FirstPrinciplesa Physoly project
EN-02energytrap · Wrong conservation law

The normal force usually does no work

  1. 01

    The tempting model

    Counting the normal force in the work–energy budget.

  2. 02

    Why it feels right

    It's a big force, so it seems like it should transfer energy.

  3. 03

    Where it breaks · how to catch it

    You included W_normal in an energy equation for sliding on a surface.

  4. 04

    The correct model

    Work is Fd cos θ. The normal force is perpendicular to the sliding displacement, so it does zero work; it redirects the motion without transferring energy.

  5. 05

    Repair · see it, then test it

    worked example Work done by the normal force as a block slides down an incline?

    Tempting move

    Some positive amount related to N.

    Correct move

    Zero — N ⟂ displacement.

    Test the repair EN-02 · transfer
    A bead moves along a fixed, smooth horizontal wire. What work does the wire's normal force do?

    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.