
Metaphors Are Us (2013) - dnetesn
http://nautil.us/issue/75/story/metaphors-are-us-rpp
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amelius
> The flotation device in the toilet water tank was rubbing against the side,
> getting stuck halfway up so that the tank didn’t fill completely.

Wouldn't that cause the tank to overflow instead?

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sah2ed
Nope. The only way for the tank to overflow is for the flotation device — the
ballcock — to be removed from the tank, causing the valve to never close.

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amelius
But if the flotation device never rises to the height where it closes the
valve, the water will keep on flowing in (?)

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sah2ed
You seem to be thinking of only one force when in fact there are at least two
forces acting on the ballcock. If you were to empty the tank instantaneously
by cutting out the bottom of the tank, what would happen to the ballcock? It
would fall on its own.

The ballcock has an upward force (buoyancy) acting on it, causing it to float
when the tank is not empty. It also has a downward force (gravity) acting on
it, causing it to fall, when the tank is empty.

So the reason why the ballcock causes the valve to open and close is due to a
pressure differential.

[https://en.wikipedia.org/wiki/Ballcock](https://en.wikipedia.org/wiki/Ballcock)

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amelius
Yes, but if the ballcock is stuck "halfway up", then I assume the valve is
open (or half-open).

Anyway, I see in the schematic that there's typically an overflow mechanism.

