Impulse is a vector — signs matter
The mistake
Computing Δp with speeds instead of signed velocities.
Why it feels right
'Same speed in and out' suggests no change.
How to catch it
You got Δp = 0 for a bounce, or ignored the direction reversal.
The first principle
Δp = m(v_f − v_i) with a chosen positive direction. A reversal doubles the change: from +v to −v gives Δp = −2mv.
worked mini-example
A ball hits a wall at 5 m/s and rebounds at 5 m/s. Impulse magnitude?
Common wrong move
0 — same speed.
Correct
|Δp| = m·(5 − (−5)) = 10m.
Test the repair MOM-03 · transfer
include N·s
Make a fresh prediction before referring to the worked example.