eli5: Why does this perpetual motion machine not work? [picture included]



Why does this perpetual motion machine not work?

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Edit: thanks for all your answers, though here are some remarks:

– I know this machine will not work, I just want to know why exactly

– I know there’s a lot of friction involved, but it’s not the reason why this won’t work in theory. I want to know what counteracts the seemingly “generated” energy that comes from the buoyancy of the balls.

[Answer](https://reddit.com/r/explainlikeimfive/comments/ku4hzo/_/gipxdid/?context=1) by /u/RhynoD:

Buoyancy works because there is more water pressure under the thing than above the thing. When the ball is underwater the water is pushing in from all directions. Gravity is pulling down on the water all around the ball, which puts pressure on the ball. Since water is more dense, there’s more force pulling the water down so it wants to fill the space the ball is occupying, which forces the ball up.

When the ball is below the water but not under water, the water is still squeezing down on all parts of the ball touching the water. Except the ball isn’t entirely underwater. There’s no water pushing on the bottom of the ball, just air. And since air is a lot less dense than the water, gravity isn’t pulling it as hard and it won’t push as hard – which is exactly why liquid water doesn’t float on air, and why stuff full of air does float on water.

That means as long as the ball is more than halfway out of the water on the bottom, the full weight of the water is pushing only on the top of the ball, pushing it down. Essentially, the ball is still “floating”, it’s just floating upside down. Which, by the way, is totally a thing that happens.

The amount of force generated by buoyancy depends on how much water is over the object (it does not matter how much water there is around it, only how deep it is). By definition, the ball coming in from the bottom is deeper than it will ever be once it’s fully immersed – therefore, it must take more force to put it into the water than you can possibly get out of it. And that doesn’t include the force required to also lift and move through the seal.

In: Physics

Because the energy needed to lift the seal around the floating ball will be more than the boyancy generated.

In a word: friction

I’m a perfect world a perpetual motion machine would run forever but because of friction there is always a small amount of energy lost in a system. Even air causes friction, that’s why skydivers don’t keep talking faster and faster to the ground.

The “seal that lets balls through but stops air from escaping” requires energy to work. At least as much energy as can be extracted from the buoyancy of the balls in the water.

Resistance on the seal from air to water? To close this efficiently the friction between the air ball and the seal would be too high.

Regardless of what machine is designed, a perpetual motion machine cannot exist according to our current understanding of the laws of physics (specifically, thermodynamics).

No system can keep all of its kinetic energy (movement) inside itself forever. There’s always at least some friction acting on it, which causes some of the kinetic energy to escape the system as heat. Eventually all that kinetic energy is used up and converted to heat that blows away, and the machine stops, unless extra energy is put into it from outside. You can build a *very* energy-efficient machine, but not one that never needs an extra injection of energy.

This will be considered true unless we somehow discover an exception to the laws of thermodynamics, but that is *extremely* unlikely to happen.

No perpetual motion machine can work in our universe. In this case, the energy created by the movement is dissipated through friction, halting the entire mechanism.

Generally speaking, it’s because there are a lot of forces that affect an object, not just the ones that are shown in a high school textbook. Usually it’s friction and air resistance that are most often forgotten.

Let’s see some problems with your example:

* The wheels are touching their axis, despite them being oiled, some energy is lost there
* When the chain that holds the balls touch the wheels some energy is also lost to friction
* The balls, chain and wheels are all subject to air resistance
* The airtight seal that lets the balls inside has to fit them very tightly in order to minimize water loss, and that implies friction. I imagine that this would be the place where most of the energy is lost
* I don’t think you can make an airtight seal that would open and close like this without losing water. This is probably the greatest design flaw with the machine
* The balls would be wet when they leave the water, so some water is also lost there
* There are no perfect generators that convert 100% of the motion to electricity, so you lose a lot of energy there.

With all this in mind, as far as we know, we cannot build a perpetual motion machine. Even if we could, we would have to make one that produces more energy that is needed to sustain itself if we want to use it as an infinite electricity generator.