Question

# Block A of mass 35 kg is resting on a frictionless floor. Another block B of mass 7 kg is resting on it as shown in the figure. The coefficient of static friction between the blocks is 0.5 while kinetic friction is 0.4. If a force of 100 N is applied to block B, the acceleration of the block A will be (g=10 ms−2):

A
0.8 ms2
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B
2.4 ms2
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C
0.4 ms2
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D
4.4 ms2
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Solution

## The correct option is A 0.8 ms−2Maximum force for which both blocks move together is Fmax=m1m2μs(m1+m2)g Fmax=7350.5(42)10=42 N Given F=100 N Hence relative-slipping occurs between A and B Only force acting on block B is kinetic friction fk=μk N=0.4[70]=28 N Hence acceleration of the 35 kg block a=2835=45=0.8 ms−2

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