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Spacecraft, satellites, etc. They also experience zero gravity, weightlessness, or "micro gravity" when outside Earth, about 20-300+ miles beyond the escape velocity zone("atmosphere"). However, you experience full gravity on the Earth. Why does gravity decrease just slightly farther away?

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    $\begingroup$ Your premise that gravity drops off to zero (or "micro") in orbit is completely wrong. There is no such thing as an "escape velocity zone". $\endgroup$ – Brandon Enright Nov 19 '13 at 22:38
  • $\begingroup$ You know what I mean ... the "imaginary line" between Earth and low-Earth orbit. $\endgroup$ – Captain Plaster Nov 19 '13 at 22:41
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    $\begingroup$ That line isn't imaginary. It's called the atmosphere. If it weren't for the atmosphere you could orbit the Earth just inches above the tallest object in your way. You would feel weightless too. You could orbit the moon just a few meters off the surface provided no craters rims or mountains were in the way. $\endgroup$ – Brandon Enright Nov 19 '13 at 22:42
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    $\begingroup$ Your misunderstanding of the physics is too great to resolve in comments. $\endgroup$ – Brandon Enright Nov 19 '13 at 23:10
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    $\begingroup$ @CaptainPlaster you have to move sideways very fast to orbit something. I highly doubt NASA wanted to waste energy just to prove this. Also if they mess up the calculations, and try to orbit too low, the astronauts crash into a hill at high speed and die. $\endgroup$ – immibis Apr 23 '14 at 13:36
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Partly because the magnitude of the gravitational force decreases as $\frac{1}{r^2}$, so as the distance from the center of the earth, $r$, increases, the magnitude decreases. The bigger reason for spacecraft is because they are constantly in free fall, and there is no way to feel gravity when you are falling freely. The spacecraft are falling and moving forward at great enough speed that they literally fall "around" the earth. When you are in orbit, what prevents you from flying off in a straight line is the centripetal force supplied by gravity. So it isn't that there is no gravity, but that there is nothing pushing back up on you to give you the feeling of being heavy.

When you are on Earth, the Earth is pulling down on you, and what prevents you from falling through the ground is the surface pushing back up on you. You perceive this upward push as your "weight". If you are falling and nothing is pushing back up on you, you feel weightless. This is why, when airplanes execute certain parabolic maneuvers, the crew inside can float around, because they are falling at the same rate as the airplane.

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  • $\begingroup$ The Earth can't push you back ... counter-force is impossible. I have never heard of gravity "pushing back" against mass(that would imply bi-directional force amplitude). Do you have any sources for this? $\endgroup$ – Captain Plaster Nov 19 '13 at 22:48
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    $\begingroup$ @CaptainPlaster The source is that you don't fall through the earth. $\endgroup$ – jinawee Nov 19 '13 at 22:53
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    $\begingroup$ @CaptainPlaster Do you know what normal force is? $\endgroup$ – jinawee Nov 19 '13 at 22:56
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    $\begingroup$ @CaptainPlaster When you fall from 3rd floor you have both situations... :). 1st you feel the "free" fall where (almost) no forces are reducing your speed and when you reach the ground, you have the effect of the normal force stopping your fall. If you think breaking your bones can be done without any force, you have to revise basic things. $\endgroup$ – laurent Nov 19 '13 at 23:50
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    $\begingroup$ @CaptainPlaster, read carefully. What prevents you from falling through the ground is the earth, with a lowercase 'e'. I.e. the ground beneath. :) This "normal force" is due to the Pauli exclusion principles for electrons conspiring with the Coulomb force to produce a kind of exchange interaction. $\endgroup$ – lionelbrits Nov 19 '13 at 23:52

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