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Homework Help: Forces surrounding Car on a Bridge

  1. Jan 11, 2010 #1
    1. The problem statement, all variables and given/known data
    An W = 7500 N automobile is stalled one-quarter of the way across a bridge (see Fig. P77). Compute the additional reaction forces at supports A and B due to the presence of the car. Take the length of the bridge to be AB.

    8-P65alt.gif


    2. Relevant equations



    3. The attempt at a solution
    I was told that if you pick one side of the bridge as your rotation point, then either A or B is already a length of 0. But I'm not sure how to proceed from here, any help would be greatly appreciated!
     
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  3. Jan 11, 2010 #2

    rock.freak667

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    Call the reaction at A, RA and at B, RB.

    So if you take moments about A, then the moment of RA about A is 0 right?

    So what is the moment produced by RB about the point A and what is the moment produced by W about A?
     
  4. Jan 11, 2010 #3
    I didn't know what a moment was so I read the explanation but doesn't that mean that the force around point B would just be a normal force of 7500 because the object is static? But that isn't the right answer.
     
  5. Jan 11, 2010 #4

    rock.freak667

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    a moment is the turning effect produced by a force which acts perpendicular to a body.

    So the moment of RB about A, would be the force multiplied by its perpendicular distance from A.
     
  6. Jan 11, 2010 #5
    Oh ok! That makes sense, however the bridge length is only given to be AB. So that would mean, if A was the rotation point, that A=0 and therefore B=7500AB right?
     
  7. Jan 12, 2010 #6

    rock.freak667

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    No the force is RB and the distance is AB , so the moment produce is (AB)*RB Right?

    Can you find the moment produced by the 7500N force?
     
  8. Jan 12, 2010 #7
    Would that be the rotational acceleration around point A?
     
  9. Jan 13, 2010 #8

    rock.freak667

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    Taking moments about A.
     
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