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Homework Help: Forces and Pulley System

  1. Sep 19, 2006 #1
    Block A has weight w_A and block B has weight w_B. Block A is positioned on the horizontal surface of a table. Block A is connected to a cord passing over an easily turned pulley that has Block B hanging down from the pulley. Once Block B is now set into downward motion, it descends at a constant speed.

    a. Calculate the coefficient of kinetic friction between Block A and the tabletop.

    I drew Block A's force diagram. For this part, I am assuming Block A is not in motion as yet; it is just sitting on the tabletop. (Is this correct?)

    Well, F_fr = mu_K * F_N and the normal force = w_A.

    F_fr = mu_k*w_A

    mu_k = F_fr/w_A

    But I think this is somewhat incorrect. Musn't I express mu_k with the known variables give. What is the Value for F_fr? :confused:

    b. A cat, also of weight w_A, falls asleep on top of block A. If Block B is now set into downward motion, what is its acceleration (magnitude and direction)? Express a in w_A, w_B, and g.

    Well, I am unsure of the F_fr expression. This is what I did for this part.

    For Block A, the expression is now F_T - F_fr = [(2*w_A)/g]*a
    For Block B, ---------------------w_B - F_T = (w_B/g)*a

    Adding these two expressions, I get

    w_B - F_fr = [(2*w_A)/g]*a + (w_B/g)*a
    w_B - F_fr = a[(2*w_A + w_B)/g]

    a = [w_B - F_fr]/[(2*w_A + w_B)/g]
    a = g*[(w_B-F_fr)/(2*w_A + w_B)]

    But I don't know what to put for F_fr as noted in the first part.

    Are my reasonings and math that I did do correct?

  2. jcsd
  3. Sep 19, 2006 #2


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    One may assume block A has zero velocity.

    To find F_fr, what is F_fr opposing? What is the significance of Block B descending at constant speed?
  4. Sep 20, 2006 #3
    F_fr = w_B, but when I plug this value into the expression for a that I found, acceleration would be 0. According to Newton's First Law, a body that has constant velocity and, therefore, 0 acceleration is acted on by no net forces. Where do I go from here?
  5. Sep 20, 2006 #4
    Soaring -- didn't your last post just answer your own question??? In your first post, you wanted to know what F_fr was. So, what is it?
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