Third-law pairs act on different bodies
- 01
The tempting model
Cancelling an action–reaction pair on a single free-body diagram.
- 02
Why it feels right
Equal and opposite forces sound like they should cancel.
- 03
Where it breaks · how to catch it
You canceled two forces that act on two different objects.
- 04
The correct model
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.
- 05
Repair · see it, then test it
worked example A horse pulls a cart forward. If forces are equal and opposite, how does it accelerate?
Tempting move
The forces cancel, so nothing moves.
Correct move
The pair acts on different bodies; the cart accelerates from the net force on the cart alone.
Test the repair NL-08 · transferMake a fresh prediction before referring to the worked example.
Then practise it on the ladder
R1Identify the forces on a blockadds: A free-body diagram lists only real forces from real agents.