Thursday, October 23, 2014

Drinking Through Straw

We've all drank through a straw before, but how does drinking through a straw work?  Are you sucking up the water?  Not really.  What happens when you suck on a straw is that you remove the air in the tube.  The atmosphere pushing on the liquid in the glass at all times.  When the air from the straw is removed, the air pressure on the water is greater than the air pressure in the straw.  Therefore, the air pushes water in the glass up the straw.

Physics tells us there's a limit to the height of a working straw.  Construct a straw several meters in length and you'll find it impossible to suck water out of it.  Why?  The force of gravity acting to pull the water back down the straw increases as more water is pushed up the straw.  When the downward force of gravity equals the force pushing the water upward, water can no longer be forced up the straw.  Make a straw too long, and it doesn't work.

My daughters and I set out to simulate this by using a regular straw.  We took a jar of water with a lid, poked a hole in the lid, slid a straw through the hole, then sealed the hole with play-doh.

Sealing the hole with play-doh prevents outside air from entering the jar.  The only air pushing the water up the straw is the air inside the jar.  The task is to try to suck up a mouthful of water.  At first a little water comes up the straw, but if the hole is sealed, that's it.  There's not enough air pressure inside the jar to continue pushing water up the straw.  Cool you get to use play-doh!!!

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