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102 votes
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If a massive object like Jupiter flew past the Earth how close would it need to come to pull people off of the surface?

TL:DR Jupiter isn't dense enough for its gravity gradient over Earth's radius to produce a 1g tidal acceleration, even right at Jupiter's surface. thanks to PeterCordes Jupiter's gravity will pull ...
uhoh's user avatar
  • 30.8k
74 votes

Why does gas form a star instead of a black hole?

Essentially gas does, it just happens to form a star first. Mass is not the only factor in creating a black hole. You also need for this mass to reach a high density. In the process of doing this, a ...
Natsfan's user avatar
  • 4,494
58 votes

Why can't we feel the Earth's revolution?

You do, but it's too small to really notice First, it's not correct to say that we don't feel Earth's rotation because it's rotating at a constant speed. Think about driving a car, or riding in an ...
ap55's user avatar
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56 votes
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Moon's unusual gravity

As you said, the mass of the Moon is 1.2 percent that of the Earth. Now, if you mean the gravitational acceleration at the surface, it is calculated like this $G\frac{M}{R^2}$, where $M$ is the mass, ...
Tosic's user avatar
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53 votes
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Is Earth's Surface "In orbit"?

1. Is material on Earth's surface not in free fall around Earth's center? No. Material on the Earth's surface -- or inside it -- is not in orbit, and so is not in free fall. You can temporarily put ...
Peter Erwin's user avatar
  • 17.1k
49 votes
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How large can a ball of water be without fusion starting?

You really need a full-blown stellar evolution model to answer this precisely and I'm not sure anyone would ever have done this with an oxygen-dominated star. To zeroth order the answer will be the ...
ProfRob's user avatar
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49 votes
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Are black holes spherical during merger?

No need to guess. There's solid research done in this field. Even Wikipedia has some info: As two black holes approach each other, a ‘duckbill’ shape protrudes from each of the two event horizons ...
Florin Andrei's user avatar
49 votes
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What makes protoplanetary disks start rotating? (Initial energy needed to rotate)

Two rocks placed in space with no relative motion are going to be attracted by gravity, and hit. 3 rocks, placed in space with no carefully rigged symmetry, will likely miss each other, as the ...
Wayfaring Stranger's user avatar
48 votes
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Can Newton's gravity equation explain why black holes are so strong?

No you can't and the behaviour of bodies with mass and of light is completely different near a compact, massive object if you use Newtonian physics rather than General Relativity. In no particular ...
ProfRob's user avatar
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45 votes
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Why do astronauts move so slowly in zero gravity?

It's more for safety than anything else. Space is a very dangerous place for so many reasons. And making mistakes can very easily cause death. Being weightless does not mean you lose mass, so ...
Rory Alsop's user avatar
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44 votes
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Does the Earth constantly lose mass?

You are wrong that "to keep any object in circular motion requires energy" In a circular orbit, the force of gravity is always perpendicular to the motion of the moon, so no work is done by ...
James K's user avatar
  • 123k
43 votes

What is the shape of orbit assuming gravity does not depend on distance?

Circular orbits are always possible for any central force law, but noncircular orbits would resemble rosettes. Here's a specific example for the case where the force is constant with distance: By ...
Sten's user avatar
  • 4,753
42 votes

Why do things float in space, though the gravity of our star is always present?

It is not true that "objects float around" in the solar system. Perhaps you have seen video from the space station, and you can see things floating. This is not because there is no gravity, but ...
James K's user avatar
  • 123k
42 votes
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Intuition about why gravity is inversely proportional to exactly square of distance between objects

Imagine "gravity" spreading out in a sphere, like light from a bulb. For each doubling of the distance, the sphere has four times the area. The surface area of the sphere is proportional to ...
James K's user avatar
  • 123k
37 votes

Can a magnetic field of an object be stronger than its gravity?

It depends on what object it's acting on. There are many objects, including stars, that have magnetic fields where Lorentz forces on charged particles like electrons and protons are stronger than the ...
Ken G's user avatar
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34 votes
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At what velocity would an object have to fall in order to become weightless?

