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We created a table in my physics class which contained the strength of gravity on different planet and objects in space. At altitude 0(earth), the gravitational strength is 100%. On the moon at altitude 240,000 miles, it's .028%. And on the International Space Station at 4,250 miles, the gravitational strength compared to the surface of the earth is 89%.

Here's my question: Why is the strength of gravity compared to the surface of the earth 89% even though it appears like the ISS has no gravity since we see astronauts just "floating" around?

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6 Answers 6

up vote 20 down vote accepted

The effective gravity inside the ISS is very close to zero, because the station is in free fall. The effective gravity is a combination of gravity and acceleration.

If you're standing on the surface of the Earth, you feel gravity (1g, 9.8 m/s2) because you're not in free fall. Your feet press down against the ground, and the ground presses up against your feet.

Inside the ISS, there's a downward gravitational pull of about 0.89g, but the station itself is simultaneously accelerating downward at 0.89g -- because of the gravitational pull. Everyone and everything inside the station experiences the same gravity and acceleration, and the sum is close to zero.

Imagine taking the ISS and putting it a mile above the Earth's surface. It would experience about the same 1.0g gravity you have standing on the surface, but in addition the station would accelerate downward at 1.0g. Again, you'll have free fall inside the station, since everything inside it experiences the same gravity and acceleration (at least until it hits the ground).

The big difference, of course, is that the ISS never hits the ground. Its horizontal speed means that by the time it's fallen, say, 1 meter, the ground is 1 meter farther down, because the Earth's surface is curved. In effect, the station is perpetually falling, but never getting any closer to the ground. That's what an orbit is. (As Douglas Adams said, the secret of flying is to throw yourself at the ground and miss.)

But it's not quite that simple. There's still a little bit of atmosphere even at the height at which the ISS orbits, and that causes some drag. Every now and then they have to re-boost the station, using rockets. During a re-boost, the station isn't in free fall; instead. The result is, in effect, a very small "gravitational" pull inside the station -- which you can see in this fascinating video.

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The Hitchhiker's Guide to the Galaxy states: "There is an art to flying, or rather a knack. Its knack lies in learning to throw yourself at the ground and miss." Both the ISS and its occupants are falling due to the Earth's gravitational field, but their tangential momentum means that they keep missing and so orbit the Earth. Since they are falling at the same rate, the occupants appear to float inside. –  Henry Jun 11 '12 at 21:47
Probably safer to talk about weightless than effective gravity. Gravity only depends on mass - if you accelerate 'gravity' hasn't changed, only your weight –  Martin Beckett Jun 12 '12 at 16:27

The astronauts are just floating around because they are each in orbit. Because they are at the same altitude as the ISS they are in the same orbit and moving at the same speed as it and so appear weightless.

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So "technically" there is 90% of gravity in the ISS, but it just appear weightless because of the orbital motion, right? –  SuperStack Jun 11 '12 at 20:46
Yes - although deep space = weightless, being a mere 300km above the Earth doesn't make any difference as far as gravity is concerned. –  Martin Beckett Jun 11 '12 at 20:48

A simpler way to put this is as follows: when you rotate an object tied to a string there is a centrifugal force pulling the object away from the centre of rotation. This is called centrifugal force. Gravity pulls the object toward the centre of gravity which is also the centre of rotation. Gravity acts like a string. When the centrifugal force is equal to the gravitational force the object is in a stable orbit. The object and all within it are weightless as the two forces cancel out.

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First of all, the word "gravity" connotes a material substance that can somehow be transferred from body to body and lack of this material causes a body to NOT gravitationally attract other bodies. After 20 years of teaching undergraduate physics, I have direct evidence of this misconception. The fix is to NOT use the word "gravity" and to replace it with "gravitational attraction." Second, gravitational attraction is required for an artificial satellite to maintain anything other than a linear, uniform speed trajectory. Third, objects don't "float" as that word (words are very important here, by the way) connotes support by some physical influence. Water holds boats up. Air holds aircraft up. Air hold balloons up. Nothing hold the ISS or its contents "up" as the entire system (craft + astronauts) is being pulled toward Earth's center. Fourth, the correct term that describes the ISS and astronaut's state of motion is "free fall." There's nothing more to it than that. The astronauts are free falling toward's Earth's center, but so is the ISS and its floor which would otherwise support them. The astronauts are NOT floating. ISS is NOT floating. Things don't float.

EDIT: There's no such condition as "weightless" either, because the term "weight" is most properly defined as the gravitational attraction between Earth and an object, and that can never be zero. OF course, it can be very, very, very small, but it cannot be zero.

You have no idea how badly words like "gravity" and "weightless" and "float" harm students' understanding of the simplest physics concepts.

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Consider a lift with its rope snapped. The lift would be falling freely. An observer is inside the lift (tough luck for him!) releasing the ball just at the moment of the free fall. Since the ball and lift would be falling freely the ball would appear to float. Thus, to the observer in the lift, it would seem as if no force is acting upon the ball, using this the observer inside can verify Newton's first law as far as observer inside is concerned. A similar situation is inside the ISS, for a person inside ISS he would be falling freely as both the ISS and he himself are undergoing the same acceleration. So in that particular inertial frame he would not be undergoing acceleration.

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Thanxx for the editing you made. –  user38345 Apr 5 at 4:44

Every object in a stable near-circular orbit around the Earth is actually falling towards the Earth at an accelerating speed or at an ever-increasing falling-speed, but because of its horizontal speed around the Earth, ten times faster than a rifle bullet for the ISS, the surface of the Earth curves away below it at the same falling-speed, and it therefore maintains a constant altitude above the Earth and never gets any closer to the ground.

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You know @Keith's answer already states this, right? –  Rory Alsop Apr 27 '13 at 8:45
"<I>actually falling towards the Earth at an accelerating speed or at an ever-increasing falling-speed</I> Acceleration is a vector. An orbiting body accelerates by changing direction vs. the fixed stars. Its speed can be constant. ISS FUBAR flies tangent to the surface. If NASA were not retrocranially inverted, its largest principle inertial moment would be equilibrium pointed at Earth's center of mass in the divergent gravitational field. –  Uncle Al Feb 6 at 1:38

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