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Consider a block placed on a rough floor. Now, the reaction force is because it is equal and opposite to the weight  . Now apply a horizontal force  so that the block just begins to slide, i.e., the frictional force is equal to the limiting friction. When this condition is satisfied, the angle which the resultant (between the normal and external force) makes with the vertical is called the angle of friction.The tangent of the angle of friction is equal to the coefficient of static friction.