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2002-Jan-28, 12:37 PM
Hello everyone, this is my first question, and hopefully I will not look *too* ignorant for asking it.

I have a question regarding gravity. Lets say for a moment that we can construct a hypothetical room where we can simply "switch" gravity on and off. If there is something lying on a table, say a wrench, and I switch off the gravity, will it start to drift immediately or will it stay in the same place until another force acts on it?

I ask because I see an image in my mind of the wrench slowly floating away once the gravity is turned off. Is this the case?

For that matter, would the earth's rotation have an effect on objects in the room if there is no gravity?

A bit of a warning - I am very weak on physics. A liberal arts major, dontchaknow.

GrapesOfWrath
2002-Jan-28, 12:52 PM
Just be careful. The gravity switch is also connected to the opaque-clothes machine. Turning it off is liable to be embarrassing.

The weight of objects on the surface of the earth is a combination of effects resulting from the gravity of the earth's mass and the rotation of the earth. One way to get rid of the latter is to perform your experiment at the north pole. That way, when you switch off gravity, the wrench will pretty much just lie there. Pretty soon though you'll start to notice that the rest of the room is spinning with the earth (every 24 hours) and the earth is turning slightly to orbit around the Sun, so it'll leave the wrench behind.

Kaptain K
2002-Jan-28, 01:54 PM
As I see it, there are three answers to your question, depending on whether local inertial referents apply.
1) Assuming local inertial referents apply: It will just sit there until the slightest breeze starts it moving.
2) If local referents do not apply, but that inertial mass and gravitational mass are not inherently 1:1:
It will stay in one place with respect to the universe, while the room move away. The velocity of the room will be a vector sum of the rotation of the Earth (0 - 1000 mi/hour, depending on latitude), the Earth's motion around the Sun (18.5 mi/sec), the Sun's motion around the Milky Way (12 mi/sec) and the Milky Way's motion with respect to the universe as a whole as represented by the Cosmic Background Radiation (~370 mi/sec). So, to an observer in the room, it will take off at a speed in excess of 1/1000th of the speed light in an almost arbitrary direction (it could be calculated in advance). If the direction is more or less up, it will reach space in less than a second (melted by atmospheric friction to a molten blob) never to be seen again. If it heads down, all that kinetic energy will be released as it bores into the ground, making a good sized crater where the neighborhood had been.
3) If inertial mass and gravitational mass are directly related, it will take-off at the speed of light (see "Billiard Ball" by Isaac Asimov) and I don't want to anywhere within a couple of parsecs when you throw the switch!

<font size=-1>[ This Message was edited by: Kaptain K on 2002-01-28 08:59 ]</font>

steinhenge
2002-Jan-28, 02:00 PM
On 2002-01-28 08:54, Kaptain K wrote:
(see "Billiard Ball" by Isaac Asimov)

I was about to bring up "Billiard Ball" myself, as I just got through re-reading it a few nights ago (in the collection The Edge of Tomorrow). I love the way that man thought....

MongotheGreat
2002-Jan-28, 02:07 PM
On 2002-01-28 09:00, steinhenge wrote:

On 2002-01-28 08:54, Kaptain K wrote:
(see "Billiard Ball" by Isaac Asimov)

I was about to bring up "Billiard Ball" myself, as I just got through re-reading it a few nights ago (in the collection The Edge of Tomorrow). I love the way that man thought....

If I remember the story correctly, Gravity wasn't turned off, but the mass was made to be zero. If gravity was simply turned off, the wrench wouldn't move unless some other force acts upon it. It'd be the same as having a wrench on a table somewhere in extragalactic space.
Mongo

David Simmons
2002-Jan-28, 02:51 PM
On 2002-01-28 07:37, Beyondarookie wrote:
Hello everyone, this is my first question, and hopefully I will not look *too* ignorant for asking it.

I have a question regarding gravity. Lets say for a moment that we can construct a hypothetical room where we can simply "switch" gravity on and off. If there is something lying on a table, say a wrench, and I switch off the gravity, will it start to drift immediately or will it stay in the same place until another force acts on it?

