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Homework Help: Foam rubber ball hanging in a string - Forces

  1. Aug 28, 2009 #1
    1. The problem statement, all variables and given/known data
    Okay, so I'm not originally an English speaker, so this may be silly reading for some of you, especially a lot of the words here, which isn't to find in the dictionaries either, but hopefully you'll at least get a picture of the task and help me solve it.

    The drag (air resistance) of a ball made of foam rubber, is:
    [tex]L = (8,7 \cdot 10^{-4} Ns^2 / m^2)v^2[/tex]
    The ball has a mass of 8,0 g.

    We attach the ball to a piece of string hanging from the ceiling, and in front of a reversed vacuum cleaner. LOL. I have no idea what to call that machine in English, but it creates an air flow the affects the ball when it's hanging there in midair.

    The air flow makes the ball move to the side, so that the string makes an angle of 38 degrees compared to straight down.

    Draw an illustration showing the forces affecting the ball. Find the speed of the air when it passes the ball.

    2. Relevant equations

    I mentioned the only equation earlier, I think.

    3. The attempt at a solution
    No idea how to start. As for the drawing, I've written the gravitational force G straight down, the wind force V directly right and the string force parallel to the string, going from the ball. Is that correct? Anything else?

    Thanks a lot.

    Edit: I guess I could have called that "machine" a fan, and made it easy for myself. Lol.
  2. jcsd
  3. Aug 28, 2009 #2


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    You have drawn the forces correctly. Now all you have to do is add them as vectors and say that their sum is zero. Don't forget to convert the mass from grams to kilograms.

    ** Edit **

    A "reverse vacuum cleaner" is perfectly understandable.
    Last edited: Aug 28, 2009
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