Why do people say that a perpetual machine is one that runs for eternity? Wouldn't a machine that runs taking no energy from outside but sometimes needs to be restarted, such as pendulum clock, be considered a perpetual machine? It just needs to be restarted one or two times a month. Given that physicists love approximations, wouldn't restarting a machine two times a month be higher order smaller term that we could throw away and call the pendulum clock a perpetual machine?
[Physics] Why isn’t a pendulum clock considered a perpetual machine
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Related Solutions
A simple symmetry argument might help here. Try to abstract everything away from your picture that is just a distraction. In the end it might look like this:
Why should this move to the left or to the right? It cannot and therefore adding a person and wheels does not change the problem.
I was surprised to see an effect that's actually real, and not a hidden motor or something like that. I believe this experiment can feasibly be repeated. The principles behind it make sense.
The driving force comes from the density difference in the cup versus in the pipes. The cup has very few bubbles in it compared to the pipe. Why? Because:
- The bubbles in the cup have an exit strategy - floating to the top. Not so in the pipe.
- The pipe has greater wall area to volume ratio. So it likely has more nucleation sites for the bubbles.
If I were recreating this, I would design the pipe to turn horizontal/vertical as close as possible to the bottom of the cup, in order to help maximize the driving pressure. Also, make sure to use the soda/beer right after opening it.
This experiment shows (not perpetual motion) that carbonation contains stored energy in some sense. More specifically, the process of a carbonated drink decaying into un-carbonated liquid and CO2 gas liberates extra energy. A small fraction of that energy is harvested here to drive the flow.
Very good science project.
In order to demonstrate that it is not perpetual motion, either allow it to run to its full conclusion, or try it again with soda that has sat out for a day. The data should support the hypothesis that the driving force to power the flow comes from stored energy in the carbonation.
Best Answer
There is no such thing as a physical quantity that is small or large in an absolute sense. Quantities can only be small or large compared to other quantities. Furthermore, comparisons must be between quantities with the same physical dimensions (which can be unitless).
For example, sometimes a velocity is much less than the speed of light: $v \ll c$, or equivalently $v/c \ll 1$. Sometimes we say "$v$ is small" but the understanding is that we mean to say "$v$ is small compared to $c$."
In your case, you want to throw out a quantity. Either it's the time between rewindings $T \approx 1\ \mathrm{month}$ (which you say is long) or the frequency of rewindings $f \approx 4\times10^{-7}\ \mathrm{Hz}$ (which you say is small). But you haven't specified what the quantity is large or small compared to.
Finally, whenever a quantity is properly neglected, it can be viewed in the paradigm $x + \epsilon \approx x$, where $\lvert \epsilon \rvert \ll \lvert x \rvert$. You can replace one number by another if they differ by small amount (compared to the actual value). But you can never just replace a number by $0$ in vacuo. For example, if your formula is $P = 3\pi (\epsilon/x)^2$, then even if $\lvert \epsilon \rvert \ll \lvert x \rvert$, $P \approx 0$ can be a very bad approximation. If the scale of the problem is set by $Q = 16 (\epsilon/x)^4$, then $P$ is in fact a large quantity, and you wouldn't be justified in saying $Q + P \approx Q + 0 = Q$.
As for perpetual motion itself, we define it to mean you don't inject energy ever. One month is short compared to "infinite time," and in fact any finite length of time is "infinitely short" compared to $\infty$. As far as infinity is concerned, restarting your clock once a month is no different from restarting it once every nanosecond.