Laws of Motion 3 Question 9

9. A block of mass m is on an inclined plane of angle θ. The coefficient of friction between the block and the plane is μ and tanθ>μ. The block is held stationary by applying a force P parallel to the plane. The direction of force pointing up the plane is taken to be positive. As P is varied from P1=mg(sinθμcosθ) to P2=mg(sinθ+μcosθ), the frictional force f versus P graph will look like

(2010) (a)

(c)

(b)

(d)

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Answer:

Correct Answer: 9. (a)

Solution:

  1. When

P=mg(sinθμcosθ)f=μmgcosθ when P=mgsinθ;f=0 and when P=mg(sinθ+μcosθ)f=μmgcosθ

(downwards)

Hence, friction is first positive, then zero and then negative.



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