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Hi! Thank you for your help on my last question. I recently viewed a concept for a supposed "perpetual motion" machine. Of course, according to the laws of thermodynamics, it shouldnt be able to work. On the other hand, there doesn't seem to be any flaw in the machine or any mechanical reason that I can think of that would prevent it from working. The design goes like this: There is a pendulum with a metal ball on the outer end, which swings on the axle of an electric generator. The pendulum is put in a vertical position and dropped. It swings about 75% of the way around in a circle as the electric generator generates a given amount of electricity . That amount of electricity is fed into another electric motor/generator with a pendulum of the same dimensions as the first. Except in this case the pendulum is at rest pointing down. The amount of electricity makes the second pendulum swing 50% so that it is in a vertical position, then the second pendulum drops (as the first one did) and the process repeats. The result is that both pendulums continue to rotate indefinitely. It is supposed to work on the principle that the same energy that is obtained from a falling object is the same amount of energy required to raise that falling object back up to its original position. But in the case that I stated, each pendulum swings 75% of the way around, but only need to raise the second pendelum 50% (allowing the motor to work with an efficiency of less than 100%) For example, if the energy generated by the pendulum going 50% of the way around is 1 Volt, then it would require 1 Volt to move that pendulum back up to its original position (but the motor would have to be 100% efficient) but in this case, the pendulum swings about 75% of the way around (so it would produce about 1.5 Volts) that 1.5 Volts is put into the second pendulum that would only require 1 Volt (so you can lose .5 Volts due to inefficiency). Then the second pendulum continues the process by swinging 75%, and feeding the electricity generated into the first pendulum which by now is at rest... etc. What is the flaw in this concept - or is there really no flaw (in that case it would work)? I hope I explained it well enought, but if you guys need clarification, please look at my diagram in teh link below. Thank you so much for your help!


My diagram:

my-diagram
Question Date: 2010-05-26
Answer 1:

The concept is flawed, for a couple of reasons. First, by swinging 75% of the way around the circle, the first pendulum loses half of the kinetic energy gained while dropping on the upswing, because it's pulling against gravity on said upswing. (If you simply had your "pendulum" spinning in a circle, you would notice for example that it has the greatest speed when it is at the bottom of the circle, because that's where the most energy is kinetic and not potential) As a consequence, you would not generate enough electrical power to rotate the second pendulum back to a fully-upright position. You would only rotate it halfway up, assuming a perfect transfer of energy.

Your second problem is that the electromagnet in your generator exerts a force on the pendulum opposing its downward swing as it generates electricity. The magnitude of that force is determined by upon the magnetic field and current being generated in the wires, and it is that same force, operating in reverse, that will move the second pendulum. The force applied by the generator on the first pendulum must be weaker than gravity, or else the pendulum would not swing downward at all. However, the force on the second pendulum must be greater than that due to gravity, or else it won't be strong enough to push the second pendulum up. This is the same way that a balance scale works, except that instead of using a lever arm you're using electromagnets, but equal weight and equal force means no acceleration and no movement.


I hope this helps!

Addendum
Hi, there is one thing I realized after I wrote that which is needed to be complete: because your lever arms are pendulum rather than simple levers, the actual ability of gravity to move one of the pendulum depends on their orientation as well as their position, because of the angle of the pendulum's position relative to the downward pull of gravity. This means that if pendulum A is at the halfway point, gravity is pulling perpendicular to the orientation of the pendulum, thereby exerting maximum torque. If at this time pendulum B is at or near the bottom of its circle, gravity is parallel or close to parallel to the orientation of the lever, there by exerting no torque (or very limited torque). In this circumstance, it is possible for the electric generator attached to pendulum A to produce enough current to move pendulum B, because it is working against a reduce force of gravity on pendulum B. However, this also means that you still won't be able to move pendulum B up to the halfway point, because at that point gravity on pendulum B matches the maximum possible on pendulum A.


Answer 2:

If I understand this correctly, each pendulum will recover 50% of its gravitational potential energy while having 50% converted to an AC current.That 50% of the energy is used to drive the next pendulum back to the top of its arc, where the process continues.

The reason why this will not work is as follows: when you are generating the electric field (EMF) with the generator, by conservation of energy the kinetic energy of the pendulum is converted to an EMF. This is how hydroelectric power is generated. The trick here is to understand the generator. A simple generator would involve pushing a bar magnet in and out of a broken current loop (a wire wrapped in a circle, but with the ends not connected). This would induce an EMF in the loop, but the currents moving in the broken loop would also apply a force against the direction of motion of the bar magnet (and consequently the pendulum driving it).

All of this to say the generation of the EMF or current in the generator would slow down the pendulum as the kinetic energy is converted to an EMF. The scheme sketched out is a fancier version of a single pendulum. With one pendulum, it will be damped by friction. With two, there will be additional losses in the wires and generator.



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