Negative Mass

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exfret
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Re: Negative Mass

Post by exfret »

I'm still confused why the CoM isn't at infinity. I mean could someone explain to me what CoM is?
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robly18
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Re: Negative Mass

Post by robly18 »

exfret wrote:I'm still confused why the CoM isn't at infinity. I mean could someone explain to me what CoM is?
CoM is the center of mass. Basically it's the center of the system. In a closed system, the center of mass retains a constant velocity.

It's hard to explain, but think of it as the averages of everything; the place where things would go if they were mish mashed into one spot.

As well as this, if you have a system, as another object gets farther and farther from the system, the direction from which it feels the gravitational pull approaches the CoM.

Just think of it as averaging the position of everything taking into account the mass of the objects.
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testtubegames
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Re: Negative Mass

Post by testtubegames »

Yeah, you can think about it in terms of balance, too. The center of mass of a plate of food is where you'd have to put your finger to balance the plate perfectly on it. Put a lot of mashed potatoes on one side... and your finger will need to move to that side, too.

Or, you can think about putting the two objects on a see-saw and having them balance there. The fulcrum is then at the center of mass. Normally -- with two discrete positive masses, say -- the two objects would go on opposite ends of the fulcrum to balance. With one negative mass and one positive mass, it's like if you were trying to balance a bowling ball and a helium balloon, both would go on the same side of the fulcrum/center of mass*. Hence a lot of this weirdness.

*Note, this analogy breaks down with actual negative masses... since gravity no longer behaves in the normal way. As we know, both negative and positive masses fall down to earth. Negative mass isn't like a helium balloon... but that helps you think about the 'balance' of center of mass, at least.
exfret
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Re: Negative Mass

Post by exfret »

Oh, it's just the centroid. So, it's the point where m1*d1=m2*d2, cuz the masses are weighted with distance in a seesaw. It's the point at which if you place an asteroid, it won't accelerate, right? Or not, because gravity is distance squared..? But, wait, wouldn't the CoM be near the negative mass if it's larger... But if they're equal, then where would the CoM be?
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testtubegames
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Re: Negative Mass

Post by testtubegames »

Not quite the point where an asteroid wouldn't accelerate -- since (in addition the squared part), that point would be closer to the smaller object. The center of mass is closer to the bigger one.

And, yes, you're right that if the negative mass has a large magnitude, then the center of mass will be closer to it. ...assuming I've done my calculations right. There are too many darn minus signs floating around here! :)

If the two masses are equal and opposite, then there's no center of mass. There's no net mass at all, so the center of mass becomes pretty meaningless. Which is nice, because we can then avoid the issues brought by dividing by zero (physics tends to be pretty nice when it comes to that).
AlternateGravity
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Re: Negative Mass

Post by AlternateGravity »

One of the things I also tried doing with negative mass is I tried using a negative mass planet to accelerate a positive mass planet out of orbit before deleting the negative mass planet and using another star to capture the planet. I found that in order to get the moving planet to orbit a star after it was already moving fast I needed to give the star it self a velocity that was similar to that of the planet. I found that the orbit of a captured planet tended to be unstable and after a while the orbit of the planet would go through its new star.
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A Random Player
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Re: Negative Mass

Post by A Random Player »

Here's an interesting article which may or may not be interesting:
http://www.reddit.com/r/Physics/comment ... st_in_our/
--> Article https://medium.com/the-physics-arxiv-bl ... 0a980320a7
On closer look, negative mass only MAY exist, and its only for a certain definition of mass.

Edit: Hold on I Schrödinger catted my first line
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testtubegames
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Re: Negative Mass

Post by testtubegames »

That's a good find -- I hadn't realized that there was a school of thought out there that negative mass *couldn't* exist. But I breathe a sigh of relief knowing that it fits in with General Relativity.

On a side note, that last bit in the summary you linked, about +-masses making a plasma that absorbs gravity waves... and could explain why we haven't detected gravity waves yet... whoa! That's really fun to think about. Maybe all these 'null' results they've been getting actually point to a very important discovery. (Probably not, of course, but hey, it's fun to dream)
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