What does tidal gravity feel like?

[FtlIsAwesome]

Suppose we have an astronaut orbiting Object X. His feet are tidally locked with it. He is close enough that there is a gravity difference between his feet and his head, say 0.3 g at his feet and -0.3 g at his head. What exactly will this feel like? What about lesser or stronger gravity differences?

 

[tiny-tim]

Hi FtlIsAwesome!

(what's negative gravity? )

Tidal gravity s t r e t c h e s the astronaut …

thats all there is to it ​

 

[FtlIsAwesome]

Oops. I kinda thought that didn't make sense, but I wasn't sure how to word it.
The astronaut feels a pull on his feet downward, and a pull on his head upward, while his center is weightless, right?

 

[tiny-tim]

Yup! …

same thing happens to the Moon …

its centre of mass is orbiting at exactly the correct speed …

the bit nearest the Earth is orbiting too slowly, and the bit furthest too fast, so both feel a force away from the centre, ie the Moon is stretched

(which is basically why the Moon lost all that energy that caused it to face us all the time)

 

[DaveC426913]

Lie with your centre-of mass at the centre of a fast-rotating carousel to achieve this same effect.

 

[phinds]

Oops. I kinda thought that didn't make sense, but I wasn't sure how to word it.
The astronaut feels a pull on his feet downward, and a pull on his head upward, while his center is weightless, right?

The only way to get his head pulled upward would be if there was an equally powerful gravitational attractor as close to his head (in terms of effect) as the gravitational attractor in the direction of his feet.

What happens, as pointed out in the posts before mine, is that his head and his feet are BOTH pulled down, just that the feet are pulled down MORE and the middle of his body somewhere in the middle. His middle is not in free fall.

 

[FtlIsAwesome]

What happens, as pointed out in the posts before mine, is that his head and his feet are BOTH pulled down, just that the feet are pulled down MORE and the middle of his body somewhere in the middle. His middle is not in free fall.

Isn't there a difference between falling toward an object and orbiting it? I can see this happening if you are falling towards it, not orbiting it.

[The Moon's] centre of mass is orbiting at exactly the correct speed …

the bit nearest the Earth is orbiting too slowly, and the bit furthest too fast, so both feel a force away from the centre, ie the Moon is stretched

 

[phinds]

Isn't there a difference between falling toward an object .

It has been a lot of years since I did basic physics, but I think it is correct to say that "orbiting an object" IS "falling toward it". If you weren't falling toward it, you would be flying off into space. I understand that this is NOT moving closer to the center, which I'm figure is likely what you mean by falling. With luck, someone more knowledgeable will jump in any explain this better. An astronaut orbiting the Earth experiences tidal forces, but they are so tiny that he doesn't "experience" them in any meaningful way. If he were orbiting a black hole, that would be a different story, but that's a difference in quantitative effect, not qualitative effect.

I can see this happening if you are falling towards it, not orbiting it.

Again, I hope someone more knowledgeable will jump in, but I believe that tidal forces are purely a function of distance from the center of mass, not dependant on whether you are orbiting or heading straight in.

 

[DaveC426913]

I was about to bring this distinction up.

If you were orbiting, you would (at least in theory) feel yourself flung around the object - that's what the upward pull would feel like. Imagine sticking your head off the edge of a carousel. You'd feel like you were hanging upside down.

 

[phinds]

I was about to bring this distinction up.

If you were orbiting, you would (at least in theory) feel yourself flung around the object - that's what the upward pull would feel like. Imagine sticking your head off the edge of a carousel. You'd feel like you were hanging upside down.

Ah ... I think I get that Dave. Thanks. I guess the spahettification would still happen (if you were close enough) but would act differently than I was thinking, because whatever force it is that was keeping you from getting any closer to the center of mass would be pulling your head off and the gravity from below would be pulling your feet off, and my earlier statement that your middle is NOT in freefall was wrong. Do I have it right now?

 

[DaveC426913]

Larry Niven wrote the Hugo award-winning http://en.wikipedia.org/wiki/Neutron_Star_(short_story)" where the protagonist went through this exact experience.

He did a nape-of-surface hyperbolic trajectory around a neutron star and only survived by positioning himself at the ...

...wait...

Read the story.

 

[phinds]

Dave, I read all the early Niven stuff as it came out so I'm sure I read that one but I stopped reading Sci Fic quite a few years ago and my memory isn't what it used to be. In fact my memory never was what it used to be.

I've gotten into cosmology and QM as a raw amateur in the last year or so and find it discouraging sometimes how much of the simple basic physics stuff I have forgotten, and even worse how much of it I can no longer figure out nearly as readily as I once was able to 50 years ago when I was in engineering school.