There's no speed at which you experience weightlessness; to stay weightless, you need to keep getting faster and faster all the time. From our perspective on Earth, everything accelerates downward at ...
Darth Pseudonym's user avatar
33 votes

Why do things float in space, though the gravity of our star is always present?

To help with James K's excellent answer, a visual representation might help. Let's look at a thought experiment - Newton's Cannonball. Let's say you have a cannon, high enough that it's being held ...
RPBCL's user avatar
  • 421
33 votes
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Why doesn't the Sun wobble towards Jupiter instead of away from Jupiter?

The part of your intuition that is correct is that Jupiter pulls the Sun towards it. The problem is that "pulls towards" does not mean "brings closer"! The gravitational force ...
nanoman's user avatar
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33 votes
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Is it possible to have a stable 3 body system that orbits in a perfect circle?

Yes and No. — It depends on what you mean by "stable". To be precise, "stable" means immune to small perturbations. "Equilibrium" can be either "stable" or &...
James K's user avatar
  • 123k
32 votes
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If the Sun disappeared, could some planets form a new orbital system?

The issue here is whether pairs of planets can become gravitationally bound to each other. In the two-body problem the trajectories or orbits are ellipses (bound orbits), parabolas and hyperbolas (...
Anders Sandberg's user avatar
31 votes
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How is it possible that Saturn's gravitational acceleration felt by Mimas is stronger than Mimas' own surface gravity?

An object on Mimas' surface would be much more attracted to Saturn than it is to Mimas. You are missing that Mimas as a whole accelerates gravitationally toward Saturn. What this means is that a ...
David Hammen's user avatar
  • 34.1k
30 votes
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Is there a better formula for gravitation, besides Newton's?

Going from Newton's theory to Einstein's theory is not simple. It's not like you can just add a term to Newton's gravity, like $\textbf{F}=-{GmM \over r^3}\textbf{r} + \textbf{f}(\textbf{r})$ and ...
Prallax's user avatar
  • 4,431
28 votes

How does gravity really work

First of all: "How gravity really works" is a deep question, and any serious scientist would quickly concede that all we have is an incomplete working model. You certainly have heard about General ...
Peter - Reinstate Monica's user avatar
28 votes

If suddenly "knocked" or perturbed from its orbit, would gravity eventually return the Earth to its original orbit?

There's a few parts to this question so there's more than one answer. Earth gets knocked a little bit out of its orbit all the time by gravitational influence of other planets in our solar system. ...
userLTK's user avatar
  • 24.1k
28 votes
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How would "dark matter", subject only to gravity, behave?

What you describe is the standard paradigm in cosmological physics, so it has been studied extensively. The basic consequence of dark matter not having significant nongravitational interactions is ...
Sten's user avatar
  • 4,753
27 votes

Is a perfectly circular orbit possible?

"Perfectly" is a funny word. Perfect circles are a mathematical abstraction. Real objects are not "perfect". So supposing a "perfectly spherical planet" is to suppose something that does not and ...
James K's user avatar
  • 123k
27 votes
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Are there places in the Universe without gravity?

In two dimensions I think I can infer in a lame, unconvincing and rigorless numerical way that there are likely to be zeros in gravity from a random distribution of objects there can be points of zero ...
uhoh's user avatar
  • 30.8k
26 votes
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If dark matter bends light, how do we know the stuff in the sky is where we think it is?

The local dark matter density is actually quite tiny, on the order of $\rho\sim10^{-19}\text{ g/cm}^3$ (see e.g. Bovy & Tremaine (2012)). This means that there is roughly $0.001$-$0.01M_{\odot}$ ...
HDE 226868's user avatar
  • 36.6k
25 votes
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Why can't supermassive black holes merge? (or can they?)

The main problem is angular momentum. In order for two gravitationally bound objects to merge (whether black holes, supermassive black holes, planets, stars, etc.), they must shed enough angular ...
NeutronStar's user avatar
  • 2,673

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