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Homework Help: Friction and Magnitude Problem

  1. Sep 26, 2008 #1
    1. The problem statement, all variables and given/known data
    The person in This picture weights 170lbs. A seen from the front, each light crutch
    makes and angle of 22degrees with the vertical. Half of the persons weight is
    supported by the crutches. The other half is sipported by the vertical forces
    of the ground on the person's feet. Assuming that the person is moving with
    a constant velocity and the force exerted by the ground on the crutches acts
    along the crutches, determine (a) smallest possible coefficient of friction
    between crutches and ground and (b) the magnitude of the compression force
    in each crutch

    2. Relevant equations

    3. The attempt at a solution
    I have first drew a free body diagram for one side and I get my
    forces to be this.

    x forces are -fk
    y forces are -mg, n

    I determined that each crutch has 42.5 lbs of force
    I converted the 42.5 lbs to 189n

    I solved the n force to be 202n by drawing a triangle and then using sin22(189)
    = 70.8n. Then getting 202n using pyth. theraom.
    Am I starting this correctly?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Sep 26, 2008 #2


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    Homework Helper

    Which force components do you plan to use for determining the coefficient of friction?
  4. Sep 26, 2008 #3
    I used fk=uk(n)
    70.8n = uk(461.8n)
    uk= horizontal force/total normal force
    I get uk to be .153 for part a

    part b I get the compression force to be 202n from the findings before.

    This seems to be correct.
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