Projectile components are independent only after forces are split
The mistake
Letting horizontal velocity affect vertical acceleration in ideal projectile motion.
Why it feels right
A faster throw clearly changes the path, so it feels like it changes every component.
How to catch it
You put horizontal speed into the vertical acceleration equation with no horizontal force.
The first principle
With only gravity, a_x = 0 and a_y = -g. Horizontal and vertical motions share time but have separate kinematic equations.
worked mini-example
A ball is launched horizontally from a table. What is its vertical acceleration just after launch?
Common wrong move
Less than g because it is moving sideways.
Correct
g downward; horizontal velocity does not change vertical acceleration.
Test the repair NL-19 · transfer
Make a fresh prediction before referring to the worked example.