Physics › Section C · Forces and Newton's laws › Topic C.3
Newton's third law
You cannot push anything without being pushed back. Newton's third law says forces always come in matched pairs, and knowing where the partner force acts explains walking, rockets and why internal forces never move a system.
In this topic
- You cannot touch without being touched
- Newton's third law
- How to recognise a third-law pair
- Why the pair never cancels
- Walking, driving and flying
- Weight and the normal force are not a pair
- Worked example: finding the partners
- Worked example: two skaters push apart
- Choosing the system
- Worked example: a tractor and trailer
- Common mistakes to avoid
- Key takeaways
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