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Homework Help: Three friction problems

  1. Oct 12, 2007 #1
    1. The problem statement, all variables and given/known data

    A 2.5 kg block is initially at rest on a horizontal surface. A horizontal force F of magnitude 5.7 N and a vertical force P are then applied to the block. The coefficients of friction for the block and surface are s = 0.36 and k = 0.23.

    Determine the magnitude of the frictional force acting on the block if the magnitude of P is 8.0 N.

    2. Relevant equations

    f = [tex]\mu[/tex] Fn

    3. The attempt at a solution

    The force due to gravity is 2.5kg(9.8 m/s^2) = 24.5 N

    The normal force would be 16.5 N.

    The magnitude of the maximum value for the static frictional force is 5.94.

    I do not know where to go from here.

    1. The problem statement, all variables and given/known data

    In about 1915, Henry Sincosky of Philadelphia suspended himself from a rafter by gripping the rafter with the thumb of each hand on one side and the fingers of the other side. Sincosky's mass was 79 kg. If the coefficient of static friction between hand and rafter was 0.65, what was the least magnitude of the normal force on the rafter from each thumb or opposite fingers? (After suspending himself, Sincosky chinned himself on the rafter and then moved hand-over-hand along the rafter. If you do not think Sincosky's grip was remarkable, try to repeat his stunt.)

    This problem is very easy and I know that it is easy. I just can't seem to get the correct answer.

    2. Relevant equations

    He is hanging. The gravitational force is 79kg(9.8m/s^2) = 774.2 N and is parallel to the surface he is gripping. [tex]\mu[/tex] = 0.65

    3. The attempt at a solution

    i did 774.2N = 0.65Fn. and got Fn = 1191 N which is incorrect.

    1. The problem statement, all variables and given/known data

    Playing near a road construction site, a child falls over a barrier and down onto a dirt slope that is angled downward at 30° to the horizontal. As the child slides down the slope, he has an acceleration that has a magnitude of 0.55 m/s2 up the slope. What is the coefficient of kinetic friction between the child and the slope?

    This one I don't know where to start.
  2. jcsd
  3. Oct 12, 2007 #2
    Don't need help anymore, figured them all out.
  4. Jan 21, 2008 #3
    I do not understand how can I solve the problem #2. I did drew free body diagram. It has gravitational force pointed down, normal force pointed up. Both forces are equal. Static fricition of 0.65 pointed up, am I right? Then, how can I find minimum normal force?
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