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Homework Help: Forces and Motion

  1. Oct 5, 2011 #1
    5. A 12N horizontal force F vector pushes a block weighing 5N against a vertical wall. The coefficient of static friction between the wall and the block is .60 and the coefficient of kinetic friction is .40. Assume the block is not moving initially.
    (A) will the block start moving?
    (B) What is the force exerted on the block by the wall?

    I draw a FBD on the block, and these are all the forces that I've found:
    Take y upwards and x positive going from left to right

    y direction:
    -mg - fs = 0
    fs = -mg = -5N

    x direction
    F - Fc (contact) = 0
    F = Fc = 12N

    Part a.
    The maximum static friction that prevents the block from moving down is
    fs = 0.6 * 12 = 7.2 N
    since magnitude of fs > magnitude of mg, the block will not move.

    Part b:
    From x direction, we have F = Fc = 12N, so the force on the block from the wall is vector Fc = (-12N)i

    Did I do it right ? Did I miss any forces on my FBD ?
  2. jcsd
  3. Oct 5, 2011 #2
    hi first part seems ok. the sign of [itex]f_s[/itex] should be positive though.
    for the second part, wall exerts force in two different directions. one is normal reaction
    as you pointed out and the frictional force which is upward......so you will need to find the
    magnitude and angle of the resultant force
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