How can horizontal motion be uniform while vertical motion is accelerated?

Imagine standing on the back of a truck moving at constant velocity and then throwing a ball vertically in the air. Relative to you the ball will go vertically up and back down so you could catch it (neglecting air resistance). Thus its vertical motion would be accelerated due to gravity but its horizontal motion would be uniform. In fact viewed by someone on the ground the path of the ball would be a parabola. Drag would the shape of the parabola to be distorted since both horizontal and vertical motion would be affected slightly.

Ok, this is a question regarding projectile motion theory.

When beginning projectile motion, we ignore the effects of air resistance which acts horizontally to oppose the motion of the projectile in flight, hence no net force, no acceleration, and thus we have uniform motion.

Vertically however, we do not neglect the effect of gravity, hence the projectile is constantly under an acceleration of 9.8m/s/s due to the force of gravity acting on it.

Horizontal motion and vertical motion are completely independent of each other in this situation. None of the vertical component effects the horizontal component.

If air resistance is taken into consideration (only horizontally and not vertically), the horizontal component will undergo a deceleration, this means the projectile won't travel as far horizontally, it will still fall at the same rate, it will just curve over more to the left if it was traveling to the right, also, the trajectory will no longer be perfectly parabolic.

Hope that helped.