In projectile motion, the quantity that remains constant is the horizontal component of velocity. This is because there is no acceleration acting in the horizontal direction (neglecting air resistance), so the horizontal velocity does not change throughout the motion. The vertical component of velocity changes due to the constant acceleration of gravity acting downward.

Explanation

• The motion of a projectile is influenced only by gravity, which acts vertically downward.
• The horizontal velocity remains constant because there are no horizontal forces or acceleration acting on the projectile.
• The vertical velocity changes linearly because of the constant acceleration due to gravity.

Thus, the horizontal component of velocity is the key constant quantity in projectile motion.