Kinetic energy is a scalar — it has no direction
The mistake
Assigning a direction to KE or adding energies as vectors.
Why it feels right
Motion has a direction, so its energy seems to as well.
How to catch it
You wrote a negative KE or split energy into components.
The first principle
KE = ½mv² depends on speed squared — always ≥ 0 and directionless. Momentum, not energy, carries direction.
worked mini-example
Two balls, equal speed, opposite directions. Total kinetic energy?
Common wrong move
Zero — they cancel.
Correct
2 × ½mv²; energies add as positive scalars.
Test the repair EN-03 · transfer
Make a fresh prediction before referring to the worked example.