Weird question regarding perpetual motion...

Yahweh

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Today I was talking on the phone with one of my sister in-laws. She is quite intelligent and a joy to talk to, today we discussed a little bit perpetual motion. I'm not entirely sure, but from what I can tell she believes perpetual motion is possible.

As we were talking, I cited a few examples. The one she was intrigued with was when I said "Assume this happens in an airless environment (to weed out the "air resistance" factor): You take a ball, you hold it a few feet in the air, drop it, and it bounces. With Newtons third law, for every action there is an equal and opposite reaction, you would assume the ball is being pushed up at the same force with which it hit, and you could assume that the ball would bounce forever. But it doesnt, it is a result of the second law of thermodynamics in action". Of course, she promply rebuttled by saying those two laws might be inaccurate (she's also an Philosophical actualist... or at least when she wants to be...).

I understand full and well about closed systems and the second law of thermodynamics, but could someone give me a little more detail as to why the ball doesnt bounce forever?
 
Hi Yahweh,

Well, to me, probably the best way of thinking about the problem is in terms of energy. So, initially, you have the ball in your hand, waiting to drop. In this case it has some gravitational potential energy. You drop it, and it speeds up, so that the potential energy is converted into kinetic energy. The ball hits the ground and bounces.

During the bounce, the ball squishes together, and in the process you get heat generated. This heat is then dissipated into the surrounding air. So, by the law of conservation of energy, some of the kinetic energy of the ball has been taken away, and when it travels upwards after the bounce, it won't reach as high as it did before.

In a nutshell, this is basically what happens in all machines- some of the energy available for motion, or for work, is dissipated into the environment as heat due to, for example, friction. And this is basically what the second law of thermodynamics says- the amount of energy available tends to decrease over time.

The second law of thermodynamics could well be "inaccurate"- however, this seems unlikely since no violation has ever been observed experimentally (at least, for macroscopic systems).

Does that answer your question?
 
There are a couple of problems with this idea.

1) A real ball will never bounce elastically. There will always be some translational kinetic energy transformed into thermal energy with each bounce.

2) This is not because of the laws of thermodynamics, but rather a function of the mechanics of how the ball bounces. In fact, it is possible to construct physical scenarios in which there is no loss of translational kinetic energy.

3) Such an example is two objects in orbit around each other. This is actually quite analogous to the idea of perpetual bouncing due to elastic collision.

4) This may sound like "perpetual motion", since this is motion continuing forever*, but this is not the kind of "perpetual motion" that is forbidden by the laws of thermodynamics. What the laws of thermodynamics forbid is the possibility of being able to extract useful energy from this motion without slowing down or stopping the motion.

5) While it is true that the laws of thermodynamics "might" be inaccurate, all that means is that perpetual motion "might" be possible. This has no bearing on the fact that there is considerable evidence to indicate that it is, in fact, not possible. Everything we know could be wrong. That doesn't mean we should just believe whatever we want, regardless of the evidence.


*Technically speaking, a planet's orbit should eventually decay. This has nothing to do with thermodynamics, though, but rather with the fact that acceleration causes radiation, which changes the energy of the accelerating objects.


Dr. Stupid
 
The ball would indeed bounce to the same height if it were a perfect system and all energy was preserved. But it is not a perfect system, and energy is lost to the following reasons:

1. Air resistance, both up and down.
2. Deformation - depending on how (in)elastic the ball material (or ground) is.
3. Heat - compression of the ball/ground material (that's why squash balls get hot)
4. Sound - boink! (actually, part of air resistance, but hey...)
5. Other losses people will berate me about.

A really bouncy ball like a snooker ball will not deform much or lose much energy to heat, so it will bounce quite well off a solid surface like tiles. You can test this for yourself - it bounces many times and much of the way back on each rebound.

Now try dropping a snooker ball on grass - grass is very energy absorbing so the ball loses all the energy almost instantly, and so doesn't bounce.

By comparison, a tennis ball is easily deformed, is not very bouncy really, gets hot easily, and has much air resistance, so it will lose energy rapidly and not rebound so far each bounce, even if dropped on a hard surface.

I'm sure you can take it from there!
 
Along the same lines, if all work uses energy, does the very nature of gravity as an effect of mass stem from a slow conversion of mass into energy itself? Do all of the basic forces steadily lose energy as time goes on?

And is this itself the second law of thermodynamics?

(physics was always my weakest area, and sadly I now teach and introductory subject in it. *sigh*)

Athon
 
Well, from my limited knowledge of physics and its jargon:

Work = Force X Distance

I don't know how this in your antecedent relates to your questions, which IMO are far more interesting. As far as I know, mass and energy are interchangeable in equations and so are space and time (mass-energy/space-time), I'm sure you're aware of this as well, just stating it so that my fingers have something to do while I'm thinking.

I'm not sure if physicists have come to any sort of consensus as to what "causes" gravitational fields outside of mass-energy (I'm not familiar with any current work on ideas dealing with gravitons and the like), but it is an interesting idea that it is due to a continuous circular cycle between mass and energy states. As for the 2nd Law of Thermodynamics, after reading some quantum mechanics, I always like thinking about everything reverting to the chaotic quantum level of space or akin to a logarithmic graph: as you reach the asymptote the value gets infinitesimally smaller, but never completely dissipitates. Also, at these miniscule space-time scales, who knows what can happen. Basic "laws of physics" seem to be defied at this level, including the laws of conservation (as far as angular momentum and mass go, someone please correct me if I'm mistaken). So there may be all sorts of violations of these fundamental "laws" that work so well at our macroscopic levels.
 
If perpetual motion were possible, it would be taxed.
Perpetual motion is not taxed.
Therefore, perpetual motion is not possible.

And if that doesn't merit a Laudie, my name isn't Hiram T.Chucklebutty.
 
Thanks for the answers, yall.

I always like it when the people on this forum make me feel dumber-er...
 
Soapy Sam said:
If perpetual motion were possible, it would be taxed.
Perpetual motion is not taxed.
Therefore, perpetual motion is not possible.

And if that doesn't merit a Laudie, my name isn't Hiram T.Chucklebutty.
Click to expand...

I see this month's selection of "Single Malt Scotch Of The Month" has arrived...:roll:
 
shemp said:
Yahweh, don't listen to them! The real reason is that every time the ball falls, Satan grabs at it and tries to drag it into Hell! Only you can put a stop to this!
Click to expand...

Here's a few posts summing up how I feel about that Satan fella:

http://www.randi.org/vbulletin/showthread.php?s=&postid=1870073092#post1870073092
http://www.randi.org/vbulletin/showthread.php?s=&postid=1870067672#post1870067672
http://www.randi.org/vbulletin/showthread.php?s=&postid=1870063853#post1870063853
http://www.randi.org/vbulletin/showthread.php?s=&postid=1870057982#post1870057982

If you want to find a guy to put a stop to this Satan, find someone who cares...
 
Even if the ball appeared to bounce forever - in the idealized case - here is the problem: you couldn't extract any work from it, so it is not a perpetual motion machine. If you tried to extract some benefit from such a system, it would wind down.

Some definitions of the Second Law of Thermodynamics:

a. A transformation whose only final result is to transform into work heat extracted from a source which is at the same temperature throughout is impossible. (Lord Kelvin)
b. A transformation whose only final result is to transfer heat from a body at a given temperature to a body at a higher temperature is impossible. (Clausius)
c. For any transformation occurring in an isolated system, the entropy of the final state can never be less than that of the initial state. (Fermi)

You need to extract work or heat from it to qualify as a "perpetuum mobile of the second kind".
 
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