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Homework Help: Springs and static friction

  1. Oct 7, 2007 #1
    a 52 N backpack is laying on a table with a spring attached to one side of it where force is applied and the spring stretches. The backpack begins to slide when the spring (k= 150 N/m) stretches by 2.50 cm. What is the coefficient of static friction between the backpack and the table?

    i set up a freebody diagram, but i guess i'm just confused about the weight given in N. I have only had problems dealing with kg. I set up adding all forces in the x and y direction and got:

    -F + fs = ma(x direct)
    -Fg + FN = ma(y direct)

    FN is normal force, fs is static force

    I got the F by using F= -kx and got 3.75N
  2. jcsd
  3. Oct 11, 2007 #2
    if the spring stretches 2.5cm =.025m so the force is
    150N/m * .025 = 3.75n

    now this is the force reqired to just overcome the static friction

    so assuming the table is horizontal the reaction force is 52N

    now 3.75N= cof of frict * 52N

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