I ask because I see an image in my mind of the wrench slowly floating away once the gravity is turned off. Is this the case?

For that matter, would the earth's rotation have an effect on objects in the room if there is no gravity?

A bit of a warning - I am very weak on physics. A liberal arts major, dontchaknow.

I think you are asking what would happen if the attraction of only the earth's gravity were turned off for only the wrench. I say this because you said "a room where gravity could be turned off."

If this is what you meant then you can see what happens by drawing a picture. Draw a circle and a horizontal line just touching the top of the circle.

The circle is the earth looking down from the north pole. The spot on the circle's circumference touching the line is traveling to the left in the direction of the line at the instant it is touching the line.

If the gravitational on the wrench only is turned off at that time, the wrench would continue along the line while you would stay with the circle. In the first second you would see the wrench slowly lift off the table by 1.28 in. In the second second it would be 2.56 in. away from the table.

As time passed you would see the wrench accelerate faster and faster away from and pull ahead of you as you continued around the circle and it continued on a straight line.

That is the case for the equator. If you were at 45 deg. north, say Minneapolis, MN, then the motion would be 70% of what it was at the equator. At the north pole the wrench would just sit there and spin around once in 24 hours if there were no friction. Of course is would do that at the north pole in the absence of any friction even with gravity. But that's another story.

If all gravity were turned off then you wouldn't see any change in the wrench, barring, as was said, wind or some other disturbing force. However, the earth and other planets would stop circling the sun and would go flying off in the direction they were traveling when gravity disappeared. Everything, everywhere would retain its velocity of that instant but you wouldn't notice much except that the sun would start receding. And then it would start to get cold.

<font size=-1>[ This Message was edited by: David Simmons on 2002-01-28 09:55 ]</font>

Hale_Bopp
2002-Jan-28, 03:14 PM
Interesting question...as you can see...it depends on what effects you are taking into account.

It would move. There are multiple effects going on as well as the initial velocity of the hammer to take into account.

One of my favorite problems in physics is having my students calculate how fast the Earth would have to rotate for the net force on an object to be zero at the equator (taking into account only gravity and the Earth's rotation). I then ask them to generalize it to a positiion of random latitude.

Amazingly enough, many of my students are concinved that NASA has rooms with a switch where they can "turn off gravity". I think it was a bad 1980s movie called Space Camp that used this as a plot device, setting back the course of science education years!

Rob

GrapesOfWrath
2002-Jan-28, 04:12 PM
On 2002-01-28 10:14, Hale_Bopp wrote:
Amazingly enough, many of my students are concinved that NASA has rooms with a switch where they can "turn off gravity".
There is the vomit comet (http://www.space.com/peopleinterviews/yaniec_991020.html), which sorta does that. Here's four kids going to such a space camp. (http://www.emba.uvm.edu/VSGC/rocket/press/4vomit.html) /phpBB/images/smiles/icon_smile.gif

Hale_Bopp
2002-Jan-28, 04:35 PM
Oh, I talk about the vomit comet. They are quite insistent about what they "know"!

Rob

Donnie B.
2002-Jan-28, 05:19 PM
The earlier posts all cover the various possibilities well, except for one thing: they all assume that only the Earth's gravity gets turned off.

If somehow the wrench was made to be completely "numb" to all gravitational interactions, its motion is going to be harder to predict, because it will no longer be in orbit around the sun, or around the galactic core; nor will it continue to accelerate along with the rest of the Milky Way toward the local cluster, and so on.

Instead, if it retains its inertia, it will continue to move in whatever direction it happened to be going at the time you threw the switch; but exactly what its apparent motion will be (relative to the room) is highly contingent on the room's position on Earth, time of day, time of year, and so on.

If it loses its inertia along with its gravitational sensitivity, then you have the "Billiard Ball" situation. Let's hope it doesn't decide to go down; a wrench moving at c (minus a small delta) would make a pretty big crater.

Anybody care to calculate the energy in a one-kilogram wrench with velocity c? How would that compare with the Chicxulub impact?

(Left as an exercise for the reader...)

