# Remind me again how gravity works? **URL:** \ **Category:** Factual Questions\ **Created:** [October 12, 2012, 2:38pm UTC](https://boards.straightdope.com/t/remind-me-again-how-gravity-works/637513 "2012-10-12T14:38:51Z")\ **Posts on this page:** 4\ **Page:** 2
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [October 12, 2012, 8:34pm UTC](https://boards.straightdope.com/t/remind-me-again-how-gravity-works/637513/21 "2012-10-12T20:34:19Z")
In the case of the Earth, the gravity actually increases slightly till you get to the mantle, then decreases linearly till you reach the core. This is because the density of the Earth is not constant, and the effect of getting closer to the denser core is greater than the effect of not feeling the gravity from the layers above you for the first part of the descent. I would expect this to be even more true of Jupiter. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [October 12, 2012, 8:44pm UTC](https://boards.straightdope.com/t/remind-me-again-how-gravity-works/637513/22 "2012-10-12T20:44:02Z")
_basks in the validation of fellow physics Dopers_ šŸ˜ƒ ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [October 12, 2012, 9:26pm UTC](https://boards.straightdope.com/t/remind-me-again-how-gravity-works/637513/23 "2012-10-12T21:26:55Z")
> [@chrisk](#): > > Did Newton solve it for the case where the density of the planet was constant, or for all possible cases? Well, for all possible cases where the density is evenly spread out. i.e., if you have a huge concentration of density in India, and a huge region of very light mineral formations in Mexico, then the shell itself is asymmetric. Someone very deep below India would feel a net upward pull, while someone deep below Mexico would feel a net downward pull. But if the shells themselves (think in terms of calculus, where the shells can be made arbitrarily thin, like an ideal onion) are symmetric, then any collection or sum of them will follow Newtonā€™s rules. > [@](#): > > Take a spherical volume centered around the planet Earth, but with equal radius to the commonly accepted radius of Jupiter. Release a low-density gas into this volume, (and use a force-field if necessary to keep it from drifting away) and call this volume the planet Super-Earth. > > As you ā€˜descendā€™ through Super-Earth to the solid surface, will the pull of gravity increase or decrease? I say, in this case, it will increase, because the effect of the sparse gas in the layers you are descending past is small enough to be outweighed by the fact that you are approaching closer to a dense core, and the effect of gravity from that core, (ie, the solid planet Earth as we know it,) will grow stronger as you approach. > > Would you agree with me so far? Ah! It took me a while to visualize this, but, yes, I quite agree. All that Newton said was that you disregard the shells above you. Since, in your model, they arenā€™t doing much anyway, then, yeah, gravity would increase as you fall through this vast extended ā€œsuper atmosphereā€ toward the surface of the ā€œreal earth.ā€ > [@](#): > > In that case, is there anything that prevents the same effect from manifesting as we descend through the Jovian atmosphere? I suspect that we donā€™t know density of the different layers of Jupiter to be certain. > > (And yes, Iā€™m pleased that I got an opportunity to use ā€˜Consider:ā€™ after youā€™d referenced Ringworld šŸ™‚ ) Ah! again! Okay, you got me: I dunno. The density almost certainly does increase toward the center of Jupiter (or as Pogo would say Jupister.) But, as you descend and disregard larger and larger external shells of matter, what remains is smaller and smaller, and thus has less and less pull. In your model of a Super-Earth, the gradation of density is extreme. In Jupiter, it may be sufficiently gradual as not to produce the effect of increasing gravitational pull. (Iā€™m almost good enough at math to answer the question, if someone could present a function of Jupiterā€™s density as a function of distance from its center. I just did some Googling and didnā€™t find it.) ETA: Oops! Missed **AndrewL** ā€™s post. Right! Sorry! Yes! ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [October 12, 2012, 9:59pm UTC](https://boards.straightdope.com/t/remind-me-again-how-gravity-works/637513/24 "2012-10-12T21:59:15Z")
> [@chrisk](#): > > Take a spherical volume centered around the planet Earth, but with equal radius to the commonly accepted radius of Jupiter. Release a low-density gas into this volume, (and use a force-field if necessary to keep it from drifting away) and call this volume the planet Super-Earth. This is a really weird and fascinating thing to envision. You realize that almost half of your super-atmosphere would be above the altitude for geosynchronous orbits? A layer of gas that would be both rotating with the earth and orbiting it, and above that, not rotating because orbital speed is too slow. Imagine the crazy weather patterns such an atmosphere would have. I guess it helps explain why the gas giants have rather short day lengths. [Previous page](https://boards.straightdope.com/t/remind-me-again-how-gravity-works/637513.md?page=1)