FirstPrinciplesa Physoly project
EN-08energytrap · Path vs. state confusion

Potential energy needs a reference level

  1. 01

    The tempting model

    Treating gravitational potential energy as an absolute number instead of a difference.

  2. 02

    Why it feels right

    mgh looks like a direct value, so h feels universal.

  3. 03

    Where it breaks · how to catch it

    You compare two potential energies computed from different zero levels.

  4. 04

    The correct model

    Only changes in potential energy matter. Choose any zero level, then use it consistently: ΔU_g = mg Δh.

  5. 05

    Repair · see it, then test it

    worked example A ball drops 2 m. Does it matter whether y=0 is the floor or the table?

    Tempting move

    Yes, because mgh changes.

    Correct move

    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.

    Then practise it on the ladder

    R17Endpoint work by gravityadds: Conservative work depends on endpoints, not the path.