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Soccer ball pressure problem

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  • Soccer ball pressure problem

    Dear Colleagues,

    I posted a message to ask for help to solve a soccer ball pressure
    problem. A student in the master program of biomedical engineering did
    a study on the impact forces in soccer heading. She used soccer balls
    in three sixes with three inflation pressures. She had an
    unproportional increase in ball mass when the inflation pressure
    increased from 12 PSI to 14 PSI. She had the same results after
    repeating the tests. She could not give her thesis committee a
    convincing explanation. To try to help her, I posted a message to ask
    for help.

    With the help from many colleageus, especially Cameron Barr in
    Biokinetics & Associates, Ltd., we finally found the problem. The
    original measurement of ball masses at 10 PSI and 12 PSI was in error.
    The air pump and the scale were in different locations. The student had
    to test ball masses on three different days. She calibrated the scale
    only before the first test (14 PSI). Another group of students used the
    scale with different calibration after that. That caused error in the
    measurement of ball masses at 12 and 10 PSI. There was a coincident
    pattern in the lab schedule when she repeated the test. That gave her
    the same results in retest. After her thesis committee requested her to
    re-test again, she thought the error was in ball mass at 14 PSI because
    the increase in ball mass from 10 PSI to 12 PSI was reasonable. No need
    to say that she still could not find the problem after repeatedly
    testing the ball mass at 14 PSI several times. Then I recieved a
    message from Cameron who pointed out the ball mass should be around 400
    g when the inflation pressure is between 10 PSI and 14 PSI. This made
    the student realize the error was in ball masses at 10 PSI and 12 PSI.
    After repeating the test three times again, she finally had the
    reasonable results:

    0PSI 10PSI 12PSI 14PSI
    Size3 0.3197 0.3221 0.3225 0.3230
    Size4 0.3566 0.3597 0.3603 0.3611
    Size5 0.4204 0.4236 0.4243 0.4251

    I sincerely thank everybody who replied my message and offered help, and
    apologize for any confusion caused by this problem.

    Bing Yu, Ph.D.
    Assistant Professor
    Center for Human Movement Science
    Division of Physical Therapy
    The University of North Carolina at Chapel Hill
    Chapel Hill, NC 27599-7135

    Tel: 919-843-8643
    Fax: 919-966-3678

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