試題詳解

2. Suppose a block is placed on a rough surface inclined relative to the horizontal and the coefficient of static friction between the block and the rough surface is μS. Let the incline angle be increased from θ = 0 until the block starts to move. What is the critical angle θ_c at which this slipping just occurs?
(A) θ_c= arctan(μ_S ) 
(B)θ_c= arccos(μ_S )
(C)θ_c=arcsin(μ_S ) 
(D)θ_c =π/2
(E) None of the above.