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Freebody diagram

  1. Sep 30, 2007 #1
    This is not a homework question. I was preparing a problem sketch when I was questioned about a freebody diagram, and realized I didn't have an absolutely dead-certain answer. I know how to work the problem; I need help in documentation, that is, what the freebody diagram should look like. To make the question simple, I'll use a gedanken instance.

    Suppose a 100 kg person is standing on a 1 kg scale that is sitting on a small platform. The small platform is located in a part of the universe that has little gravity. The platform is accelerating upward at 10 m/s^2.

    I draw the freebody diagram for the scale as having 2 forces acting on it: 1010 N upward and 1000 N downward. The net force is 10 N which, for m = 1 kg, gives a = 10 m/s^2). That seems OK.

    Now, my conundrum. Should I draw the freebody diagram for the person as having a single upward force of 1000 N?

    As I said, I know how to solve the problem; I'm uncertain how to document it.

    As Helen Thomas likes to say, I'll have a follow-up question. Thanks.
  2. jcsd
  3. Sep 30, 2007 #2

    Doc Al

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    Sounds good to me.
  4. Sep 30, 2007 #3


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    When drawing a force on a free body diagram, one thing to ask yourself is "What object is applying this force on our object?" If you can't answer that question, then it might not be such a force on the free body diagram.
  5. Sep 30, 2007 #4
    Thanks guys. Here's the follow-up. I'll identify where my little brain gets fried.

    Let there be 3 observers of this event:
    (1) the person on the scale
    (2) a person standing on a nearby platform which is not accelerating
    (3) a person standing on the accelerating platform near the scale
    Observers 2 and 3 are generally aware of the mass of observer 1 since they are all similar size; however, observers 2 and 3 cannot see either the scale readout or whatever causes the acceleration.
    THE QUESTIONS: Do all 3 observers make the same freebody diagram for the scale and observer no. 1 system? Should they?

    MY ANSWER: Clearly observer 1 makes the freebody diagram as described before. I think observer 2 does also and nobody has contradicted me...yet. However, I think observer 3 should make the same freebody diagram and I have been challenged on that and I don't have a good answer. Is it, in fact, possible for observer 3 to come up with the same diagram based on his knowledge of the system?

    So, there we are. No more followups. Thanks for your input.
  6. Sep 30, 2007 #5

    Doc Al

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    I don't see any difference between observer 1 and observer 3: they both are in the same accelerating, non-inertial frame. Only observer 2 is in an inertial frame.

    I would say that all three observers should draw the same free body diagram, since they all observe the same real forces. But the accelerating observers must use a modified form of Newton's 2nd law that includes so-called "fictitious" forces.
  7. Oct 1, 2007 #6
    Thanks. I see my error.
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