Straw Length and Atmospheric Pressure
I was told that you cannot suck liquid up a drinking straw if
it is too long. I do not understand why. It seems to me you would just
suck harder. Does this have anything to do with atmospheric pressure?
Bulls eye, Daniel! If you think that you are not sucking the liquid up in
the straw, but rather the atmospheric pressure is pushing it up, it will
make sense. Because the force of the weight of the atmosphere works in all
directions, it exerts a force on the liquid in the container from which you
are drinking. This force is evenly distributed in the liquid and finite -
about 14.7 pounds per square inch or 29.92 inches mercury or 1013.2 millibars. It is that total atmospheric
pressure that limits you ability to get a liquid to rise in a straw.
Could a person at sea level cause a liquid to rise higher than a person at
the top of a high mountain? Sure. Because there is more atmospheric
pressure a sea level. Would that ability be affected by a changing pressure
system? Absolutely... but it would be hard to notice the difference while
drinking a Coke at the drive-in.
In science we generally do not think of "sucking" - a pulling force in a
liquid. It is more useful to think of a push... and when there are uneven
pushes on an object (or a liquid) it tends to move. The liquid is pushed up
You have two things working against you with a longer straw,
gravity and friction. The longer the straw, the more weight
you need to pull up into the straw (gravity works against you
here). Also, the longer the straw, the more friction there is
of the greater amount of liquid against the inside of the straw.
Theoretically, it is true that you can just suck harder, but there
will be a physical limit to your ability to do so if the straw
is long enough.
Atmospheric pressure actually helps you, because it presses down
on whatever liquid surface (such as the top of the drink in your
glass) you are drinking from. However, atmospheric pressure is a
small force, in this case, and does not help you very much.
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012