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Homework Help: Object on an incline with friction

  1. Sep 13, 2010 #1
    An object of mass 3 kg lies on a rough plane inclined at 45 degree to the horizontal. The coefficient of friction is 0.2 and a force of 10 N acts at 30 degree to the object. Calculate the friction. According to the booklet the answer is 3.24 N

    My work

    F = u R

    Normal reaction = 10 sin 30 + 30 cos 45 = 26.2

    U = 0.2 x 26.2 = 5.24 N

    I am confused HELPPPPPP.. where is the fault, is there something that i have not considered
  2. jcsd
  3. Sep 13, 2010 #2
    Re: Mechanics

    What is the 30 degrees measured relative to? Is it relative to the plane the block is sitting on or relative to the horizontal?
  4. Sep 13, 2010 #3
    Re: Mechanics

    It is relative to the block(object)
  5. Sep 13, 2010 #4
    Re: Mechanics

    So you have a block sitting on a slope. Is the block moving?
  6. Sep 13, 2010 #5
    Re: Mechanics

    Your normal reaction has a sign error. Also that cos 45 should be sin 45. I know numerically they are the same thing, but for what it actually represents, it should be sin.

    I'll give you a hint, think about what the normal force is conceptually, not mathematically
    Last edited: Sep 13, 2010
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