• Card 33 / 36: You find experimentally that the force required to start sliding motion of a block of mass 20 kg on a horizontal surface is 12 N. If the same surface is inclined to a 30 degree slope with respect to gravity, what force (in addition to gravity) is required to initiate sliding motion of the object?
    A) 0 N
    B) 196 N
    C) 12 N
    D) 108 N
    E) 170 N

    Answer:
    D) 108 N

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Explanation:

One approach to the problem is to first calculate the coefficient of static friction = 12 N/(20 kg x 9.8 m/s2) = 0.06. When the surface is inclined at 30 degrees, the normal force of the surface on the object is reduced to cos(30)x 9.8 m/s2 x 20 kg = 170 N. Hence, the transverse force required to initiate sliding is 0.06 x 170 = 10.2 N. This figure is less than the transverse force due to gravitational acceleration = sin 30 x 196 = 98 N. Hence 98+10 = 108 N is required to start the block sliding up the incline.

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Mechanics I ME102

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Attribution:  Stephanie Redfern, Ranjeet (Ron) Agarwala, and Dr. Steve Gibbs. Mechanics I. The Saylor Academy 2014, http://www.saylor.org/courses/me102/
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