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Homework Help: Dense about Density?

  1. Mar 1, 2004 #1
    I said my good-byes too soon, it seems...the course actually has 200 more pages of physics torture before we reach the chemistry part of the course. Unpleasant news, yes, but I know I can take it...can this forum?

    BTW, I got 100% on my test today, thanks to the help I received on these boards! But no rest for the wicked! Press on, press on! We are studying fluid dynamics now.

    Q. If you release a Ping-Pong ball beneath the surface of water, it will rise to the surface. Would it do the same if it were submerged in a big blob of water floating weightless in an orbiting spacecraft?
    A. I think it would possibly do so, since the water will still be pressing on it from all angles, no? and thus pushing it to the edge of the blob?

    Q. What physics principle underlies this observation: The windows of older passenger trains sometimes break when a high-speed train passes by on the next track.

    Thanking you in advance.
    A. Is this like a tornado, POPPING the window out because it sucks the air out?
  2. jcsd
  3. Mar 1, 2004 #2
    1. I can't imagine any reason it would. Water isn't pushing it from all sides any more than would happen if I put it on a table and stuck two books next to it. Gravity is the source of water pressure.

    2. I'm sure there must have been a time when you were walking on the sidwalk and a huge big rig just drove right by you. I'll bet you felt it, too. The concept's the same.

    Glad to hear you did well on your test. Is the stuff you've picked up still torture, or is it getting easier in general?

  4. Mar 2, 2004 #3


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    Cookiemonster's answers are good but I want to say 2 things:

    Actually there is a slight pressure on something surrounded by water- it's basically surface tension- even when weightless.
    However, holly said "since the water will still be pressing on it from all angles and thus pushing it to the edge of the blob". If the water is pressing (equally) from all sides, why would it move?

    Second: "Bernoulli's principle": the faster the air is moving, the lower the pressure.
  5. Mar 2, 2004 #4
    1. I thought the ping-pong ball might move because the orbiting capsule will surely have a pressurized cabin...in effect, artificial atmospheric pressure? I thought that maybe if the capsule wasn't rotating, just flying along with one side towards the earth, the air pressure would be pressing a bit more at the bottom of the blob because the air has weight, and that would pop the ball out of the top of the blob. If there wasn't any air pressure, then there wouldn't be a blob, but rather millions of bloblettes, was my thought.

    2. Thank you both, I can now see by the real answer that they are looking for the principle and not asking for a reason the window would break.

    BTW, this forum is why I can now do better in understanding the course material. Before, I felt desperate and hopeless, but I feel now that I can get help, altho' I'm sorry to be so irritating to the members and regretful I have so few "Eureka" moments in my thinking.
    Last edited: Mar 2, 2004
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