@LearningCHelpMeV2 No, upthrust means the force due to the weight of fluid displaced i.e. Achimedes' principle and this is a constant force. By contrast drag is a force due to energy being dissipated in the liquid and it depends on the speed the body is moving through the liquid.

I need some help with a matter of gravity.

But would you feel jerk in this situation?

I suspect that all changes to the acceleration have a tidal effect, whether they're changes over distance, or changes over time.

OTOH, in perfect circular orbital motion, you don't feel the change in the direction of the centripetal acceleration. So I'm a little confused...

I was thinking that maybe the Gallilean moons could tell us something about this, since they experience tidal heating. OTOH, their orbits are all quite circular, with less eccentricity than Earth's, although of course the distances between the moons vary.

What you'd feel is a proper jerk! :-)

In your frame the proper acceleration is always zero so its time derivative is always zero. So for an observer travelling along a geodesic the jerk in their frame (proper jerk) is always zero.

Thanks, @John. That makes sense. However, only the COM of the planet is actually doing geodesic motion, which is why there are tidal forces on the points not at the COM. Can we still ignore the jerk at those other points?

No, because the proper acceleration of those points won't be zero and would be time dependent for the sort of complex system you've drawn.

Ok. So now we need to calculate how big the jerk effects are relative to the tidal effects. What fun. :(

BTW, please feel free to write an answer to that question on Astronomy.SE. :)

@JohnRennie hi @JohnRennie can you come to our room when free?

@PM2Ring isn't it just what you get applying d/dt to the expression for tidal accelration?

@uhoh Well, applying d/dt to the acceleration itself. And you get the tidal acceleration by applying d/ds to the acceleration, where s is arc length. The tidal force is often given as proportional to the size of the body divided by the cube of the separation between the bodies, but that approximation is poor when the size is large relative to the separation. And of course it's pretty useless in this 3 body scenario.

To make things even more complicated, these bodies aren't rigid spheres, so their shapes wobble a bit due to these tidal distortions. But maybe we can ignore that. ;)

Even JPL ignores the effects of Earth tides on the location of observatories. But they aren't modelling this crazy 3 body chaos stuff. :)

WHats a good introductory texxt foir cardy verlinde formula?

(hey all)

:P

@MoreAnonymous 1. Sunday is traditionally a slow day for chat (this is very much a place of procrastination :P) 2. Cardy-Verlinde is a very specific request, and I'd be surprised if there's something like an "introductory text" for it - have you considered just reading Cardy's and Verlinde's corresponding papers?

I don't even know what is Cardy Verlinde

@ACuriousMind Usually I avoid papers :P I find books much more user friendly. I'm suprised since I think this was discovered in the 1960s

@Slereah Yea me too! Its some bizzare formula about FLRW entropy in 2D (thats my understanding)

@MoreAnonymous what? Wiki says 1986 for Cardy's original formula and 2000 for Verlinde's generalization.

@ACuriousMind Ah lol ... got my dates wrong. I could start with the Cardy formula atleast :P

really, the topic is new and specific enough that I wouldn't expect anything "introductory" on it (and if you don't want to read actual papers when why are you interested in such a specific thing to begin with? :P)

@ACuriousMind Someone asked me: How does your work fit with the Cardy-Verlinde formula

something to look up

Nlab has an entry if it helps : ncatlab.org/nlab/show/Verlinde+formula

@Slereah that's a different formula

Well I tried

we all did

And now we're in this chatroom

:P

@ACuriousMind is there some review you would recommend atleast?

I don't know anything more about this formula than what is on Wiki :P

@ACuriousMind I didn't even know the date on wiki :P