I have made a plan for a perpetual motion device, but I am sure that it won't work because otherwise the world would be relying on it right now. I know that perpetual motion defies the laws of physics.

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The blue represents a container of water, the brown wheel is made of wood, halfway into the water.

Half of the wheel is in the container of water, and half is not. This seems like it should work to me (a complete noob at physics), because the part of the wheel that is in water will always try to float upwards, and the side outside of the water will be pulled on by gravity and try to fall downwards.

Assuming that we found a way to not let any water leak out of the container, why wouldn't this work? Extra credit to anyone who can make a explaination without using equations or complicated terms, so I (a noob) can understand. Thanks!!


This question is different from the one it is marked as duplicate of because that one has an answer that is explaining to someone that has experience with physics, whereas I have none. Also, the main question of that question is about Buoyancy, whereas mine is about the machine itself.

  • $\begingroup$ @K I was writing a simple answer when it was made duplicate . I will put it in the comments and read it fast, because comments may be deleted, $\endgroup$ – anna v Jan 15 '15 at 5:03
  • $\begingroup$ Perpetual moving devices have been made, similar to your concept, using buoyancy. See this Perpetual moving devices have been made, similar to your concept, using buoyancy. See this youtube.com/watch?v=03x95eXPM38 for example. These devices stop when friction or other forces dissipate the acquired kinetic energy of the system, so they are fun but useless, except to prove once more that energy cannot come out of nothing. $\endgroup$ – anna v Jan 15 '15 at 5:06
  • $\begingroup$ Perpetual means "continuously/for-ever", and within human measures they exist. After all the planetary system is a "forever moving" , except for the various dissipations of energy that happen at time scales much larger than a human life. $\endgroup$ – anna v Jan 15 '15 at 5:07
  • $\begingroup$ Now your specific drawing has problems in that to be water tight the wheel has to touch the container thus frictional forces will be high. The difference between the two sides is the buoyancy on the left, otherwise gravity pulls down both sides. Suppose zero friction for fun. As you have drawn it it will be in a metastable state unless angular momentum is given to the wheel. $\endgroup$ – anna v Jan 15 '15 at 5:07
  • $\begingroup$ Once a motion starts by hand it will move for a while, but then the water viscosity will break the motion dissipating the kinetic energy. These type of devices can only be imagined to be moving forever if there are no dissipations of energy, but certainly can not be useful for energy production $\endgroup$ – anna v Jan 15 '15 at 5:08

I think it's because the buoyant pressure forces act on the outer surface of the wheel; however, because it's circular, the pressure at every point is pointing toward the axis, so there can be no resultant moment. Think of it this way: the pressure forces that are pushing the lower-left quadrant up are also pushing that same quadrant to the right, so the torque cancels out.

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