OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Recall and apply Newton’s third law (to include: application to situations…
Full specification wording
Newton's laws: recall and apply Newton’s third law (to include: application to situations of equilibrium and non-equilibrium)
Newton's third law says that when body A exerts a force on body B, body B exerts an equal-sized force on A in the opposite direction.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 19 of 20
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Newton's third law says that when body A exerts a force on body B, body B exerts an equal-sized force on A in the opposite direction. The two forces act on different objects, so they never cancel each other. A book resting on a table is in equilibrium because the upward push of the table on the book balances the downward pull of gravity on the book; both of those forces act on the book. The third-law partner of the book's weight is the book's gravitational pull on the Earth, not the table's push. In a non-equilibrium case, such as a rocket accelerating, the exhaust pushes the rocket forward while the rocket pushes the exhaust backward with an equal force.
What earns the marks
- States that the two forces in a third-law pair are equal in size and opposite in direction.
- Identifies that the paired forces act on two different objects, so they do not cancel.
- Explains equilibrium by balanced forces acting on the same object, such as the table's push and the book's weight both acting on the book.
- Applies the law to a non-equilibrium case, such as a rocket and its exhaust pushing on each other with equal and opposite forces.
Common mistakes and how to put them right
Claims that the table's push on a resting book cancels the book's weight as a third-law pair.
Put it right: Identify instead that the third-law partner of the book's weight is the book's pull on the Earth.
Treats equal and opposite third-law forces as producing no motion at all.
Put it right: Recognise instead that each force acts on a different object, so each object can accelerate.
Describes the two forces of a pair as acting on the same object.
Put it right: State instead that they act on two different bodies.
Exam tips
- Name the two objects involved in each force pair before deciding whether the forces cancel.
- Check whether the forces in a question act on the same object or on different objects.
- Use the phrase 'equal in size, opposite in direction, acting on different objects' when justifying a third-law answer.
Try the lesson
The start of the Newton's laws: recall and apply Newton’s third law (to include: application to situations of equilibrium and non-equilibrium) lesson, made from checked teaching notes for this specification point.
Have a go first
A person stands still on the ground. Which force is the Newton's third law reaction to the person's weight?
Listen
Newton's third law says that when body A exerts a force on body B, body B exerts an equal-sized force on A in the opposite direction. The two forces act on different objects, so they never cancel each other. A book resting on a table is in equilibrium because the upward push of the table on the book balances the downward pull of gravity on the book; both of those forces act on the book. The third-law partner of the book's weight is the book's gravitational pull on the Earth, not the table's push. In a non-equilibrium case, such as a rocket accelerating, the exhaust pushes the rocket forward while the rocket pushes the exhaust backward with an equal force. A strong answer earns marks for these points. States that the two forces in a third-law pair are equal in size and opposite in direction. Identifies that the paired forces act on two different objects, so they do not cancel. Explains equilibrium by balanced forces acting on the same object, such as the table's push and the book's weight both acting on the book. Applies the law to a non-equilibrium case, such as a rocket and its exhaust pushing on each other with equal and opposite forces. Exam tip: Name the two objects involved in each force pair before deciding whether the forces cancel.
Key words
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Newton's third law
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Force pairs acting on different objects
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Equilibrium from balanced forces on one object
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Non-equilibrium motion from unbalanced forces
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See every definition
- Newton's third law
- When body A exerts a force on body B, body B exerts an equal-sized force on A in the opposite direction.
- Force pairs acting on different objects
- The two forces act on different objects, so they never cancel each other.
- Equilibrium from balanced forces on one object
- A book resting on a table is in equilibrium because the upward push of the table on the book balances the downward pull of gravity on the book, both acting on the book.
- Non-equilibrium motion from unbalanced forces
- In a non-equilibrium case such as a rocket accelerating, the exhaust pushes the rocket forward while the rocket pushes the exhaust backward with an equal force.
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