Constant speed is not zero acceleration
The mistake
Assuming constant speed in a circle means no acceleration.
Why it feels right
“Acceleration” colloquially means speeding up, and the speed isn't changing.
How to catch it
You set a = 0 for something moving in a curved path at steady speed.
The first principle
Velocity is a vector. In uniform circular motion its direction changes, so there is centripetal acceleration a = v²/r directed toward the center.
worked mini-example
A ball on a string circles at a steady 3 m/s. Is it accelerating?
Common wrong move
No — the speed is constant.
Correct
Yes — the velocity's direction turns, giving inward acceleration v²/r.
Test the repair NL-02 · transfer
Make a fresh prediction before referring to the worked example.