Many students have difficulty with the concept
that the only force acting upon an upward moving projectile is gravity. Their
conception of motion prompts them to think that if an object is moving upward,
then there must be an upward force. And if an object is moving upward and rightward,
there must be both an upward and rightward force. Their belief is that forces cause
motion; and if there is an upward motion then there must be an upward force.
They reason, "How in the world can an object be moving upward if the only
force acting upon it is gravity?" Such students do not believe in
Newtonian physics (or at least do not believe strongly in Newtonian physics).
Newton's laws suggest that forces are only required to cause an acceleration
(not a motion). Recall from the Unit 2 that Newton's
laws stood in direct opposition to the common misconception that a
force is required to keep an object in motion. This idea is simply not true! A
force is not required to keep an object in motion. A force is only required to
maintain an acceleration. And in the case of a projectile that is moving
upward, there is a downward force and a downward acceleration. That is, the
object is moving upward and slowing down.
To further ponder this concept of the downward
force and a downward acceleration for a projectile, consider a cannonball shot
horizontally from a very high cliff at a high speed. And suppose for a moment
that the gravity switch could be turned off such that
the cannonball would travel in the absence of gravity? What would the motion of
such a cannonball be like? How could its motion be described? According toNewton's first law of
motion, such a cannonball would continue in motion in
a straight line at constant speed. If not acted upon by an unbalanced force,
"an object in motion will ...". This is Newton's law
of inertia.
Now suppose that the gravity switch is turned
on and that the cannonball is projected horizontally from the top of the same
cliff. What effect will gravity have upon the motion of the cannonball? Will
gravity affect the cannonball's horizontal motion? Will the cannonball travel a
greater (or shorter) horizontal distance due to the influence of gravity? The
answer to both of these questions is "No!" Gravity will act downwards
upon the cannonball to affect its vertical motion. Gravity causes a vertical
acceleration. The ball will drop vertically below its otherwise straight-line,
inertial path. Gravity is the downward force upon a projectile that influences
its vertical motion and causes the parabolic trajectory that is characteristic
of projectiles.
A projectile is an object upon which the only force is gravity. Gravity acts to influence the vertical motion of the projectile, thus causing a vertical acceleration. The horizontal motion of the projectile is the result of the tendency of any object in motion to remain in motion at constant velocity. Due to the absence of horizontal forces, a projectile remains in motion with a constant horizontal velocity. Horizontal forces arenot required to keep a projectile moving horizontally. The only force acting upon a projectile is gravity!