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Homework Help: Turning effect of forces and min force

  1. Mar 17, 2008 #1
    diagram.jpg

    to find the moment of force X about A,is the working
    20x1.4=0.8X
    X=35Nm clockwise.


    and what does it mean when the question asks "what is the minimum force X that must be applied in order to stop the door from turning?"

    does that mean it has to be in equilibrum?
     
  2. jcsd
  3. Mar 17, 2008 #2

    Doc Al

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    X is the force; the moment of the force is just the force times the moment arm (distance to pivot).


    The force F exerts a moment tending to turn the door; force X must exert an opposite moment to cancel it.

    Yes, rotational equilibrium.

    (FYI: It's always a good idea to state the problem exactly as given.)
     
  4. Mar 17, 2008 #3
    okay the question is "what is the minimum force X that must be applied in order to stop the door from turning?"

    (p.s.,i'm not quite sure as i didnt attend school today and this is part of my homework from my physics workbook)
     
  5. Mar 17, 2008 #4

    Doc Al

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    And you already solved for that force X. :wink: (Even if you didn't recognize it as such.)
     
  6. Mar 17, 2008 #5
    oh,i see.but then the question is phrased in two parts.does this mean that i can just bring forward the answer?
     
  7. Mar 17, 2008 #6

    Doc Al

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    What are the two parts? State the complete problem exactly as given.
     
  8. Mar 17, 2008 #7
    a) Calculate the moment of the force X about A.

    b) what is the minimum firce X that must be applied in order to stop the door from turning?
     
  9. Mar 17, 2008 #8

    Doc Al

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    Good. In the light of my previous posts, can you answer each part?
     
  10. Mar 17, 2008 #9
    so does that mean that both questions the answers are the same?
    because you previously said that i actually answered the question without me knowing
     
  11. Mar 17, 2008 #10

    Doc Al

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    No, the answers are not the same. The first part asks for a moment; the second part for a force.
    Right. You actually solved for the force (X) when you thought you had solved for the moment.

    I interpret the first question as a general question that helps you answer the second question. For a force X that is a certain distance from the axis, what's the moment?
     
  12. Mar 17, 2008 #11
    so the answer is 28Nm,right?
     
  13. Mar 17, 2008 #12

    Doc Al

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    That's the moment needed to produce equilibrium, but I don't think that's what they are looking for in question a. (They don't even bring up the idea of preventing the door from turning until question b.)

    I think they just wanted a general answer. The moment of a force (F) at a perpendicular distance (d) from an axis is Fd. So just plug in what you know about the force X. (At this point, X is just some unknown force. But you do know the distance from the axis.)
     
  14. Mar 17, 2008 #13
    ah i see
    thanks fr the help!
    i got to go now :)
    once again thanks!
     
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