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Homework Help: Difficult Force Problem with changing angle ( )

  1. Oct 23, 2008 #1
    Difficult Force Problem with changing angle (plz help!!)

    1. A force F is applied to a crate of mass m on a floor where it has a coefficeint of static friction . Angle theta is intially 0 degrees but is gradually increased so that the force vector rotates clockwise in the figure. During the rotation the magnitude F of the force is continuously adjusted so that the crate is always on the verge of sliding. For coeff. of static friction = 0.70 (a) plot the ratio F/mg versus theta and (b) determine the theta sub infinite at which the ratio approaches an infinite value. (c) Does lubricating the floor increase or deacrease theta sub infinite or is the value unchanged? (d) what is theta sub infinite if the coefficient = 0.60?

    3. Please help me set up the problem! I am very confused and don't comprehend the question. If you can just help me start the problem I can probably take it from here. We are having an emergency study session w/ our class tonight and none of us understand this.
  2. jcsd
  3. Oct 24, 2008 #2


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    Homework Helper

    Re: Difficult Force Problem with changing angle (plz help!!)

    One way to approach understanding this problem is to draw a force diagram and write things in terms of the Forces. For instance the x component of force is balanced out by the weight and the y component of the force times u.

    Then rearrange your formula with F/mg = ...?

    You can then evaluate this relationship as theta varies and as to how you might vary u.
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