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Thread: Centripetal Force/Air Pressure Question

  1. #1
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    Centripetal Force/Air Pressure Question

    Please see the following link and the answer page that follows:

    http://www.physics.umd.edu/lecdem/ou...rch12/q231.htm

    My question concerns the reasoning for the answer to the first tube.

    (SPOILER alert...if you want to answer the question on your own first, DON'T scroll down until after you see the link.)


















    Again, for the answer to the first tube, I need help explaining the action-reaction system involved in basic terms that high school students can visualize.

    Any help would be appreciated.

  2. #2
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    It might be easisest to first consider the air flow that is travelling underneath the flange and ask will the air travel faster around it or slower. Thinking of the airflow around an airplane wing will also help.

    [I kinda disagree with the flat flange since air entrainment will create a slight vacuum at the hole, but the hair dryer does not provide enough air flow, apparently, to create enough vacuum to see a rise in the column of fluid.]
    We know time flies, we just can't see its wings.

  3. #3
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    OF,
    The air that is travelling across the top of the first tube is accelerated upwards by the curvature of the flange. Therefore there is a downwards reaction, seen as an increased pressure in the tube and a downwards movement of the manometer.

    I was surprised that there was no movement of the third tube. The air that is moving across that flat flange is not accelerated at all, but Bernouille says that the moving air has more kinetic and less dynamic energy than the still air around it, or within the tube, so that its dynamic pressure will fall. The third manometer should rise. But it doesn't, or at least, it can't be seen to rise inthe pic. I suspet that the velocity of the air just wasn't enough.

    Oops, George said that already.

    John

  4. #4
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    I agree with both of you about tube 3. I, too, thought that it would move upward.

    I'm having trouble with a more basic explanation for tube 1, still. For example, if an able sits on a table, it applies a downward force on the table as a result of it's weight. The table applies and upward normal, or support force. Applying Newton's 3rd law (F a on b = - F b on a), we can say that F of apple on table = - F table on apple.

    How can I use similar terms for tube 1?

  5. #5
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    Quote Originally Posted by Orion's Fan View Post
    I agree with both of you about tube 3. I, too, thought that it would move upward.

    I'm having trouble with a more basic explanation for tube 1, still. For example, if an able sits on a table, it applies a downward force on the table as a result of it's weight. The table applies and upward normal, or support force. Applying Newton's 3rd law (F a on b = - F b on a), we can say that F of apple on table = - F table on apple.

    How can I use similar terms for tube 1?
    Tilt the apple's table.

    There is an imbalance of forces caused by the bend in the flange. One side will encounter more resistance than the other side. This will allow greater air flow on the less resistive side and back pressure on the more resistive side.

    It would be easier to see the problem on the first flange by cuping your hand behind the hole. Your hand will create a pressure region in front of it and the liquid would go down even more than with the flange alone. The flange is simply a reduced version of your hand.
    We know time flies, we just can't see its wings.

  6. #6
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    OF,
    You posted at least four hours after mine, so I presume you read it.
    Should I repeat it?


    A force accelerates the moving air upwards, Newton's 'action' (Fa on b). There must be a corresponding downwards reaction (Fb ona) which presses on the upward curving ramp AND the air in the tube. Does that help?
    OR
    The action of accelerating the moving air is caused by a force. That force must have reaction force in the opposite direction. If you could weigh the rig, it would be slightly heavier while the wind was blowing. That force can be seen as added weight or added presure in the tube.

    JOhn

  7. #7
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    My intuitive take on the first tube is much simpler. The first tube is an example of backpressure. The airflow is sort of blocked by the upward flange. It's essentially the same effect as if you put the palm of your hand in front of the fan. There's going to be some backpressure.

    Of course, pressure pushes in all directions, so this backpressure will push downward into the first tube.

  8. #8
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    It looks like the same aerodynamics of an airplane.

    I think it's a good demonstration of the lift forces.

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