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A block of mass M slides down on a rough inclined plane with constant velocity. The angle made by the incline with horizontal is θ. The magnitude of the contact force will be-

1. Mg
2. Mgcosθ
3. √(Mg sinθ + Mg cosθ)
4. Mg sinθ √(1+μ)

Correct answer: Mg

Solution

The contact force is the resultant of the normal reaction and friction. Since the block moves at constant velocity, all forces balance, so the contact force equals and opposes the weight: magnitude = Mg.

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