[Clarified description of wrench's motion]

<font size=-1>[ This Message was edited by: Donnie B. on 2002-01-28 12:23 ]</font>

GrapesOfWrath
2002-Jan-28, 05:36 PM
On 2002-01-28 12:19, Donnie B. wrote:
Instead, if it retains its inertia, it will continue to move in whatever direction it happened to be going at the time you threw the switch; but exactly what its apparent motion will be (relative to the room) is highly contingent on the room's position on Earth, time of day, time of year, and so on.
No, I think that's wrong.

Clearly, before you throw the switch, its motion relative to the room is zero. That's going to be the case just after you throw the switch, too. The rooms motion will gradually change, though. As the Earth turns, or the Earth orbits around the Sun, the room's motion changes, relative to the rest of the universe.

2002-Jan-28, 05:39 PM
Wow, there was a bit more to that than I had hoped. Thanks for your answers, though.

What I was imagining was exactly that, just "switching off" the earth's gravity, so to speak.

But if the wrench did have all gravity turned off, like you are describing, why would it fall down at c? Wouldn't another object, say the table or whatever, simply nudge it along, or would it immediately appear to accelerate away?

MongotheGreat
2002-Jan-28, 05:53 PM
Just because an object feels no gravity does not mean it will accelerate at all, especially not to c. The idea behind Asimov's story was if an object's mass could be changed to zero. Nothing about the surrounding gravity would change an object's mass.

Azpod
2002-Jan-28, 06:00 PM
On 2002-01-28 09:51, David Simmons wrote:
If all gravity were turned off then you wouldn't see any change in the wrench, barring, as was said, wind or some other disturbing force. However, the earth and other planets would stop circling the sun and would go flying off in the direction they were traveling when gravity disappeared. Everything, everywhere would retain its velocity of that instant but you wouldn't notice much except that the sun would start receding. And then it would start to get cold.

Actually it would get very warm in about 8 minutes, since the Sun would no longer have gravity to counterbalance the pressures from its hydrogen-fusing core. It would turn into a solar-mass hyrdogen bomb, vaporizing everything within a lightyear or so...!

So no, you wouldn't see the wrench move at all for about 8 minutes. Then the shock wave would hit, and in the millisecond before you are heated to 10,000 degrees C, you would see the rapidly melting wrench moving very quickly indeed.

Donnie B.
2002-Jan-28, 06:03 PM
On 2002-01-28 12:36, GrapesOfWrath wrote:
Clearly, before you throw the switch, its motion relative to the room is zero. That's going to be the case just after you throw the switch, too. The rooms motion will gradually change, though. As the Earth turns, or the Earth orbits around the Sun, the room's motion changes, relative to the rest of the universe.

Quite so. In this case, I didn't intend to imply that it would instantly shoot off in some random direction, but that when it did start to move, its exact direction would be hard to predict.

Gsquare
2002-Jan-28, 06:10 PM
On 2002-01-28 07:37, Beyondarookie wrote:

Lets say ...we can simply "switch" gravity on and off. If there is something lying on a table, say a wrench, and I switch off the gravity, will it start to drift immediately or will it stay in the same place until another force acts on it?

Hello Beyond,
Grapes of Wrath stated it correctly; that if gravity in the room was switched off, there would still be centripetal force from the earth's rotation acting exactly opposite to the direction of gravity. The magnitude of this force, though small, depends on your latitude.

Therefore, if you are on the equator when you switch off gravity in the room, the wrench will slowly begin to fall (accelerate) upward. If the ceiling is 10 feet high, the wrench would hit the ceiling in about 13.5 seconds and would stick.

Warning: Be sure to get out of the way when you switch gravity back on; the wrench will come plowing back down to the floor in less than one second.

One other point: This 'falling upward' effect will only occur if the method you used to switch off gravity did NOT involve eliminating the inertial mass of the wrench.
If your gravity eliminating device is of this rare variety, please e-mail me immediately.

