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Mass on Bridge, Find Force of Friction when bridge is barely inclined

  1. Feb 25, 2013 #1
    Look at the picture attached. There is a mass on a drawbridge, which is inclined at half a meter. We have the Normal force, the coefficient of static friction, force of gravity and the force of friction pushing the mass upwards to the end of the draw bridge that is most inclined. We also have [itex]\theta[/itex], from the joint of the bridge. Put the answer in terms of

    What is the force of friction in terms of [itex]\theta[/itex] and force of gravity, F_g
    theta and force_gravity


    The correct answer is Theta*F_g. How did they get this answer without any trig identities? Did it all cancel out?
     

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    Last edited: Feb 25, 2013
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  3. Feb 25, 2013 #2

    PhanthomJay

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    Re: Mass on Bridge, Find Force of Friction when bridge is barely incli

    It is not sufficient to say that the drawbridge is inclined "half a meter" without giving more info. Perhaps you mean the angle theta is rather small? What do you get for a result? Please show your work.
     
  4. Feb 25, 2013 #3
    Re: Mass on Bridge, Find Force of Friction when bridge is barely incli

    The answer is at the bottom, and yes the intention is to tell you that [itex]\theta[/itex] is small. Also, I do not know the work to arrive at this answer, that is why I am asking.
     
  5. Feb 25, 2013 #4

    PhanthomJay

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    Re: Mass on Bridge, Find Force of Friction when bridge is barely incli

    There is an approximate but very close relationship between theta and sin or tan theta when theta is small and expressed in radians. But first, you should apply Newton's first law along the direction of the incline after first drawing a free body diagram showing the forces acting along the incline, to see how your answer compares to the book answer.
     
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