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Bernoulli's Principle of lift

  1. Mar 15, 2006 #1

    I am a student taking Conceptual Physics in College. We have just completed going over Bernoulli's Principle of lift. Please bear with me on this as it will most likely seem elementary to most of you. Below are several claims that I've heard and that my logical reasoning tells me to be incorrect. If you may be so kind, please help a frustrated student out by helping me understand any of these concepts.

    Claim #1: The reason why windows shatter during high winds is because the pressure of the wind on the outside portion of the window (i.e. the area being "hit" with the wind) is much lower than the pressure of the wind on the inside of the window (the area not being directly affected by wind). The high-pressure on the inside thus "pushes" the window and cracks it.

    My logic #1: It is not the high pressure of the inside portion of the window that cracks the window. It is the speed of the air being blown by the wind. That is to say, wind, just as any other object, is subject to Newton's First Law of motion. The air in motion has a tendency to stay in motion just like any other object (e.g. a rock thrown at the window has nothing to do with air-pressure, but rather, the direct force exerted by the rock).

    Question #1: Does the air not exert a direct force upon the window on impact? If I were to take a very powerful fan and start blowing air parallel to the window (also lowering the pressure[?]) would the window also shatter?

    Claim #2: It is said that the reason for why an umbrella sometimes brakes (by being inverted) during a high wind is because the pressure blowing on top of the umbrella is lower than the pressure directly below the umbrella, which exerts a great force at pushing the umbrella up and breaking it.

    My Logic #2: I don't have as much of a problem with this statement, except if it weren't for one thing. See question.

    Question #2: What if I take a fan and blow directly on top of the umbrella (not sideways)? Although I have never conducted this experiment, I have reasonable doubt that the umbrella will also "come up" upward. Is this true or not?

    Question #3: Take a small ship and attach a sail to it. Now turn the sail in the direction of the wind. The effects will be obvious. The ship will move in the direction of the wind. Why? According to Bournelli's Principle, the pressure is actually lower at the side of the sail where the wind makes the impact. Should not the higher pressure from the other side push the sail in the opposite direction of the wind?

    Thank you in advance for taking the time to look over these questions.
  2. jcsd
  3. Mar 15, 2006 #2


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    Hi whereami, welcome to PF!

    Re the first case:

    The way the wind is blowing would make a difference if the window blows out or in.

    If the pressure is lower outside because of wind blowing tangent to the pane or a sudden loss of atmospheric pressure or whatever, the window will blow out if the difference is enough to overcome the structural strength of the glass.

    If the wind is blowing directly into the window, you will have a stagnation point somewhere on the window, and as a result, a localized high pressure.

    Remember, that ANY point force is merely an idealization of a pressure distribution. In the real world there are no point forces because there are no points. Even throwing a rock at a window breaks the window because the pressure distribution of the impact overcomes the window's local structural strength.

    Similar situation for case 2. Wind blowing at the bottom of an umbrella slows down to a stop, increasing the pressure. The other side remains at atmospheric pressure (disregarding turbulence).

    Case 3: I think you have it backwards. Wind hitting the sail slows down, therefore the pressure increases.
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