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Homework Help: Force, acceleration, gravity question

  1. Oct 4, 2005 #1
    Hi, I've been working on my homework assignment, and there are a couple tricky questions that I hope someone can give me a few pointers on.

    Question:
    Compressed air is used to fire a 60 g ball vertically upward from a 5.3 m tall tube. The air exerts an upward force of 3.3 N on the ball as long as it is in the tube. How high does the ball go above the top of the tube?

    I have tried a few things:
    First, I tried finding the acceleration in the tube and then using that to find the speed at the end of the tube.

    F=ma
    a=55.0m/s/s
    then vf^2=vi^2+2a*d
    vf= 17.38 m/s

    Then to find height, I used vf^2=vi^2+2a*d with vf from the tube as the new vi, and vf=0 because it's at the max height.
    Using 0=17.38^2+2(-9.8)d
    d=15.41 m (which is incorrect)

    I also tried d=.5*gt^2 and got d=35m (also incorrect)

    If anyone can give me a better direction, I would be very greatful,
     
  2. jcsd
  3. Oct 4, 2005 #2

    Doc Al

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    Staff: Mentor

    To find the acceleration, first find the net force on the ball. (You forgot about gravity.)
     
  4. Oct 4, 2005 #3
    That makes sense, but following the same steps using net force as applied-gravity (net force= 2.712N), I get d to be 24.4 m, which is still not the correct answer.
     
  5. Oct 4, 2005 #4

    Doc Al

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    Staff: Mentor

    That's the answer I get. (Your method is fine.)
     
  6. Oct 4, 2005 #5
    The program that I am imputing the answers into still tells me that I don't have the correct answer, so I think that it might be missing something still.
     
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