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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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Acceleration, Changing Velocity and Net Force
An accelerating object is an object that is changing its velocity - either slowing down, speeding up or changing directions. When focusing on one dimension of an object's motion (the horizontal dimension in this question), an acceleration in that direction will be the result of slowing down or speeding up.
Accelerations are caused by an unbalanced or net force. If the individual forces in a given dimension balance, then there is no acceleration in that dimension. If the individual forces are unbalanced, then there is an acceleration in that dimension.
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Many students confuse velocity and acceleration. For the horizontal motion of a projectile, there is a zero acceleration. But don't be fooled! The fact that the horizontal acceleration is zero does not mean that the horizontal velocity is zero. A horizontal acceleration of zero simply means that there is no change in the horizontal velocity of a projectile.
Many students also have difficulty with the need to separate a projectile's motion into two independent directions. A projectile simultaneously moves in the horizontal and the vertical direction. Since perpendicular components of motion are independent of each other, these two dimensions of motion can be discussed separately. It is critical that a student does not confuse information about the horizontal direction of motion with information about the vertical direction of motion. For the horizontal motion of a projectile, there is no force, no acceleration and no change in velocity. For the vertical motion of a projectile, there is a net force (gravity), and acceleration (the acceleration of gravity) and a change in velocity. Avoid confusing these two distinctly different conclusions regarding the two directions of motion.
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