FirstPrinciplesa Physoly project
MOM-02momentumtrap · Energy vs. momentum misuse

Energy and momentum are not interchangeable

  1. 01

    The tempting model

    Conserving kinetic energy in an inelastic collision (or momentum via energy).

  2. 02

    Why it feels right

    Both are 'conserved quantities', so they seem to travel together.

  3. 03

    Where it breaks · how to catch it

    You set ½mv² conserved in a collision where objects stick or deform.

  4. 04

    The correct model

    Momentum is conserved in all collisions (no external impulse); kinetic energy is conserved only in elastic ones. Use the right invariant.

  5. 05

    Repair · see it, then test it

    worked example Two clay balls collide and stick. Which is conserved?

    Tempting move

    Kinetic energy.

    Correct move

    Momentum — KE is lost to deformation/heat.

    Test the repair MOM-02 · transfer
    Two carts collide on a level track with negligible external impulse and bounce apart. What is guaranteed?

    Make a fresh prediction before referring to the worked example.

    Then practise it on the ladder

    R27Classify the collisionadds: Momentum conservation does not imply kinetic-energy conservation.