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Homework Help: Free-body Diagram

  1. Feb 26, 2014 #1
    Not sure about the correct wording for this question!

    A brick of mass M sits on a rubber pillow of mass m . Together they are sliding to the right at constant velocity on an ice-covered parking lot.

    (a) Draw a free-body diagram of the brick and identify each force acting on it.

    (b) Draw a free-body diagram of the pillow and identify each force acting on it.

    (c) Identify all the action-reaction pairs of forces in the brick-pillow-planet system.

    Assume the brick is m1 and the pillow is m2

    For a), the brick should have an upward force and Fg downward. Should the upward force be F21 or n or both?

    For b), the pillow should have a normal force upward, force due to gravity downward, and a downward force (F12?).
  2. jcsd
  3. Feb 26, 2014 #2


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    Hello Confusius,
    Welcome to PF and please use the template. It helps you, it helps us and it helps us help you.
    That way you can easily conclude that you forgot to mention what F12 etc. stand for.
    It also helps decide whether m1 m2 are your assumptions or are part of the given variables. (I know, I know; point is they are not assumptions -- you just give them a name)

    Under 1. they ask for diagrams. Can you post yours ? Or describe them unambiguously (takes a lot of words to do so unambiguously, but can be done here) ?
  4. Feb 26, 2014 #3


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    Yes that is all correct however you should identify the value of F21 , F12, gravity forces , and normal force of the ice on pillow in terms of m1, m2, and g.
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