1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar

    Staff: Mentor

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

    robphy

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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.
    Tom
     
  6. Sep 30, 2007 #5

    Doc Al

    User Avatar

    Staff: Mentor

    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.
    Tom
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Freebody diagram
  1. Phase diagram (Replies: 2)

  2. PV Diagram (Replies: 4)

Loading...