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Definition of a Projectile:
A projectile is an object for which the only force acting upon it is gravity. The force of air resistance is negligibly small or nonexistent for a projectile.
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A projectile is an object upon which the only force of gravity. As such, an object experiencing normal forces or air resistance forces is not an object consistent with the definition of a projectile. Being that gravity is the only force, there is an imbalance of forces with the net force being the force of gravity. Thus, one can conclude that a projectile will accelerate under the influence of this net force, changing its velocity over the course of its motion.
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Many students of physics are fooled into thinking that a projectile must be moving downward and cannot be moving upward. After all, they think, the only force is gravity and it acts downward. Such thinking is faulty because it presumes that the direction of motion is determined by the direction of the net force. But don't be fooled! Newton's Laws suggest that an object can be moving upward and slowing down if the only force is a downward force. Forces do not determine the direction of motion, but rather the direction of acceleration. A projectile can be moving in any direction; but it must be accelerating in the downward direction.
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