# Tag Info

1

I'm not entirely sure what you mean about 'pulling hydrogen', all bodies, whether they be planets or literally human bodies, will pull hydrogen via gravity. Earth can lose the H it attracts as H is so light that it can have speeds greater than the escape velocity (just due to random thermal motion). Perhaps Jupiter is sufficiently massive that this happens ...

2

Jupiter will never (not on any timescale like the lifetime of the Sun anyway) accrete enough mass to begin hydrogen fusion. It would need to accrete 12 times its current mass to undergo a brief period of fusing its interior deuterium and to accrete more than 70 times its current mass to attain a central temperature high enough to sustain hydrogen (pp chain) ...

21

The answer is that inside a spherically symmetric shell of matter (your hollow earth or massive beach ball) there is no gravitational force anywhere - you will not "fall" in any direction, whether you are at the centre or not, regardless of the radius of the sphere. This is a classic result of both Newtonian Gravity, and Einstein's General Theory of ...

26

If the mass/charge is symmetrically distributed on your sphere, there is no force acting on you, anywhere within the sphere. This is because every force originating from some part of the sphere will be canceled by another part. Like you said, if you move towards on side, the gravitational pull of that side will become stronger, but then there will also be ...

0

The mean radius of the earth's orbit has not changed perceptably in the last 1000 years, or even in the last 10,000 years. In the long run, though... The dominant mechanism in the long run is the solar wind, which is estimated to cause a mass loss of 7-9 x 10^-14 per year. As solar mass is lost, conservation of angular momentum causes the earth's orbit to ...

2

I can't give a truly mathematical answer, but consider Saturn's rings. There's 2 key differences between proto-planet dust and Saturn's rings and that's that Saturn's rings are inside the planet's Roche limit so they won't ever have the cohesion to clump, and Saturn's rings are remarkably flat. So, what happens with proto-planet dust is that it would ...

0

Nothing wrong with your reasoning. Planets aren't plane mirrors, they are more like the first case that you discuss. The reflected light from the Sun does not come straight back to the observer, it is reflected over a range of angles. The luminosity of the planet will actually be $$L \simeq a \pi R_p^{2} \frac{L_{\odot}}{4\pi d^2}$$ where $R_p$ is the ...

1

Yes. There are a few ways this happens. Ultimately, it's an energy budget issue. There is less "mass" being radiated, and more "energy" being radiated. Of course, they're fundamentally related. Here are some of the ways this can happen: Loss of energy due to tidal dissipation Radiative Heat Transfer / Radiant heat loss Atmospheric particle escape ...

3

You appear to be using energy to move all the "stuff" out of a hole, and then filling in the hole again with "special magnetic stuff" to extract the same energy as electricity, with inevitable losses... Perhaps you could practice by pumping water from Lake Ontario up into Lake Erie, and then selling the electricity you get when the water flows through the ...

2

It is absolutely ridiculous. The deepest drilling on earth, the Kola Superdeep Borehole, is $12.262\,\mathrm{km}$ deep, the earths diameter is about $13000\,\mathrm{km}$, that is the deepest hole on earth is one thousandth of earth's diameter! Furthermore, the inner of the earth is hot, your coils would melt, your magnets would not be magnetic (as they have ...

5

All bodies with temperatures higher than 0Kelvin radiate away electromagnetic energy according to black body radiation. The power emitted goes according to the Stefan-Boltzmann law. The law states that the total energy radiated per unit surface area of a black body across all wavelengths per unit time (also known as the black-body radiant exitance or ...

2

What if this planet existed and you fell in? Let's take a look at that and see. I'll spare listing all the reasons why a planet like this can't exist and wouldn't last long if it did; those are no fun. So instead, let's say this planet does, for some reason, exist in some galaxy far far away and it's made of something strong enough that it'll be around for ...

-1

Take a baseball and put it at about ground level and drop it into the hole. The gravity will pull on the ball and will accelerate and plunge through the planet, the void, and on it's way up again (or down from your perspective), gravity will slow it's ascent to the point that it'll just about make it to ground level on the other. The ball will spring back ...

6

Emilio Pisanty's answer is a great one to this particular answer, i.e. there is no in principle bar from the laws of physics to a structure like yours and it would have the zero gravity inside property that Emilio describes. But, from a materials perspective, it would be pretty much impossible for a planet like this to form unless very small. The stablest ...

4

According to the shell theorem, the gravitational force inside such a hollow spherical planet would be 0 On the outside, gravitational force would be as if the planet was a regular planet. At some point during your fall, the gravitational force pulling you to the center would decrease to 0. If there's atmosphere inside the planet, it would slow you down ...

21

Yes, this is possible. It is perfectly fine for a mass configuration to produce, for points outside a sphere of radius $R$ centred at $\mathbf r_0$, a gravitational field identical to that of a point mass at $\mathbf r_0$, and still be completely empty inside a smaller sphere of radius $a$ around $\mathbf r_0$. The spherical-shell model you describe is ...

1

What is the shape of a self-gravitating rotating body? I didn't read through all answers but skimmed them instead. So far as I've read, are missing a piece. As @Alan Rominger pointed out the gravitational acceleration at any point on a self-gravitating sphere varies with latitude, like it does on the Earth. But the shape the body is complex and does not ...

1

Short version: The Earth is round because it was molten once. It would definitely matter if our planet were a cube, because gravity near the points would be off from "vertical." Longer version: Smaller objects in our solar system are indeed more oddly shaped - they often look like a bunch of small blobs stuck together. They slowly build up in size as bits ...

Top 50 recent answers are included