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1988-03-05
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THAT GUY, DANIEL BERNOULLI by Alan Jones Physics and Chemistry SIG
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In the January edition of Discover magazine there was an article about
Cecil Adams. Adams makes his living answering obsure scientific
questions. In the article there was one question left unanswered. What
makes the shower curtain sometimes drift inward when you are taking a
shower?
This was surprising since the answer is simple to anyone familiar with
*Bernoulli's Principle*. This principle states that, as the air
velocity increases, the pressure decreases, and vice versa. Actually,
in tech- nical language, it states that the total energy of a particle
in motion is constant at all points on its path in a steady flow.
Examples of where this principle is used by modern technology are
numerous. The one I am going to deal with today is *flight*.
The operation of a venturi tube provides a good illustration of
Bernoulli's Principle. Manometers placed along the tube can be used to
illustrate the variation of pressure with respect to restriction. It is
at the point of greatest restriction that the fluid travels fastest and
exerts the lowest pressure. Now take the top surface away, you will
notice no change in the point of minimum pressure. The venturi at this
moment will have a profile very similar to the upper surface of an air-
craft wing.
When air strikes the leading edge of the wing, its flow is obstructed
and the velocity reduced. Some of the air must pass over the upper
surface, and some over the lower surface. But all of the air must reach
the trailing edge at the same time. Looking at a cross-section of a
wing you will notice that the upper surface has a greater curvature.
Since we stated above that air starting at the leading edge must reach
the trailing edge at the same time, and because of the curvature the
upper surface distance is greater, than the air across the upper
surface must travel faster. From Bernoulli╒s Principle we see that when
the fluid (air) travels faster there is a drop in pressure. So now we
have a greater force acting on the wing from below. And as with
everything in nature the object must move in that direction. Thus the
aircraft has *lift* and is able to gain altitude. It can be seen from
this explanation, simple as it is, that an airplane is not lifted into
the air but is pushed upwards due to the greater pressure below the
wing.
One easy way to show this phenomenon is to take a piece of paper and
place it on a table top. Now carefully blow across the top of the
paper. You will see it lift off the table. Be careful that you don't
blow underneath the paper or you will alter the true results of the
experiment.
Try and see how many examples and experiments you can discover. Maybe
you could post your results here.
Back to our shower curtain. The reason that it drifts inward is
because of the higher velocity air caused by the movement of the shower
water. Thus a lower air pressure on the inner surface. Simple??
Has anyone else observed an everyday occurance and wondered what
scientific principle might lie behind it?? If so lets hear from you.
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