Work-energy uses net work
The mistake
Equating the work by one force to ΔK while ignoring other forces that also do work.
Why it feels right
The force named in the problem feels like the only force that matters.
How to catch it
You use W_applied = ΔK even though friction, gravity, or a spring also acts along the displacement.
The first principle
The work-energy theorem is W_net = ΔK. Add the work by every force, or use an energy budget with the right storage terms.
worked mini-example
A 10 N pull moves a box 3 m while friction does -6 J. Find ΔK.
Common wrong move
30 J.
Correct
W_net = 30 J - 6 J = 24 J.
Test the repair EN-09 · transfer
include J
Make a fresh prediction before referring to the worked example.