The diagram above illustrates a ball of mass m sliding down a plank inclined at an angle of θ to the horizontal. The kinetic friction between the ball and the plank is F and acceleration of free fall is g
The normal force on the ball is
The normal force is the force that extends perpendicular (90o) to the surface.
W is the weight of the ball and Fn is the normal force.
From the above diagram, resolving W to the the Fn (extended or broken line), yields: mgcosθ
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