FirstPrinciplesa Physoly project
MOM-03momentumtrap · Sign convention error

Impulse is a vector — signs matter

  1. 01

    The tempting model

    Computing Δp with speeds instead of signed velocities.

  2. 02

    Why it feels right

    'Same speed in and out' suggests no change.

  3. 03

    Where it breaks · how to catch it

    You got Δp = 0 for a bounce, or ignored the direction reversal.

  4. 04

    The correct model

    Δp = m(v_f − v_i) with a chosen positive direction. A reversal doubles the change: from +v to −v gives Δp = −2mv.

  5. 05

    Repair · see it, then test it

    worked example A ball hits a wall at 5 m/s and rebounds at 5 m/s. Impulse magnitude?

    Tempting move

    0 — same speed.

    Correct move

    |Δp| = m·(5 − (−5)) = 10m.

    Test the repair MOM-03 · transfer
    include N·s

    Make a fresh prediction before referring to the worked example.

    Then practise it on the ladder

    R23Impulse from a force-time graphadds: Impulse is area under the force-time curve.