Donnie B.
2002-Jan-28, 06:13 PM
On 2002-01-28 12:53, MongotheGreat wrote:
Just because an object feels no gravity does not mean it will accelerate at all, especially not to c. The idea behind Asimov's story was if an object's mass could be changed to zero. Nothing about the surrounding gravity would change an object's mass.

Hi, Mongo,

I would argue that a truly inertialess object acts like one with no mass. Any force acting upon it (a tiny breeze, even the Brownian motion of air molecules) would impart enough force to accelerate it to c. What's to prevent it?

It might not be dangerous, though, as any other force could send it off in another direction with just as much speed.

Maybe what we'd see is the wrench simply disappearing, as it went jittering around the room like popcorn in a popper, at c, far too fast for the eye to follow. As long as it stayed within the "turn-off-inertia" region we'd be safe. But if got outside the region, or if we turn off the field, look out -- it's headed off in some random direction at c.

That's exactly what Asimov was writing about.

Don't worry, though, I don't think we'll see such a device anytime soon. By conservation of mass-energy, our machine would consume enough energy to accelerate a kilogram to lightspeed... not battery-operated, I'd guess.

MongotheGreat
2002-Jan-28, 06:36 PM
On 2002-01-28 13:13, Donnie B. wrote:
Hi, Mongo,

I would argue that a truly inertialess object acts like one with no mass. Any force acting upon it (a tiny breeze, even the Brownian motion of air molecules) would impart enough force to accelerate it to c. What's to prevent it?

It might not be dangerous, though, as any other force could send it off in another direction with just as much speed.

Maybe what we'd see is the wrench simply disappearing, as it went jittering around the room like popcorn in a popper, at c, far too fast for the eye to follow. As long as it stayed within the "turn-off-inertia" region we'd be safe. But if got outside the region, or if we turn off the field, look out -- it's headed off in some random direction at c.

That's exactly what Asimov was writing about.

Don't worry, though, I don't think we'll see such a device anytime soon. By conservation of mass-energy, our machine would consume enough energy to accelerate a kilogram to lightspeed... not battery-operated, I'd guess.

Okay, but what makes the wrench have no inertia? I geuss it would depend on how that anti-gravity machine worked. If it's as put forth originally, that the gravity is simply turned off, then the wrench wouldn't be inirtialess, it would just be in the absence of a gravitational field.
If the switch activated a region where there was still gravity, but no inertia, then what you described could be accurate.
I'm just trying to state the differences between lack of gravity and lack of mass, or, inertia. A wrench in a gravity free environment would not act like a massless particle. Only if the mass was made to zero would it act like that.

Mongo

David Simmons
2002-Jan-28, 08:01 PM
On 2002-01-28 13:36, MongotheGreat wrote:

Okay, but what makes the wrench have no inertia? I geuss it would depend on how that anti-gravity machine worked. If it's as put forth originally, that the gravity is simply turned off, then the wrench wouldn't be inirtialess, it would just be in the absence of a gravitational field.
If the switch activated a region where there was still gravity, but no inertia, then what you described could be accurate.
I'm just trying to state the differences between lack of gravity and lack of mass, or, inertia. A wrench in a gravity free environment would not act like a massless particle. Only if the mass was made to zero would it act like that.

Mongo

I agree. I think dragging in lack of inertia just complicates a question about gravity.

And speaking of the "vomit comet," some of the trainees get nauseous under sustained, even for a short period, essentially zero gravity.

Does anyone know if the pilots flying the plane have ever gotten sick?

Donnie B.
2002-Jan-28, 08:16 PM
On 2002-01-28 13:36, MongotheGreat wrote:
Okay, but what makes the wrench have no inertia? I geuss it would depend on how that anti-gravity machine worked. If it's as put forth originally, that the gravity is simply turned off, then the wrench wouldn't be inirtialess, it would just be in the absence of a gravitational field.
If the switch activated a region where there was still gravity, but no inertia, then what you described could be accurate.
I'm just trying to state the differences between lack of gravity and lack of mass, or, inertia. A wrench in a gravity free environment would not act like a massless particle. Only if the mass was made to zero would it act like that.

