Say we have a friction less pendulum and it extends over a 1/2 of a foots length with 3 bobs. These bobs are kept in motion by magnets of the opposite force that have been placed on a metal tray that has a copper coil above it but not touching it. for those who know how those electric generators work you know that an outside mechanical input is required to get the conversion to electrical energy outputted. So would this mean that in a controlled environment a pendulum will act as your mechanical input and as it moves you get electricity as an out put but not at a very high output rate but instead at a stable and constant rate. If you need clarification on this please feel free to tell me.
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asked Dec 15, 2017 at 17:33
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2$\begingroup$ There ain't no such thing as a free lunch, even with magnets... $\endgroup$– Jon CusterCommented Dec 15, 2017 at 17:58
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2$\begingroup$ It can't have a constant output rate: Getting electricity from the pendulum movement simply means that the pendulum mechanical energy is being drained from the pendulum and converted; so its energy (and, thus, the system's output) will steadily decrease. $\endgroup$– stafusaCommented Dec 15, 2017 at 18:27
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3 Answers
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It seems like you're trying to beat conservation of energy, but I think you would be hard-pressed to find a formulation of "the physics of a pendulum" that didn't require the thing you're trying to beat.
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$\begingroup$ define conservation of energy. what i was thinking is that if in a controlled environment, for example space, would that help the pendulum stay constant(the magnets would keep it moving) $\endgroup$ Commented Dec 15, 2017 at 19:41
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$\begingroup$ @NathanielBarnhill just because things move doesn't mean you can extract work from it which is what people usually want to do with perceptual motion machines. Lot of things can move forever in space, you don't even need magnets or anything special. Newton's first law of motion says that an object moving at constant speed will continue at constant speed unless another force is placed on it. $\endgroup$ Commented Dec 15, 2017 at 19:47
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$\begingroup$ @A. C. A. C. Well, yes i know this but how would you mimic that environment is the question. if mimicking that environment is possible than lots of possibilities will open up. That's why i use magnets so much. They pull inwards but also push outwards and can generally keep things moving if you use your timing properly. $\endgroup$ Commented Dec 15, 2017 at 19:54
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$\begingroup$ @NathanielBarnhill you are fundamentally not getting that you can't get free energy from this even if it moves forever. $\endgroup$ Commented Dec 15, 2017 at 20:00
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$\begingroup$ Yes i know but i never did say free, and if in some way i did i just meant more conservative or a very reduced input required. I do understand your point though $\endgroup$ Commented Dec 15, 2017 at 20:12
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This model will convert the mechanical energy of the pendulum into electricity, but it will not "generate" energy.
Mechanical, electromagnetic, and gravitational forces all obey conservation of energy. Energy may be "stowed away" as potential energy, but there is no energy creation. If you include loss due to friction, any such (closed) system will lose work energy over time.
In the model you outline, Lenz' law is responsible for converting mechanical to electric energy, but not generating new energy from nothing.
If you're not satisfied, make that model and test it out. I think you will find that the pendulum will slowly (or quickly) begin to lose kinetic energy, and eventually not move much. Residual movement may occur from energy imparted from outside the system however, in the case of a wobbly table for example.
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Any energy that you extract will slow down the pendulum until it will eventually halts and no more energy can be extracted.