Mass and weight are different quantities
The mistake
Using kilograms as a force, or reporting weight in kg.
Why it feels right
Everyday scales display a mass estimate in kilograms even though their sensing element responds to force.
How to catch it
A force term carries units of kg, or you added a mass to a force.
The first principle
Mass (kg) is inertia; weight (N) is the gravitational force mg. On the Moon your mass is unchanged but your weight is ~1/6.
worked mini-example
A 5 kg bag — what is its weight on Earth?
Common wrong move
5 kg.
Correct
W = mg = 5·9.8 = 49 N.
Test the repair NL-09 · transfer
include N
Make a fresh prediction before referring to the worked example.