FirstPrinciplesa Physoly project
EN-09energytrap · Wrong conservation law

Work-energy uses net work

  1. 01

    The tempting model

    Equating the work by one force to ΔK while ignoring other forces that also do work.

  2. 02

    Why it feels right

    The force named in the problem feels like the only force that matters.

  3. 03

    Where it breaks · how to catch it

    You use W_applied = ΔK even though friction, gravity, or a spring also acts along the displacement.

  4. 04

    The correct model

    The work-energy theorem is W_net = ΔK. Add the work by every force, or use an energy budget with the right storage terms.

  5. 05

    Repair · see it, then test it

    worked example A 10 N pull moves a box 3 m while friction does -6 J. Find ΔK.

    Tempting move

    30 J.

    Correct move

    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.

    Then practise it on the ladder

    R16Net work changes kinetic energyadds: Work-energy uses the sum of all work terms.