A free-body diagram shows real forces only

01
The tempting model
Adding a phantom “force of motion” in the direction of travel.
02
Why it feels right
Moving objects feel like they must carry a forward force to keep going.
03
Where it breaks · how to catch it
You drew an arrow along the velocity with no object pushing there.
04
The correct model
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).
05
Repair · see it, then test it
worked example A puck glides across frictionless ice. What horizontal forces act?
Tempting move
Gravity, normal, and a forward 'motion force'.
Correct move
Only gravity and the normal force; horizontally there is nothing, so it coasts.
Test the repair NL-01 · transfer

A satellite coasts after its engine shuts off. Ignore drag. Which force belongs on its free-body diagram?
Gravity toward Earth A forward coasting force Gravity and a forward force No forces

Make 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.