Mistake Master
Mistake Master · AP Physics C · Unit 2 · Topic 2.3 · Step-Through Animation
The Pair That Never Shares a Diagram
You'll learnto find any force's third-law partner by swapping A and B in its 'on A by B' name.
A horse pulls a cart; the cart pulls back exactly as hard. If that were a tug-of-war, nothing would ever move. Learn the on–by naming convention that makes every third-law partner mechanical — and keeps it off the wrong diagram.
THIRD-LAW LAB
Before you start
What you're looking at
A horse hitched to a cart, with the pull-and-pull-back force pair drawn at the hitch between them.
The question
If every pull is met by an equal, opposite pull, how does the pair ever start moving?
Watch for
Third-law partners drawn on separate diagrams — equal-and-opposite forces that never share one body.
What the symbols mean
Forces are named 'on A by B'; swap A and B to find the partner; cart 100 kg, horse 500 kg, tension 600 N.
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