The force of friction depends on TWO key things: 1) N = normal force acting on object and 2) u = coefficient of friction (how "rough" the surface is):
F(fric) = force of friction = u*N
Assuming the coefficient of friction doesn't change, we can calculate the force of friction in the following way. Call the angle of the incline A:
N = m*g*cos(A)
Here m = mass of object and g= gravity = 9.81 m/s^2.
So the equation for the force of friction for an incline plane is:
F(fric) = u*m*g*cos(A)
The direction of the force depends on the direction the object is moving. Friction always OPPOSES the motion, so it will point in the OPPOSITE direction to the motion.
There's your answer! I hope that helps.
