Projectile Motion and Orbiting
How is projectile motion different from orbiting?
Projectile motion is for objects very close to the Earth's surface.
They are in the air for a very short time. They move such a short
distance, compared to the distance around the Earth, that one can call
the Earth flat. Projectiles move such a short distance above the
Earth's surface that the gravitational force on the projectile does not
change with height. It is an approximation that works very well, so
long as the object does not go high enough to notice a change of force or
far enough to notice that the Earth is round.
Orbital motion is for objects that move too high above the surface or
travel too far to use the projectile estimate. Gravitational force
decreases as height above the Earth's surface increases. When in an
orbit, the Earth definitely looks round. One has to use the more
complex formulas that apply to orbital motion.
A simpler example of simple approximations is seen in driving. If you
drive at sixty miles per hour, the true speed probably shifts up and
down between 58mph and 62mph. It is much easier to just say the speed
is a constant 60mph, and it will work just as well when calculating how
far you have traveled after twenty minutes. In a more complex
situation, this simple approximation will not work well.
Dr. Ken Mellendorf
Illinois Central College
Geometrically, projectile motion is parabolic, whereas orbital motion
is circular or elliptical. In some cases, orbital motion can be
parabolic, which is also known as escape trajectory. In all cases,
projectile and orbital motion are governed by the laws of physics,
where both are influenced by their velocities and the gravitational
forces between the two bodies of masses.
In physics, it is also known that the distance between two bodies have
a direct influence in their motion. Below a certain distance,
especially near the surface of a much, much larger body of mass, the
gravitational force is so strong that the projectile motion is parabolic.
Hence, "what goes up must come down." At a farther distance, the
gravitational forces between two bodies of masses become weaker, but
is still strong enough to "hold" the two bodies "together" in a motion
that is circular or elliptical. In the case of hyperbolic motion, the
velocity of one body in motion, is so fast that it overcomes the
gravitational force between the two bodies of masses and therefore
"escapes" the force of gravity, hence the term "escape trajectory".
Hope that helps.
Projectile motion means the body follows a path so that comes back
to the ground (lands) right away.
Orbit means the body goes around the earth without hitting the
ground for at least one full trip.
Both are affected by gravity, but the projectile is not going fast
enough or getting high enough to go into orbit.
R. W. "Bob" Avakian
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology
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Update: June 2012