Near the surface of the Earth, block A, with mass mA, is initially at rest on a

Question

Near the surface of the Earth, block A, with mass mA, is initially at rest on a horizontal floor. Block B, with mass mB, is initially at rest on the horizontal top surface of A. The sum of the two masses is mA + mB = 10 kg. Block A is pulled with a horizontal force F as shown in the figure. If the coefficient of static friction between the two blocks is ?s = 0.8, and if block B begins to slide off block A when F is greater than 98 N, what is the kinetic coefficient of friction, ?k, between block A and the floor?

answer is .2

Explanation / Answer

normal reaction = 10g

friction = uk10g

acceleration = 98 - uk98 / 10

mb* acceleration = usmbg (SINCE IT JUST SLIPS FRICTION = MAX STATIC FRICTION)

98(1-uk) / 10 = 0.8*9.8

1-uk = 0.8

uk = 0.2

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