Ask MetaFilter questions tagged with gravity

How was it proved that mass exerts the gravitic forces?

Lorc — Mon, 06 Jul 2009 11:00:14 -0800

How was it proved that gravitation was a function of mass? If we can estimate planetary masses from the strength of their gravity, including that of the earth, then we must be pretty confident that we've nailed the relationship between the two.

I know that the force of gravity is proportional to the mass of an object and inversely proportional to the distance. And certainly when we look at the universe there's evidence of strong gravity wherever we see a lot of mass. But I also know that science is robust and does not just accept obvious observations without testing them to destruction. How do we know that there's a causal relationship rather than it being circumstantial? And that it exists at all scales, not just with planetary masses?

It seems to me that you'd have to test known masses and be able to measure that object's gravity in order to prove anything. Is that what they did, and if so, how was it done? Is there a gravitational equivalent of Rutherford's gold foil and Robert Millikan's oil-drops?

Could there be a planet so big that it is bigger than itself?

blue_beetle — Thu, 18 Jun 2009 11:02:22 -0800

Would it be possible to have a planet with significantly more surface area than the earth and similar gravity (at ground level) by decreasing its density? I know that gravity decreases with the square of the distance, so does that mean that the earth is in the sweet spot for the size/density necessary to produce 1G? Or could that be scaled up to double or triple the surface area and still have a viable (livable) planet?

Strong man in the moon?

Gungho — Thu, 18 Jun 2009 09:44:25 -0800

With the advent of NASA's return to the moon I have a question of whether or not The Apollo astronauts could have recovered from a less than perfect landing. The new Lunar Reconnaissance Orbiter (LRO) can map the moons surface with much greater detail than in the time of the Apollo landings. During the descent of the Apollo 11 Lunar lander Niel Armstrong had to fly over an unexpected boulder field. Assuming he didn't, and landed the LEM, and it tipped over, could the two astronauts tip it back up so that it could launch again?

I'm going to throw you right into the sun.

Cool Papa Bell — Wed, 15 Apr 2009 16:26:34 -0800

Fictional gravity: Let's say I was the complete master of all things gravity and I wanted to hurl a person into the sun. Stipulations: I am in complete, omnipotent, magical control of gravity. I can increase and decrease gravity at will, relative to all objects, across all distances.

I want to hurl a person into the sun. Specifically, I want to increase the gravitational attraction between the sun and a specific person, such that the person is lifted off the ground, achieves escape velocity and eventually is pulled right into the sun.

When I do this, it will not have an effect on anything besides the targeted person and the sun. No other objects will have their gravity tweaked with.

Time is a factor, inasmuch as I'd like this person to be leaving the breathable atmosphere within ... oh, let's say 10 minutes after I flip the switch. Think of it like a survivable rocket launch (until he runs out of oxygen, that is). Beyond that, it could take forever for him to actually reach the sun. But he's out of eyeball range pretty damn quickly.

This will happen at noon for the targeted person, outside, so there's a straight line from him to the sun. He just goes straight up into the air and doesn't need to be pulled through walls, ceilings or the planet itself.

What is the minimum amount of gravitational increase, in percentage, to do this?

Muscle tensions in space?

jfricke — Mon, 23 Mar 2009 12:09:08 -0800

If my working desk were in space, would I still experience tensions in my back? This might be a very stupid question but I had a discussion with my girlfriend today about whether I would still experience tensions in my back after working for long periods on my desk, if my desk were in space or in a room with no gravity.

I believe that without gravity there are no muscle tensions or at least it is less likely to have them. Am I wrong? And if so, why?

There's a problem with (my understanding of) gravity.

woodblock100 — Sat, 17 Jan 2009 08:20:06 -0800

What is the contradiction in the discussion of gravitational forces in the ... It has been a very long time indeed since Physics 101, but even with a quick refresher on various web sites this afternoon, I can't understand the contradiction in this example:

We're all familiar with the physics class demonstration of two objects of different mass falling at the same rate (in a vacuum). In any particular gravitational field, the feather and the steel ball both drop side by side. No problem.

If we repeat the same experiment on the moon, which has a weaker gravitational field due to its smaller mass, the two objects will fall more slowly, but still side by side. No problem.

