Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 11/07/2016 14:12:09
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Could you simplify gravity for me? My problem is if the ISS has microgravity only 400km above Earth. How does the Sun have an effect on us and even more unbelivelby have a gravitional effect on the outer planets (keeping the planets in orbit) when it losses its affect so quickly?
Asked by Dan
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[chapter podcast=1001386 track=16.07.05/Naked_Scientists_Show_16.07.05_1005381.mp3](https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.thenakedscientists.com%2FHTML%2Ftypo3conf%2Fext%2Fnaksci_podcast%2Fgnome-settings-sound.gif&hash=f2b0d108dc173aeaa367f8db2e2171bd) ...or Listen to the Answer[/chapter] or [download as MP3] (http://nakeddiscovery.com/downloads/split_individual/16.07.05/Naked_Scientists_Show_16.07.05_1005381.mp3)
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The forces we can exert on objects are not uniformly distributed throughout those objects. In the case of gravitation the force is evenly distributed throughout the objects under its influence. This means that gravity is more efficient since it operates at the microscopic quantum level. I hope this helps.
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My problem is if the ISS has micro gravity only 400 k above earth... it loses its effect so quickly?
The ISS (and everyone on it) is accelerated by Earth's gravity, at pretty much the same acceleration as someone standing on Earth's surface.
The Earth's surface has a radius of around 6,370 km, and gravitational acceleration is around 9.8m/s2.
The ISS orbit has a radius of around 6,370+400 = 6,770km, or 6% more.
The acceleration due to gravity at the ISS orbit is 88% of the value at the Earth's surface, or 8.7m/s2 (by the "inverse square law"). This is enough to keep the ISS in orbit, to keep bending its path in a circle so it doesn't go flying off into space. And it bends the path of the astronauts by the same amount, so they don't get noticeably closer or farther away from the walls of the ISS. ie it looks like microgravity, even though gravity is still quite strong.
So gravity would not be noticeably lower if you were standing on a 400km high extension of the Eiffel tower.
The Earth's gravity is still quite strong at the distance of the Moon (around 300,000km) - enough to keep the Moon in orbit.
How does the sun have an effect on us and even more unbelievably have a gravitational effect on the outer planets (keeping the planets in orbit) when it looses its affect so quickly?
The Sun's gravitational field also obeys the inverse square law. So Mercury orbits more quickly than Earth, which orbits more quickly than Jupiter. The Sun even plays a significant part in the motion of cometary objects in the Oort cloud.
In theory, the Earth's gravitational field extends to the stars, but at large distances, the Earth's gravitational field can be ignored in the presence of the much larger mass of Jupiter and the Sun.
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Microgravity is a misleading term; the astronauts inside the ISS are floating around not because gravity there is appreciably weaker but because they are weightless, which is not the same as "zero/low gravity". They are weightless because an orbiting spacecraft is in a state of perpetual freefall - it is constantly falling towards Earth but missing. As they are inside the spacecraft, they too are in a similar state of freefall; hence, relative to their surroundings they are weightless and float about. The effect would be the same if you were inside a lift at the top of a very tall building and somehow the cable was cut. The lift would fall and so would you, leaving you weightless inside the lift...
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We discussed this question on our show
We put this question to professor Andrew Norton, astronomer from the University of Cambridge...
Click to visit the show page for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/naked-scientists/show/20160705/) Alternatively, [chapter podcast=1001386 track=16.07.05/Naked_Scientists_Show_16.07.05_1005381.mp3](https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.thenakedscientists.com%2FHTML%2Ftypo3conf%2Fext%2Fnaksci_podcast%2Fgnome-settings-sound.gif&hash=f2b0d108dc173aeaa367f8db2e2171bd) listen to the answer now[/chapter] or [download as MP3] (http://nakeddiscovery.com/downloads/split_individual/16.07.05/Naked_Scientists_Show_16.07.05_1005381.mp3)