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Old 05-12-2011, 08:55 PM   #1
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Physics question

These are the kinds of things my kids come up with:

Assuming two hydrogen atoms starting out motionless at opposite ends of a static universe, with nothing else in the universe and standard gravity. Would they eventually hit each other, and if so, what speed would they hit each other at, and would they still be accelerating when they hit?

(I've no idea; I got a C in physics!)

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Old 05-12-2011, 09:01 PM   #2
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Two motionless hydrogen atoms, in a static universe, in the vacuum of space, there is no gravity, so therefore they would not collide.

Actually, I have no idea but I did stay in a Motel 6 once.
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Old 05-12-2011, 09:04 PM   #3
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Old 05-12-2011, 09:53 PM   #4
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Yes, they would collide. They would be accelerating when they collide. Without more data, unable to determine speed at time of collision (although I am not entirely certain about this last sentence).
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Old 05-12-2011, 10:09 PM   #5
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The atoms will remain motionless until acted upon by an external force, which there are none. They will not collide.

But, I'd rather talk about in-laws, politics, finance and sex.
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Old 05-12-2011, 10:10 PM   #6
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Old 05-12-2011, 10:16 PM   #7
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Wouldn't they be attracted to each other by gravitational forces?
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Old 05-12-2011, 10:16 PM   #8
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The atoms will remain motionless until acted upon by an external force, which there are none. They will not collide.

But, I'd rather talk about in-laws, politics, finance and sex.
If there are no external forces, would not gravity cause them to move toward each other?
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Old 05-12-2011, 10:17 PM   #9
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Yes, they would collide. They would be accelerating when they collide. Without more data, unable to determine speed at time of collision (although I am not entirely certain about this last sentence).
Yet there's the whole quantum-mechanics issue surrounding the chemical/electron bonding to make H2... at some point the two hydrogen atoms would have to stop moving to form a molecule before the two protons collided.

I guess your kids are going to have to calculate when the gravitational attraction will be augmented by the bonding force yet balanced by the protons' mutual repulsion. Surely there's a formula for that somewhere!
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Old 05-12-2011, 10:19 PM   #10
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If there is nothing else in the universe except the two atoms and conventional gravity, there is gravitational attraction between them which would eventually bring them together. The problem is that at extreme distances the gravitational force would be infinitesimally small, so the acceleration towards each other would be infinitessimally small. Also, the "universe" as we know it is not a fixed size, so there is a problem with the formulation of the problem "at opposite ends of a static universe" since we do not know what size the "universe" has been fixed at, nor how it is made static.
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Old 05-12-2011, 10:22 PM   #11
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Yet there's the whole quantum-mechanics issue surrounding the chemical/electron bonding to make H2... at some point the two hydrogen atoms would have to stop moving to form a molecule before the two protons collided.
I had not considered this point. I just assumed that the hydrogen atoms have a neutral charge and no electrostatic repulsion, but on a subatomic level, that's not correct.

I think it is safe to say that they would accelerate toward each other.
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Old 05-12-2011, 11:08 PM   #12
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I think it is safe to say that they would accelerate toward each other.
I'd just hope to get partial credit for the essay question and skip the math...
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Old 05-13-2011, 12:01 AM   #13
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Assuming two hydrogen atoms starting out motionless at opposite ends of a static universe, with nothing else in the universe and standard gravity. Would they eventually hit each other, and if so, what speed would they hit each other at, and would they still be accelerating when they hit?
Homework?

In a classical model, with no forces acting on them other than the gravitational attraction of each other, and starting from rest, the force vectors point each atom directly at the other, so they won't miss. This would assume an empty static universe except for the two atoms.

Treat each atom as a neutral point mass. Each atom is falling from effectively infinity to the bottom of the gravitational potential well represented by the other atom. (Hint: The final velocity prior to impact will be the same as the escape velocity for the two body system.)
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Old 05-13-2011, 12:16 AM   #14
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Since the universe would be spherical, the gravitational force exerted on one atom by the other in any given direction is balanced by the same magnitude of force by that atom from the opposite direction, so neither atom will be set in motion.
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Old 05-13-2011, 01:13 AM   #15
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Originally Posted by M Paquette View Post
Homework?

In a classical model, with no forces acting on them other than the gravitational attraction of each other, and starting from rest, the force vectors point each atom directly at the other, so they won't miss. This would assume an empty static universe except for the two atoms.

Treat each atom as a neutral point mass. Each atom is falling from effectively infinity to the bottom of the gravitational potential well represented by the other atom. (Hint: The final velocity prior to impact will be the same as the escape velocity for the two body system.)
+1/2 because that is what my first thought was. (I think the sub atomic force would cancel each other out, plus I can't remember what they are).

+1/2 cause I know Paquette is way smarter than myself about this stuff.
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Old 05-13-2011, 02:24 AM   #16
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Questions like this makes one stop and think. But the the real question is - will we remember to start again?
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Old 05-13-2011, 06:23 AM   #17
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The only addition I would make to M Paquette's explanation is that, given the extraordinary time frame involved, there is a high likelihood that one or both atoms would disintegrate before they collided.
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Old 05-13-2011, 07:25 AM   #18
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They will slowly accelerate toward each other until they reach the speed of light, at which point they will alter the time-space continuum and jump through a wormhole, back to the future.
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Old 05-13-2011, 07:29 AM   #19
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According to my calculations, the X sub B bar acting in unison with pose Pi and the sti Zifor would eventually create a hardship on the pose Pi and the resulting collision would be voided by the Wee biz bag. So there you have it.
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Old 05-13-2011, 08:13 AM   #20
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Hydrogen is the lightest of compound with atomic number/weight of 1. Therefore, with standard gravity of 32.2 ft/s in a static universe, they will never meet. Unless Oxygen introduce two Hydrogen and become liquid H2O.
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