Frictional Force is mathematically defined as: Ff = μ*m*g*cos(θ) , where μ is the coefficient of friction, m is the mass of the object, g is the acceleration due to gravity and θ is the angle of the inclined plane. But in terms of direction, this makes no sense! Suppose there is an object on an inclined plane. It is going to travel down the plane because its Weight Force is pulling it towards the Earth. At the same time, a Frictional Force will act in the opposite direction to the motion of the object. The Frictional Force, however, is calculated using the object's Normal Force times cos(θ) (times μ), which acts perpendicular to the surface of the plane. But this means that the Normal Force is not acting in the same axis as the Frictional Force, so how can Normal Force times cos(θ) (times μ) be used to calculate Frictional Force? Let me elaborate: In order to calculate Normal Force in the same axis as Frictional Force, you should use μ*m*g*sin(θ), not μ*m*g*cos(θ). Would it not make more sense this way? I'm either very frustrated or very confused. Maybe a diagram with your answer can help. Thank you.