# Vectors and Projectiles - Mission VP7 Detailed Help A projectile MUST ____. 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. A projectile will move in a way that is typical of an object upon which the only force is gravity. Using a knowledge of physics principles from previous units allows one to make some conclusions about how projectile motion can be described. Since the only force on a projectile is gravity, there is an unbalanced force which causes the object to accelerate. Since the unbalanced or net force is downward, a projectile will accelerate downward. The direction that it moves is irrelevant since forces only determine the direction of the acceleration, not the direction of the motion. Being that a projectile is accelerating, one can conclude that it is changing its velocity. 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.  