Well, gravity and inertia are very tightly interconnected, at least.

If you think of gravity as a force, then it's a very strange one, because it always provides just enough force on an object to overcome the object's inertia and provide precisely the same acceleration, no matter the mass of the object. This holds to an extremely high precision.

Einstein resolved this by describing gravity as a manifold, or field; but it's still tied to inertia. Remember the famous thought experiment in which we discover that we can't tell the difference between an elevator sitting on earth (sensation of weight due to gravity) and one being steadily accelerated in space (sensation of weight due to inertia).

If you recall Asimov's story, the inventor of the machine wasn't trying to make a mass-decreaser; he was trying to make a gravity nullifier.

So we are justified in thinking that a machine that eliminates gravity may also eliminate inertia -- assuming such a machine is possible. (Remember, my posts were qualified: "if the machine eliminates inertia...")

Of course, there are other ways to cancel the effect of gravity locally, without eliminating inertia. A hot-air balloon, for example, or orbital free-fall; or, you could poise an Earth-mass black hole 4000 miles above the room. But you might have a few side-effects from that one...

Donnie B.
2002-Jan-28, 08:20 PM
On 2002-01-28 15:01, David Simmons wrote:
Does anyone know if the pilots flying the plane have ever gotten sick?

I would be surprised if they hadn't, at least on their first few flights. However, they do have the advantage of being strapped into their seats. I have read that space-sickness was less of a problem in the early Mercury and Gemini flights because the astronauts weren't able to move around as much.

David Simmons
2002-Jan-28, 09:36 PM
On 2002-01-28 15:16, Donnie B. wrote:

Well, gravity and inertia are very tightly interconnected, at least.

If you recall Asimov's story, the inventor of the machine wasn't trying to make a mass-decreaser; he was trying to make a gravity nullifier.

So we are justified in thinking that a machine that eliminates gravity may also eliminate inertia -- assuming such a machine is possible.

Yes, I think I see what you are driving at. And maybe Asimov too, I haven't read The Billiard Ball which is surprising because Isaac is one of my favorites.

Anyway, the gravitational force is:

f = G*m1*m2/r^2

If you want to selectively get rid of the force on the wrench, m2 in this case, you can't make G = 0 because then there wouldn't be any gravity at all, nowhere, no time, nohow.

Likewise you couldn't make m1, the earth mass, equal to zero because then the room, the observer and everything else would fly off because the the earth's rotation.

So the only thing left is to make the wrench massless, but then any force at all, no matter how slight, would instantly give it the velocity of light. Of course, since it would be in the atmosphere it would turn into a meteor in no time.

But I still think that unecessarily complicates things for the original poster who stated that he, or she, was very weak in physics.

GrapesOfWrath
2002-Jan-29, 04:02 AM
On 2002-01-28 13:10, Gsquare wrote:
Grapes of Wrath stated it correctly; that if gravity in the room was switched off, there would still be centripetal force from the earth's rotation acting exactly opposite to the direction of gravity.
I'm going to invoke the law of quotes here, and insist that the correct term has to be "centrifugal force," not centripetal force.

Gsquare
2002-Jan-29, 02:09 PM
OOps, sorry Grapes, centrifugal.
I thought you might be more concerned with my determination as to why the gravity switch off device that allowed centrifugal force while elimainating gravity in the room was the rare variety.

<font size=-1>[ This Message was edited by: Gsquare on 2002-01-29 09:29 ]</font>

GrapesOfWrath
2002-Jan-29, 02:45 PM
Sorry, I should have stuck a smilie in there. I'm just concerned that you might have been avoiding the term "centrifugal force" just because it was fictitious. Of course, in this context, gravity is also fictitious.

On 2002-01-29 09:09, Gsquare wrote:
I thought you might be more concerned with my determination as to why the gravity switch off device that allowed centrifugal force while elimainating gravity in the room was the rare variety.
Yeah, I guess I am. /phpBB/images/smiles/icon_smile.gif

'Cause you can transport things to a point where gravity is "nullified" but inertia is not, no? And I don't want to hear any of that bushwa about gravity extending to infinity--that is so seventeenth century.

