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Homework Help: 3 printers rest on top of each other on a table [FORCES]

  1. Apr 3, 2013 #1
    1. The problem statement, all variables and given/known data
    Three printers [X, Y, Z] are placed on top of each other on a table. What force does the middle printer exert on each of the other two printers.

    x is on top, y in middle, and z on bottom.

    x = 4 kg, y = 3 kg , and z = 2 kg
    2. F = ma

    3. The attempt at a solution

    I can consider the top two printers as one so the weight would be 7 kg.

    F = (7kg)(9.8 m/s^2 )
    = 68.6 N

    That would be the force exerted on z(the bottom printer).

    The normal force would be exerted on the top printer so:

    F = (5kg)(9.8 m/s^2 )
    = 49 N

    However this doesn't make sense since, the objects are in equilibrium so they're suppose to be equal...Please help?
  2. jcsd
  3. Apr 3, 2013 #2


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    Staff: Mentor

    68.6 N is the force between the middle and the lower printer, and required to support the upper two printers.
    You can use the same argument to calculate the force between the upper and the middle printer - you can even save one step, as you don't have to combine objects there.
  4. Apr 3, 2013 #3
    The forces have to balance out so would be just 68.6 between the middle one and the upper one.

    Also, since I'm combining objects in my previous calculations. wouldn't it be incorrect because I'm taking into account the forces of two objects when I should be just finding the force exerted by the middle one(y). The thought just came in my mind.
  5. Apr 3, 2013 #4


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    Staff: Mentor

    This would not give balanced forces, as you have nothing to balance the gravitational force of the middle printer then.

    I don't understand what you mean.

    Try to solve an easier problem first: Two printers, standing on a table. The upper one has a mass of 4kg, the lower one has a mass of 3kg. What is the force between them? Does this give balanced forces on the upper printer?
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