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Homework Help: Need help with friction/plane probelm

  1. Dec 9, 2007 #1
    A block of mass (2kg) is on a plane whose angle of inclination is slowly increased from zero. The block begins to slide down the incline at an angle at 37 degrees.
    a) Calculate the magnitude and indicate the direction of the frictional force acting on the block when the angle is only 30 degrees.
    b) Calculate the coefficient of static friction between the block and the incline.
    c) At an angle of 40 degrees, the block slides 4m down the inlcine in 2 seconds with a constant acceleration. What is the coefficient of kinetic friction between the block and the incline.

    all i can think to do is mass(gravity)sin theta/ mass(gravitiy)cos theta = 0.577
    really need some help here
  2. jcsd
  3. Dec 10, 2007 #2

    Shooting Star

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    Remember, for static friction, F is less than or equal to (ks)*N, where ks is the co-eff of static friction. Also, the component of weight along the plane is mgsin(theta), and the normal reaction due to weight is (ks)mgcos(theta) or (ku)mgcos(theta).

    a) At 30 degs, the block is not moving, and so the grav force along the plane must be balanced by the force of friction, and both must be equal.

    b) When the block is just about to move, frctional force F= (ks)*N.

    c) At 40 degs, the block is moving, and the total force along the plane is mgsin(theta) - F_kinetic. Accn = total force/mass.
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