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What is the fastest way to empty a full jug of water: tipped upside down or poured at a particular angle? (continued)

Mike Follows
Sutton Coldfield, West Midlands, UK

Tip the jug too little and the water flows slowly; tip it too far and the flow becomes a series of glugs. Somewhere between the two should be an optimum. But the optimum will change as the jug empties and the water level falls, so the fastest pour should involve continually adjusting the angle.

The reason is that the opening has two jobs: water must leave while air enters to replace it. When the jug is inverted, water can block the opening until the pressure inside drops enough for air to force its way back in as a bubble. Another surge of water escapes and the cycle repeats.

At a suitable angle, water can flow through the lower part of the opening while air enters continuously through the upper part. The flow need not be smooth – what matters is that air and water can pass through the opening simultaneously rather than taking turns.

Some containers solve the problem with a second opening or vent. Air can then enter by one pathway while water leaves by another, allowing the main opening to be devoted entirely to water. Birds exploit a similar principle: air sacs help maintain largely one-way air flow through their lungs, helping meet the high oxygen demands of flight.

So, for an unvented jug, the trick is to keep water flowing out without shutting off the air flowing in.

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