Kaptain K
2002-Jan-29, 05:25 PM
And I don't want to hear any of that bushwa about gravity extending to infinity--that is so seventeenth century.
Of coure not. Gravity cannot extend past "c" x time since creation of the gravitating object. The gravitational field of the Sun is approximately 5 billion LY in radius and expanding at the rate of one LY/year.

SeanF
2002-Jan-29, 06:19 PM
On 2002-01-29 12:25, Kaptain K wrote:

And I don't want to hear any of that bushwa about gravity extending to infinity--that is so seventeenth century.
Of coure not. Gravity cannot extend past "c" x time since creation of the gravitating object. The gravitational field of the Sun is approximately 5 billion LY in radius and expanding at the rate of one LY/year.

You sure about that? Gravity is an inherent quality of matter, isn't it? Weren't the discrete particles of cosmic dust emitting gravitons (or however it actually works) long before they coalesced into Sol?

I suspect the gravitational effects of the Sun's mass reach out far beyond a mere 5 billion light years . . . in fact, if the expansion of intergalactic space can also expand gravitational fields, I would say they do reach to infinity! /phpBB/images/smiles/icon_smile.gif

_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-01-29 13:23 ]</font>

MongotheGreat
2002-Jan-30, 03:53 PM
On 2002-01-29 13:19, SeanF wrote:

On 2002-01-29 12:25, Kaptain K wrote:

And I don't want to hear any of that bushwa about gravity extending to infinity--that is so seventeenth century.
Of coure not. Gravity cannot extend past "c" x time since creation of the gravitating object. The gravitational field of the Sun is approximately 5 billion LY in radius and expanding at the rate of one LY/year.

You sure about that? Gravity is an inherent quality of matter, isn't it? Weren't the discrete particles of cosmic dust emitting gravitons (or however it actually works) long before they coalesced into Sol?

I suspect the gravitational effects of the Sun's mass reach out far beyond a mere 5 billion light years . . . in fact, if the expansion of intergalactic space can also expand gravitational fields, I would say they do reach to infinity! /phpBB/images/smiles/icon_smile.gif

_________________
SeanF

Please note that Kaptain K said the gravitational influence of the sun, not the particles that make up the sun, which has been in existence for only these past 5 billion years, as the conglomeration known as the sun.

GrapesOfWrath
2002-Jan-30, 05:51 PM
On 2002-01-30 10:53, MongotheGreat wrote:
Please note that Kaptain K said the gravitational influence of the sun, not the particles that make up the sun, which has been in existence for only these past 5 billion years, as the conglomeration known as the sun.
There's no difference though, as far as the mass is concerned, right? Or their effect, at such large distances.

SeanF
2002-Jan-30, 06:06 PM
That's been my question ever since the first time I heard it posited that gravity travelled no faster than c and thus could only reach a certain distance from the object in question at a particular time.

But, since the gravity comes from the mass itself, and we believe (don't we?) that mass cannot be created or destroyed, all the mass in the universe has existed as long as the universe itself -- and, since matter can't move faster than c either, no two particles of matter can be farther apart than their respective earliest gravitons.

Therefore, the gravitational attraction of the mass which makes up the sun reaches as far as any matter in the universe exists, essentially into infinity.

Right?

Personally, I like thinking of gravity as warps in space-time rather than emitted particles, anyway, although it makes the concept of an anti-gravity device a little harder to fathom . . . /phpBB/images/smiles/icon_smile.gif

Kaptain K
2002-Jan-30, 06:18 PM
...since the gravity comes from the mass itself, and we believe (don't we?) that mass cannot be created or destroyed...
Not quite. mass-energy cannot be created or destroyed. IF (very big if) our current theories are correct, the mass of the Universe at BB+1sec was about 1 gram and the temperature was nearly "off scale". We'll save "inflation" and the gravitational disconnection of the Universe for another time.

GrapesOfWrath
2002-Jan-30, 07:32 PM
But we're talking about gravity here--a mass of one gram, and a lot of energy has a lot of gravity, no?