Dynamics and the Laws of Motion - Action Analysis for Animators - page 21

Let's consider for a moment the effect of gravity on a cannonball shot from a
cannon. Without the effect of gravitational forces or friction as a result of air
resistance, the cannonball shot horizontally from a high place would simply
travel in a straight line away from the earth and continue on its way into
infinity, unless it encountered another object to stop or deflect it.
FIG 2.2 Escape velocity must be reached to send objects into orbit or beyond the gravitational pull of
the earth.
However, gravitational forces do exist, and these forces determine that the
cannonball will not continue to travel on and on but instead will fall back
to earth. Where and when the cannonball falls to earth, or indeed if the ball
falls to earth, is dependent on the different forces exerted and acting on the
cannonball. The variations in these forces mean that different results will be
achieved in each case.
At low speeds the cannonball will fall to earth; the lower the speed, the nearer it
will fall to the cannon from which it was projected. At much higher speeds that
are equal to a threshold velocity, the cannonball will not fall back to earth but
will remain in orbit around the earth. This is a result of the balance among the
velocity of the ball, the result of the explosion within the cannon that propels
the ball away from the earth, and the gravitational pull of the earth that pulls
the ball toward the earth. If these forces are in balance, a stable orbit is achieved.