects, so they do not cancel each other out. A reliable method is to write the pair as “force of A on B” and “force of B on A”, then check that the sizes are equal and the arrows point in opposite directions. For example, when a rocket pushes exhaust gases downwards, the gases push the rocket upwards with an equal force. A strong answer earns marks for these points. State that two interacting objects exert forces on each other. State that the two forces are equal in magnitude. State that the two forces are opposite in direction. Identify the forces as acting on different objects, for example force of A on B and force of B on A. A common slip: Treating the two forces as acting on the same object and therefore balanced. Instead, identifying the two different objects and the force each exerts on the other. Exam tip: Write the force pair in the form “force of X on Y” and “force of Y on X” to make the two objects clear.
See it
Key words
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Newton’s Third Law
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Equal and opposite force pairs
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See every definition
- Newton’s Third Law
- Whenever two objects interact, the forces they exert on each other are equal and opposite.
- Equal and opposite force pairs
- The two forces are equal in magnitude and opposite in direction, and are always the same type of force.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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