In physics, the motion of a projectile can be broken down into two components: horizontal and vertical. In most cases, the vertical component is the motion caused by gravity acting on the object. For every object, gravity causes a constant acceleration towards the Earth at a rate of 32.2 ft/s^{2}, or 9.8 m/s^{2}. Horizontal motion is the movement of a projectile across a horizontal plane based on the force acting on it. Across short distances, the vertical and horizontal motions of the projectile are perpendicular to and independent of each other.
Newton’s second law of physics states that an object will continue to move at the same velocity as long as no external forces act on it. This means that the horizontal motion of an object will remain constant as long as it remains unaffected by an additional push or pull. If an external force is perpendicular to the motion of the object, as with gravity, a change in direction will occur, but the actual velocity will remain constant. This applies to short distances only because the Earth is assumed to be flat, so that the effect of gravity remains constant.
To be considered horizontal motion, a projectile must be shot in a straight line, not at any angle. The velocity of the projectile can be changed, but what remains the same is that it is shot perpendicular to the Earth’s surface. The projectile is subjected to a constant vertical force of gravity, no matter what horizontal force is used to shoot the projectile, which means that the projectile will always land in the same amount of time. By changing the initial velocity of the projectile, or the force used to shoot it, the projectile will travel longer or shorter distances horizontally in the same amount of time.
As the distance travelled by an object increases, the curve of the Earth begins to take an effect on its horizontal motion. The reason behind this change is that the angle of the horizontal motion of the object changes compared to the Earth’s surface. This means that gravity is no longer perpendicular to the movement of the object, so it will affect the object’s motion. When working with longer distances and/or with projectiles fired at an angle, such as a missile, the horizontal and vertical components must be determined to be able to solve for the distance the projectile can travel.