A free-body diagram shows real forces only
The mistake
Adding a phantom “force of motion” in the direction of travel.
Why it feels right
Moving objects feel like they must carry a forward force to keep going.
How to catch it
You drew an arrow along the velocity with no object pushing there.
The first principle
List only forces from real agents in contact or fields: gravity, normal, tension, friction, applied. Motion persists without a forward force (Newton's first law).
worked mini-example
A puck glides across frictionless ice. What horizontal forces act?
Common wrong move
Gravity, normal, and a forward 'motion force'.
Correct
Only gravity and the normal force; horizontally there is nothing, so it coasts.
Test the repair NL-01 · transfer
Make a fresh prediction before referring to the worked example.