In projectile motion, horizontal velocity is the rate at which an object is traveling parallel to the earth. Scientists calculate horizontal velocity using the formula v = d/t, the same formula used to determine the speed of an automobile. In projectile motion, horizontal velocity does not change; however, the forces of gravity give the object vertical acceleration, causing it to stop when it hits the ground.
On earth, the vertical acceleration due to gravity remains constant at 32.2 feet per second per second. If a quarterback throws a football in an upward pass, it has both vertical and horizontal velocities. Ignoring forces of friction due to air and wind resistance, the horizontal velocity remains the same throughout the ball's arched movement. However, the deceleration from gravity causes the vertical velocity to decrease until it reaches zero at the ball's maximum height and then to accelerate in a downward direction as it continues to move toward the receiver. If these forces are working according to the quarterback's plan, the ball is at the exact height for the receiver to complete the pass.
The military uses horizontal velocity and projectile motion in warfare. It allows them to calculate the distance a bullet travels before hitting the ground. If an airplane drops a bomb, its horizontal velocity is the same as the plane. In order for the bomb to hit its target, the bomber must consider both horizontal velocity and vertical acceleration.