Zero velocity is not zero acceleration
The mistake
Concluding a = 0 at the instant an object is momentarily at rest.
Why it feels right
If it's not moving, it seems like nothing is changing.
How to catch it
You set acceleration to zero at a turning point (top of a toss, ends of an oscillation).
The first principle
Acceleration is set by the net force, not the current speed. At the top of a toss v = 0 but a = −g; at the extremes of SHM speed is zero yet acceleration is maximal.
worked mini-example
At the peak of its flight, what is a thrown ball's acceleration?
Common wrong move
Zero — it's momentarily stopped.
Correct
−g downward; gravity never switches off.
Test the repair NL-03 · transfer
Make a fresh prediction before referring to the worked example.