Hi! I´ve been trying to follow the derivation for friction on an inclined plane in Tricks of the Windows Game Programming Gurus, but I´m afraid that there´s a little thing I don´t understand when the author implements the algorithm in pseudocode. In the while loop there´s a sentence that saids: x_vel += y_plane*gravity*(sin(angle) - frictionk * cos(angle)); When the author explained the x and y components of the force, provided that the object is not moving, he wrote: Fy = n - m*g*cos theta = 0 Fx = n - m*g*cos theta = 0 So he said that n = m*g*cos theta Then he replaced this expression on Fx to show the critical angle and said that: Fx = m*g*sin theta - (m*g*cos theta)*frictional_coefficient and provided that Fx = m*a, dividing the Fx term by ''m'' he gets: a = g*(sin theta - frictional_coefficient*cos theta) Up to this point I agree with him, but I don´t get why did he write the sentence I wrote at the beggining: y_vel += y_plane*gravity*(sin(angle) - frictionk * cos(angle)); