   Chapter 13 - Forces     Gravity
Acceleration due to gravity

Gravity causes all objects to accelerate toward the earth at 9.8 m/s/s.  This is true of ALL objects, no matter how things
seem.  The reason all thing do not fall at that speed is the fact that we have an atmosphere on our planet.  This causes air
resistance (Fluid friction) that increases as an object increases with speed.  When the force of gravity and the air
resistance balance out the object stops accelerating and has reached its terminal (ending) velocity.  Because objects have
different shapes  and different weights they will have different terminal velocities.
Gravity is a force of attraction between all matter
in the universe.  Newton determined that the
force of gravity between two objects is
determined by the masses of the two objects and
the distance between the objects.  He used
some math to not only show the formula,  but to show that if the formula was correct how fast both an apple and the moon would fall to the earth due to their different distances!  When he compared his calculations to real life they worked!  This is to say that regardless of whether a projectile is moving downwards, upwards, upwards and rightwards, or downwards and leftwards, the free-body diagram of the projectile is still as depicted in the diagram at the right. By definition, a projectile is any object upon which the only force is gravity.
Many students have difficulty with the concept that the only force acting upon an upwardly
object is moving upwards, then there must be an upward force. And if an object is
moving upwards and rightwards, there must be both an upwards and rightwards force.
Their belief is that forces cause motion; and if there is an upward force then there must
be an upward force. They reason, "How in the world can object be moving upward if the
only force acting upon it is gravity?" Such students do not believe in Newtonian physics
(or possibly don't have a clue what Newtonian physics is). Newton's laws suggest that
forces are only required to cause an acceleration (not a motion). To further ponder this concept of the downward force  and a downward acceleration for projectiles, 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 to Newton's first law of motion, such a cannonball would continue in motion in a straight line at constant speed. In the absence of all forces, "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 cliff. What effect
will gravity have upon the motion of the
cannonball? Will gravity effect 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 effect its
vertical motion. Gravity causes a vertical
acceleration, causing the ball to drop vertically
below its otherwise straight-line, inertial path.
Gravity is the downward force upon a projectile
which influences its vertical motion and causes
the parabolic trajectory which is characteristic of
all 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 are not required to keep a
projectile moving horizontally. The only force acting
upon a projectile is gravity!
 See a Video of how this works from MITThe gun and the monkey