- Bernoulli's Principle states that where the velocity of a fluid is high, the pressure is low, and where the velocity is low, the pressure is high.
- As fluid moves from a wider pipe to a narrower one, the
volume of that fluid that moves a given distance in a given time period does
not change. But since the width of the narrower pipe is smaller, the fluid must
move faster in order to achieve that result. One-way to illustrate this is to
observe the behavior of a river: in a wide, unconstricted region, it flows
slowly, but if its flow is narrowed by canyon walls (for instance), then it
speeds up dramatically.
- The above is a result of the fact that water is a fluid, and
having the characteristics of a fluid, it adjusts its shape to fit that of its
container or other solid objects it encounters on its path. Since the volume
passing through a given length of pipe during a given period of time will be
the same, there must be a decrease in pressure. Hence Bernoulli's conclusion:
the slower the rate of flow, the higher the pressure, and the faster the rate
of flow, the lower the pressure.
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Knowing the amount of water
flowing in a river is important because the river's speed increases as more
water flows through it. Double the water means double the speed, so a mild
rapid becomes a dangerous one during the rainy season. Rafters also must know
the flow because water is heavy, weighing 1,000 kilograms per cubic meter (62
pounds per cubic foot), and in rapids it exerts tremendous pressure on a raft.
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