Match the following formulas about calculating friction: a) Always works. FN can be found by summing all the y-dir forces, recognizing that the acceleration in the y-dir is (probably) zero, and solve for FN b) Works whenever the surface is horizontal and there are no y-direction forces except for gravity and the normal force c) Works whenever the surface is an inclined plane and there are no y-direction forces except for gravity and the normal force 1. Ff = mg 2. Ff = mg 3. Ff = FN