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A rock is on an inclined surface. The rock is originally at rest, but starts to slide down the incline. The magnitude of the force on the surface due to the rock is F SR , and the magnitude of the force on the rock due to the surface is F RS . If we compare these forces, we find (A) F SR RS always. (B) F SR = F RS when the rock is at rest but then F SR > F RS . (C) F SR = F RS always. (D) F SR > F RS always.

A rock is on an inclined surface. The rock is originally at rest, but starts to slide down the     incline. The magnitude of the force on the surface due to the rock is FSR, and the magnitude of the force on the rock due to the surface is FRS. If we compare these forces, we find
(A) FSR RS always.
(B) FSR = FRS when the rock is at rest but then FSR > FRS.
(C) FSR = FRS always.
(D) FSR > FRS always.

Grade:upto college level

1 Answers

Navjyot Kalra
askIITians Faculty 654 Points
8 years ago
The correct option is (B) FSR = FRS when the rock is at rest but then FSR > FRS.
Consider the rock is on an inclined surface having an angle θ with ground. The magnitude of the force on the surface due to the rock is FSR, and the magnitude of the force on the rock due to the surface is FRS, which is shown in below figure.
Figure:
232-1432_1.PNG.
When the rock is at rest the two forces FSR and FRS will be equal to maintain balance. But when the rock starts to slide down the incline the tangential force FSR Sinθ will dominate upon the force on the rock due to the surface FRS. Therefore if we compare these forces, we find FSR = FRS when the rock is at rest but then FSR> FRS.
From the above observation we conclude that, option (B) is correct.

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