Bernoulli's principle thus says that a rise (fall) in
pressure in a flowing fluid must always be accompanied by a decrease
(increase) in the speed, and conversely, if an increase (decrease) in , the
speed of the fluid results in a decrease (increase) in the pressure. This is
at the heart of a number of everyday phenomena. As a very trivial example,
Bernouilli's principle is responsible for the fact that a shower curtain
gets ``sucked inwards'' when the water is first turned on. What happens is
that the increased water/air velocity inside the curtain (relative to the
still air on the other side) causes a pressure drop. The pressure difference
between the outside and inside causes a net force on the shower curtain
which sucks it inward.
A more useful example is provided by the functioning of a
perfume bottle: squeezing the bulb over the fluid creates a low pressure
area due to the higher speed of the air, which subsequently draws the fluid
up. This is illustrated in figure 7.2.
Bernouilli's principle also tells us why windows tend to
explode, rather than implode in hurricanes: the very high speed of the air
just outside the window causes the pressure just outside to be much less
than the pressure inside, where the air is still. The difference in force
pushes the windows outward, and hence explode. If you know that a hurricane
is coming it is therefore better to open as many windows as possible, to
equalize the pressure inside and out.
Another example of Bernoulli's principle at work is in the
lift of aircraft wings and the motion of ``curve balls'' in baseball. In
both cases the design is such as to create a speed differential of the
flowing air past the object on the top and the bottom - for aircraft wings
this comes from the movement of the flaps, and for the baseball it is the
presence of ridges. Such a speed differential leads to a pressure difference
between the top and bottom of the object, resulting in a net force being
exerted, either upwards or downwards. This is illustrated in the Figure 7.3.