# Trying to understand orbiting objects in space

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• mgsullivan24
mgsullivan24
TL;DR Summary
I could really use help understanding stuff
I will try to hide how unintelligent and uneducated I actually am but I’m sure it’ll be showing throughout this question. My apologies. I really don’t know how else to find answers to questions I have besides coming to you all.

In a nutshell, I’m trying to understand why an astronaut floats inside the space station or a shuttle.
Google told me that at 200 miles out the effects of earths gravity are still at about 90%. I can understand “constant fall” idea from the balance of gravity and centripetal force. That would let the astronaut match the vessel in what I would call the horizontal movement.
But what about on the vertical axis. Why aren’t the astronaut and the vessel trying to orbit at different altitudes since their weight is so different? Why isn’t the astronaut being pulled to the ceiling or floor?

Thanks

Orbital mechanics dictates that an object that is moving TANGENTIAL to the Earth falls towards Earth but because it is also moving tangentially, the composite motion is a circle or ellipse around the Earth. While doing this the whole thing is weightless because it is in "free fall" which mean that it is freely falling towards Earth but again, because of the tangential component, it doesn't actually fall to Earth, it just moves weightlessly. Everything in it or on it is of course, also weightless.

mgsullivan24
phinds said:

Orbital mechanics dictates that an object that is moving TANGENTIAL to the Earth falls towards Earth but because it is also moving tangentially, the composite motion is a circle or ellipse around the Earth. While doing this the whole thing is weightless because it is in "free fall" which mean that it is freely falling towards Earth but again, because of the tangential component, it doesn't actually fall to Earth, it just moves weightlessly. Everything in it or on it is of course, also weightless.
Thank you for the reply. I have looked at that and it does make sense for the “weightlessness” of an object in orbit. Maybe I’m not understanding the correlation though. Why don’t the two objects try to move differently since the weights/mass are so different?

mgsullivan24 said:
Why don’t the two objects try to move differently since the weights/mass are so different?
That is the way that gravity works. The force of gravity is proportional to the mass: ##F=mg##. And Newton’s 2nd law is: ##F=ma##. Combining those equations you get that ##a=g##, regardless of ##m##. So everything in free fall accelerates the same.

mgsullivan24, jbriggs444, russ_watters and 1 other person
mgsullivan24 said:
Thank you for the reply. I have looked at that and it does make sense for the “weightlessness” of an object in orbit. Maybe I’m not understanding the correlation though. Why don’t the two objects try to move differently since the weights/mass are so different?
Here's an experiment to show that, once you remove air resistance, all objects have the same acceleration under the Earth's gravity. This experiment was famously carried out on the Moon by the Apollo 15 astronauts - the Moon has no atmosphere.

In general, this guy's videos are a good source for basic experimental physics.

This principle extends so that different objects will stay in the same orbit if they start in the same orbit.

mgsullivan24 and phinds
mgsullivan24 said:
TL;DR Summary: I could really use help understanding stuff

Why isn’t the astronaut being pulled to the ceiling or floor?
There IS an effect which causes this to happen but it's very small in a regular spacecraft because, as we already established, there is a significant gravitational field, at even large distances from Earth. Instead of an astronaut in a spaceship, imagine two masses, orbiting the earth, next to each other and in a circular orbit. Their common tangential speeds and orbital heights are each related by the basic orbital laws. If they are at, say, a few metres different orbital heights then they will each experience slightly different g force. If they start at the same tangential speed then there is an inconsistency which is taken care of by the outer one slightly moving away and the inner one moving slightly inward (they will then be following the 'laws'.) Their mutual centre of mass may be following a perfectly circular orbit but they will move apart steadily in their own orbits.
An astronaut is in one place and the centre of mass of the ship is in another so, once she lets go, she will drift away (in her own orbit) until she hits the side of the ship and they will then follow the same orbit because of the tiny restraining force. Those two orbits will take the two masses on orbits that cross again (and again, if they don't actually collide.
PS the astronaut is not "pulled to the ceiling"; she just follows a path until she hits it.
PPS two objects up there in nearly common orbits will be affected by the mutual attraction due to their masses. They may drift apart but can eventually come back together in their own mutual orbit. Take the Moon - Earth combination in orbit round the Sun.
It seems that bits of space ship that come detached may hang around and return to the ship due to miniscule mutual attraction. But the time scale for this would be very long and (of course) the returning bits would be travelling very slowly

mgsullivan24
Dat's a scary image, IMO:

PeroK said:
In general, this guy's videos are a good source for basic experimental physics.

The picture makes it look like he is doing the experiment bare-handed in a vacuum chamber, but thankfully (after I skimmed through the video), he is not. Phew!

sophiecentaur said:
There IS an effect which causes this to happen but it's very small in a regular spacecraft because, as we already established, there is a significant gravitational field, at even large distances from Earth. Instead of an astronaut in a spaceship, imagine two masses, orbiting the earth, next to each other and in a circular orbit. Their common tangential speeds and orbital heights are each related by the basic orbital laws. If they are at, say, a few metres different orbital heights then they will each experience slightly different g force. If they start at the same tangential speed then there is an inconsistency which is taken care of by the outer one slightly moving away and the inner one moving slightly inward (they will then be following the 'laws'.) Their mutual centre of mass may be following a perfectly circular orbit but they will move apart steadily in their own orbits.
An astronaut is in one place and the centre of mass of the ship is in another so, once she lets go, she will drift away (in her own orbit) until she hits the side of the ship and they will then follow the same orbit because of the tiny restraining force. Those two orbits will take the two masses on orbits that cross again (and again, if they don't actually collide.
PS the astronaut is not "pulled to the ceiling"; she just follows a path until she hits it.
PPS two objects up there in nearly common orbits will be affected by the mutual attraction due to their masses. They may drift apart but can eventually come back together in their own mutual orbit. Take the Moon - Earth combination in orbit round the Sun.
It seems that bits of space ship that come detached may hang around and return to the ship due to miniscule mutual attraction. But the time scale for this would be very long and (of course) the returning bits would be travelling very slowly
Thank you! Also, to Perok and Dale for also commenting.
I think I get it, well I’m seeing where I am getting lost. Being uneducated I don’t know about orbital laws and such. My understanding of physics is very basic and limited.

My brain tends to think in “forces” I’ve experienced. What I’m understanding is that orbiting objects act in a way that is not intuitive to how I understand force and gravity on a strictly anecdotal basis. To me it seemed like it is basically a balancing act, like gravity holding objects in position based on their weight or mass. I see that’s not exactly how it works; and/or their mass differential doesn’t Cause as much effect as I assumed.

Dale

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