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Homework Help: Diameter of football player's Piston

  1. Sep 19, 2012 #1
    1. The problem statement, all variables and given/known data

    A 54.0kg cheerleader uses an oil-filled hydraulic lift to hold four 120kg football players at a height of 1.30m . If her piston is 19.0cm in diameter, what is the diameter of the football players' piston?

    2. Relevant equations

    I think the only equation needed is (F1/A1)=(F2/A2). I didn't think that density or the height being raised was relevant in this problem

    3. The attempt at a solution

    So first I rearranged the formula so that ((F2A1)/F1)= A2.

    I know that the force is the weight of the cheerleaders(54 kg) and the four football players( 480). The value g cancels out, so I'm left with ....

    (480pi(r1)^2)/54= pi(r2)^2

    so by subbing in for r1 ( .095m) I get...


    I then rearrange the formula to solve for r2


    then to get the diameter I multiply by two to get .566m or 56.6 cm

    This however, is not the right answer. Can somebody please help?
  2. jcsd
  3. Sep 20, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF;
    It depends on how she is holding who up with the lift ... and what the relationship is between her piston and the player's piston. You need to be explicit about the setup.

    You seem to be saying that the pressure on the player's piston is the same as the pressure on the cheerleaders. Why?

    Notice that the pi's cancel out in the first equation you have them in?
    The ratio of the radii is also the ratio of the diameters too.

    Lastly; how do you know your answer is incorrect?
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