Inquiring Minds


Questions About Physics

Question:
I just wish someone could explain it to me (talking vveerryyy ssllloooowwwlllyyly) so that I can have at least a clue why atoms appear to be little perpetual motion machines that are somehow unaffected by gravity, friction etc. and just keep on going and going (like that bunny). Don't other moving things in this world quickly come to a stop when force is no longer externally applied. Doesn't there HAVE to be an external force or some sort that's being applied (more or less evenly) to all things though out the universe(s). Or possibly that all things are being driven via the BB though (th' Ether) that somehow provides the energy.. If not that where does the driving energy come from?? Wouldn't atoms have run out of fuel/momentum in 12-15 billion years for pete's sake if the energy is from within??? I should think SO! If atoms have movement is it just caused by external heat.. is it that simple? At absolute zero electrons still move right? or is it that they move only at very very very near Abs zero but wouldn't if they could ever get down to Abs zero.

By the way.... could two universes pass through each other in the same way as two galaxies can :-) oh never mind.


Answer:

Your question puts you in good, if a bit ancient, company. It was Aristotle who first said that the natural state of objects was to be at rest and that moving objects would eventually come to rest and stay that way unless they are acted upon by another object. What he really was observing (but not understanding) is friction.

It was Isaac Newton, perhaps one of the greatest geniuses of all time, who finally deduced that this is not really the case. He elucidated three laws. His first law in this case is the most relevant. This law states that any object which is initially moving will continue to move (in a straight line and at constant velocity) unless something causes this to change. Similarly, an object which doesn't move will continue to not move until something makes it move.

It is one of the most fundamental facts of the universe that energy is 'conserved'. Conserved is physics-ese for "it can slosh around in different forms, but it never changes". The most commonly observed (and easiest to understand) is motion (or 'kinetic') energy. How much energy an object has is related to its velocity. Faster moving things have a greater energy.

So what happens at the atomic level is you have atoms bopping around (let's restrict our thinking for the moment to a gas like air). If you look at any particular air molecule, it moves with a particular velocity in a particular direction. In the absence of gravity or other air molecules, it would go forever. However, it eventually hits another air molecule. The two molecules careen off in different directions, each with different velocities and different directions. But the total energy before and after is the same. So, if the first molecule was moving quickly and the other wasn't moving at all, after the collision, the first molecule would be moving more slowly and the other would be moving (when it wasn't before). Consequently, the total energy is unchanged. And if this 'unchanging energy' principle is true for each collision (and it better be, or they'll make me go back to graduate school), then it is true for all of the atoms together. If you were able to keep an eye on a particular molecule and watch it, you'd see it bouncing around off different molecules (like the ping-pong balls in a bingo machine). Sometimes it'd go more quickly. Sometimes it'd go more slowly. When it goes more slowly, some other molecule has speeded up. When it goes more quickly, some other molecule has slowed down.

You can see this (sort of) if you play pool. When you make a break, you see the cue ball has slowed down, but the other balls speed up. This analogy isn't so perfect, as the balls eventually slow down due to friction with the table and the air. A better way to see this requires that you go to an arcade. If you can get a couple of air-hockey tables going and steal the pucks out of two of them, you can toss the pucks around on the table. If the pucks are new, they'll bounce a lot before air friction slows them down. You can see that they keep moving for a long time, although the velocity and direction of each puck may change.

Finally, you might ask why things that aren't atoms or molecules slow down. Well that's because the object loses energy in different ways. Think about a kid on a sled. As the sled goes across the snow, it slows down. That's because as the sled goes over the snow, it slightly heats up the snow molecules. These molecules start to shake in place. Since energy is required to make the snow molecules shake, it gets siphoned from the moving sled. Since the sled has less energy (and remember that energy is related to velocity), it must have less velocity (and the sled slows down). But even in this case the energy is the same before and after. It's just now you have turned sled-moving energy into snow-molecule-shaking energy.

So the short answer is no, you don't need to put energy into a system in order to keep things moving. If you can simply keep the energy contained, things will go on moving forever.

BTW...if you 'could' slow something down to absolute zero, they'd stop moving. In principle, you can do this by directing the energy to go elsewhere. In practice it's bloody difficult but has (nearly!) been achieved.

And I don't have any idea about colliding universes. I'm not smart (or drunk) enough to be able to even envision such a thing.

Cheers...

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last modified 2/8/2001   physicsquestions@fnal.gov