The Physics of Draining a Container

If you have ever bought a 2.5 gallon Poland Spring container you know that they don’t work very well as purchased. The water will flow out of the nozzle at a slow pace for a few cups of water and then come to a halt. In order to get the water to flow at an acceptable rate you must punch a hole in the top of the container. This is because when there is no hole at the top of the container a vacuum builds in the interior of the top of the container and a pressure gradient force caused by the difference between the low pressure at the top of the container and atmospheric pressure outside of the container causes air to move in through the same hole that is letting out water. This causes the water to drain from the container in a discontinuous way and at a slow pace.

Putting an additional hole at the top of the container allows the air to enter the container though a hole that is separate from the hole through which water is exiting. This allows the water to exit in a continuous stream at a much faster pace.

- Post written by John Mahon