A.2 · 3 · A.2 Forces and momentum · Newton’s laws
Newton's third law
The law of action and reaction: forces come in pairs, on different objects.
SL+HLMedium14 minFree
Hook
You push a heavy trolley forward. Why do you feel it pushing back on you?
Newton’s third law
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A person pushes a trolley with 5 N to the right. The trolley pushes the person with…
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Why do an action force and its reaction force not cancel each other?
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A boot kicks a football. The reaction to the boot’s force on the ball is…
Apply
A rocket accelerates forward in space, where there is nothing to push against. Explain how, using Newton’s third law.
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Remember earlier lessons
From lesson A.2 · 2
The gradient of a p–t graph is…
From lesson A.2 · 1
Inertia is…
Summary card
Key points
- • Forces come in pairs: action and reaction.
- • They are equal in size and opposite in direction.
- • They act on different objects, so they never cancel each other.
Formulas
- FP→T = − FT→P
Key terms
- Action–reaction pair :
- two forces of the same type between two objects, on different objects
Common errors
- • Saying action and reaction cancel.
- • Putting both forces of a pair in the same free-body diagram.
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