But if you look at this from the other direction, and think of the planetary body as 'falling' toward the objects - which presumably it is, albeit in an infinitesimal amount - then something seems to be wrong. The body with more mass - the earth - falls towards the objects faster than the body with less mass - the moon. More mass = more acceleration.

So which is it to be: objects of different mass accelerate at the same rate in a particular gravitational field, or different rates? What's going on?

Sprinting in high G

IvyMike — Mon, 25 Aug 2008 09:52:26 -0800

If the earth's gravity was increased by 10%, would sprint runners be slower or faster? Argument that they would be slower: It would be harder to run, duh.

Arguments that they would be faster: Moonmen move as if in slow motion. The runner would spend less time in the air. The runner would be able to "push off" with more force, similar to the reason Indy cars find it advantageous to generate downforce.

Tai Chi in zero-g?

nímwunnan — Tue, 05 Aug 2008 15:42:04 -0800

Would Tai Chi work in outer space? Not that I really expect anyone to have any experience doing Tai Chi in zero-g (but if any forum had that person, it'd be MeFi), but I'm really curious. Tai Chi uses gravity or weight as a motive force so much that I'm really hard-pressed to imagine how it would work without gravity. I can imagine Tai Chi principles while fighting in zero-g, but not the form by itself. I do Chen Man Ching style, btw.

Help me avoid gravity...somewhat.

ostranenie — Wed, 21 May 2008 13:01:46 -0800

What are those things called that you see on badge clips and rock climbing walls that automatically retract a nylon cord? It's like a spring-loaded spool of cord that will give under tension, but as soon as tension is released the cord gets "sucked" back up into the spool. The ones on the rock climbing gyms (or American Gladiators(tm)) attach to a body harness and make you almost weightless, so if you fall you won't get hurt that bad. The badge clip things make it to where you just let go of your badge after swiping it or showing it and >>zip<< it returns to the clip.

Bottom line: I'm looking for one that's stronger than a badge clip but not as large or strong as a harness holder. Adjustable tension and/or cord length would be GREAT but are not requirements. Mainly I'm just looking for what they're called. Thanks!

Big Bang Baffles Bonzai

Bonzai — Wed, 20 Feb 2008 23:55:07 -0800

Why is the Big Bang possible? I know that a black hole is formed when X amount of matter is in one place. I know that nothing (except Hawking radiation) can escape a black hole.

So if all the matter in the universe was once contained in a single point how could it have possibly escaped from all that gravity? Obviously it did, but ... how?

What mystical powers do four kids have?

bonaldi — Wed, 21 Nov 2007 10:29:14 -0800

You know the playground trick whereby four of you can "levitate" a fifth person using just your fingertips (you first hold your hands over his head and count to 20)? How does that work, then?

How would a metal bar built around the world react to gravity?

studentbaker — Thu, 01 Nov 2007 12:38:36 -0800

How would a metal bar built around the world react to gravity? I’m not sure how to properly phrase this question. But, how would a metal rod with a slightly larger diameter built around the earth react to gravity? Say it’s built as to avoid extremes in altitude, mountains and the sort. Would it hover in place, spin at a set point, spin randomly, or buckle? Is there anyway to know the answer other than building one?

I created a simple visual to help explain what I mean.

Thicker air and lower gravity

Ritchie — Mon, 08 Jan 2007 03:35:06 -0800

Imagine a world in which the atmospheric density is somewhat greater than ours, but gravity is somewhat lower. How might our experience be different on that world? For example, I imagine that the greater air resistance would mean a lower terminal velocity for falling objects. Is this correct or am I confused?

Assuming a mix of gasses in the same proportion to our own atmosphere, would the higher pressure of oxygen at sea level cause physiological problems, or would the higher pressure of nitrogen force you to decompress gradually if you, say, went up in a balloon, or risk an attack of the bends? What other physiological effects might occur?

How would gunfire, explosives and other incendiary reactions be affected?

How would climate and weather differ? Would clouds form more readily? Would they form at higher or lower altitudes?

What sort of technologies might be favored?

What other interesting effects might this environment have?

A world with lower-than-Earth gravity probably can't retain a denser-than-Earth atmosphere, I realize. Be gentle in the face of my naivety.

How do you get a flat surface of water?

