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Name: Vamshi
Status: student
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Question:
How is projectile motion different from orbiting?


Replies:
Vamshi,

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
Physics Instructor
Illinois Central College


Hi Vamshi,

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.

Alex Viray


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
Instructor
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology



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