Third-law pairs act on different bodies
The mistake
Cancelling an action–reaction pair on a single free-body diagram.
Why it feels right
Equal and opposite forces sound like they should cancel.
How to catch it
You canceled two forces that act on two different objects.
The first principle
Newton's third-law partners act on different bodies, so they never cancel on one object's diagram. Balance forces only within a single body's diagram.
worked mini-example
A horse pulls a cart forward. If forces are equal and opposite, how does it accelerate?
Common wrong move
The forces cancel, so nothing moves.
Correct
The pair acts on different bodies; the cart accelerates from the net force on the cart alone.
Test the repair NL-08 · transfer
Make a fresh prediction before referring to the worked example.