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

Key point: Independence of x and y!. The vertical and horizontal motions are independent. This means that both balls hit the ground at the same time.

Basic consequence: Two-dimensional projectile motion can be viewed as two independent  one-dimensional motions.