# why would the siphoning effect not be perpetual motion?

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My coworker is telling me that perpetual motion exists and shows me a video of a guy using the siphoning effect on some water cans.

I’m telling him that perpetual motion, physically, cannot exist.

But why would it not work?

In: Physics

Because sooner or later the container you are siphoning from runs out, and then the siphon stops.

If you post a link to the video I can probably tell you why it is either fake or won’t work.

Siphoning only works if your target is at a lower elevation than your source. Or, more expressively presented, if the fluid column going *down* is heavier than the fluid column going *up*. It basically borrows a bit of potential energy from the fluid going down the bend and uses it to lift the intake fluid over the bend. It’s very much the same thing as throwing a long bit of rope over a pulley down into a basement and watching it spool up the pulley and down in the hole from its weight.

Which video? Without special effects the water level will just equalize between the tube and jug.

You can’t siphon upwards, the energy required to lift the liquid being siphoned would be greater than the gain of harnessing the theoretical siphon.

Siphoning works by moving water (or other fluid) from high potential energy to low potential energy, but by going through a *higher* state on the way. Gravitational potential energy is directly proportional to height, so we can just go with height = energy.

The core reason siphoning works is that at the highest point of the siphon, the energy gained by a unit of water going to the lower container is more than the energy required to lift another unit of water from the higher container up to the top.