Smedleyman — Wed, 11 Oct 2006 13:54:31 -0800

Does gravity make small pools of water curve slightly (I’m thinking a small lake size) or is surface tension enough to flatten it out? More importantly - how would I calculate whether the resistance from the surface tension is enough to keep it flat and when/what size that breaks off? Is there any way to create a perfectly, or very near perfectly flat surface of water?

Sex in Space?

blueplasticfish — Fri, 22 Sep 2006 12:21:34 -0800

Has anyone ever had sex in space? I am wondering if sex has occured in space, or at least some variety of microgravity. I have Googled around looking for something conclusive and haven't found anything.

Life on the moon!

terpsichoria — Tue, 25 Apr 2006 05:18:28 -0800

Why do videogames with physics simulation always have things fall too slowly? Looking at games like Half-Life 2, Oblivion, F.E.A.R. and so on - games with physics systems to dictate the way objects behave under gravity - I always notice that things seem to move far more slowly than they should.

In HL2 i can pick up a crate and fling it into the air, and compared to me picking up a pack of loo roll and dropping it at home (scientific tests, these!), the on-screen object falls unnaturally slowly. Considering that you can drop a metal crate, wooden box or whatever and measure its weight and how long it takes to fall in real life, surely all it would take to have things fall at closer to the right speed in a game is to plug the real numbers for objects' approximate weights and a rough figure for acceleration under gravity (minus air resistance) into the physics engine? Am I missing something, is it a conscious artistic decision (to let the player see the effects of their mayhem-causing ways better) or what?

Gravity and relativity.

grahamwell — Tue, 28 Mar 2006 02:47:18 -0800

If General Relativity is accepted as true, why do Physicists talk about (and look for) Gravitons? My confusion here is that the two approaches seem to be mutually exclusive - either gravity is a function of 'warped space-time', or it is a conventional force mediated by a particle. Can it be both? If so, isn't one explanation redundant?

If not, can I take it that researchers who talk about Gravitons, as in this bizzare press release, don't accept General Relativity?

A lot of questions here, I guess I'm really asking for a laymans account of how both approaches could be reconciled. Any good explanatory links as to the current thinking about gravity would also be appreciated.

Lumpy Earth---gravity's effects on us?

amberglow — Sat, 18 Feb 2006 09:56:35 -0800

Gravity/Life/SocietyFilter: So the Earth is all lumpy, with more or less gravity in some places. Has anyone ever corroborated it with life/social/world things? Many questions inside... --is life worse there for people or better?
--is a place like the Middle East, which has seen almost continual strife since recorded history a place of denser gravity? or of the opposite? (it's also the where civilization first developed) or was it lighter and now heavier?
--have breakthroughs in various fields tended to happen in lighter places or heavier? have wars? have long stretches of peace? have healthier populations? etc?
--was the change from the Dark Ages to the Renaissance a result of the lightening of gravity in the area?
--does it have any effect on us? i would think it would, no? Is there a depressing or generally bad effect of more gravity on us?

We're very much creatures of our environments, and there are places with lots of energy and buzz (NYC, Bombay, etc), and places that are sleepy and quieter, even when geography matches (Pittsburgh, or Amsterdam, etc). And we're affected by things like the moon and its movements, and by sunshine and the weather and pressure, etc.

Why not gravity too?

Guess the number and win a sculpture!

~rschram — Thu, 10 Mar 2005 09:08:54 -0800

How does this work? I saw a similar piece recently in person, and the explanatory note said that the materials were "Pins held together by friction and gravity."

Is "weightlessness" an artifact of a free-fall orbit?

socratic — Sun, 12 Dec 2004 11:28:22 -0800

Science hour question: My niece asked me this and I didn't have a good answer. People in orbit around the earth are "weightless." But what would happen if an object in space were kept stationary at 150-200 miles up? Would an astronaut in such a stationary but very high station in fact feel significant gravity? Is "weightlessness" (in near-Earth space) an artifact of a free-fall orbit? C'mon rocket scientists, I know you're out there.

If the sun's gravity is so strong as to keep whole planets circling it in orbit, how come light and heat can escape?

amberglow — Sun, 14 Nov 2004 15:33:48 -0800

ScientificIlliterateFilter: If the sun's gravity is so strong as to keep whole planets circling it in orbit (even ones million of miles away), how come light and heat (incredibly fragile things compared to planets) can escape as rays (or whatever they are)?