Two balls are launched simultaneously from the same height. Ball A is released from rest, and drops straight down. Ball B is given an initial horizontal velocity.

Which ball hits the ground first?

1. Ball A
2. Ball B
3. Both balls hit the ground at the same time

One of the keys to understanding projectile motion is the idea of the independence of x and y. The motion in the vertical direction is completely independent of the motion in the horizontal direction. In this case that means both balls hit the ground at the same time.

Any two-dimensional projectile motion situation can be viewed as two independent one-dimensional motion problems.