FirstPrinciplesa Physoly project
MOM-06momentumtrap · Energy vs. momentum misuse

Explosions conserve momentum while kinetic energy can increase

  1. 01

    The tempting model

    Assuming an explosion violates momentum conservation because pieces fly apart.

  2. 02

    Why it feels right

    The motion appears from nowhere, so momentum seems to be created.

  3. 03

    Where it breaks · how to catch it

    You say total momentum after an internal explosion is nonzero for a system initially at rest.

  4. 04

    The correct model

    Internal energy can become kinetic energy, but internal forces occur in equal/opposite pairs. Without external impulse, total momentum is conserved.

  5. 05

    Repair · see it, then test it

    worked example A firework at rest explodes into fragments. What is total momentum immediately after?

    Tempting move

    Positive because fragments move outward.

    Correct move

    Zero vector; fragment momenta add to the initial zero.

    Test the repair MOM-06 · transfer
    An object initially at rest explodes into two fragments. One fragment has momentum +6 kg·m/s. The other has momentum:

    Make a fresh prediction before referring to the worked example.

    Then practise it on the ladder

    R21Explosion momentumadds: Internal energy can increase kinetic energy without changing total momentum.R29Spring explosion on a trackadds: Stored spring energy can become kinetic while momentum stays fixed.