Parabolic Motion of Projectiles
A projectile is an object upon which the only force is gravity. Gravity, being a downward force, causes a projectile to accelerate in the downward direction. The force of gravity could never alter the horizontal velocity of an object since perpendicular components of motion are independent of each other. A vertical force does not effect a horizontal motion. The result of a vertical force acting upon a horizontally moving object is to cause the object to deviate from its otherwise linear path. This is depicted in the animation below.
According to Newton's law of inertia, an object in motion in a horizontal direction would continue in its horizontal motion with the same horizontal speed and direction unless acted upon by an unbalanced horizontal force. The animation above shows a green sphere moving to the right at constant speed. The horizontal distance traveled in each second is a constant value. The red sphere undergoes a vertically accelerated motion which is typical of an object upon which only the force of gravity acts. If these two motions are combined - vertical free fall motion and constant horizontal motion - then the trajectory will be that of a parabola. An object which begins with an initial horizontal velocity and is acted upon only by the force of gravity will follow the path of the blue sphere. It will travel the same horizontal distance in each consecutive second but will fall vertically a greater distance in each consecutive second. The result is a parabolic path as shown in the animation above.
For more information on physical descriptions of motion, visit The Physics Classroom Tutorial. Detailed information is available there on the following topics:
Independence of Perpendicular Components of Motion
What is a Projectile?
Characteristics of a Projectile's Trajectory
Describing Projectiles with Numbers