Potential energy needs a reference level
The mistake
Treating gravitational potential energy as an absolute number instead of a difference.
Why it feels right
mgh looks like a direct value, so h feels universal.
How to catch it
You compare two potential energies computed from different zero levels.
The first principle
Only changes in potential energy matter. Choose any zero level, then use it consistently: ΔU_g = mg Δh.
worked mini-example
A ball drops 2 m. Does it matter whether y=0 is the floor or the table?
Common wrong move
Yes, because mgh changes.
Correct
No. ΔU = -mg(2 m) either way.
Test the repair EN-08 · transfer
include J
Make a fresh prediction before referring to the worked example.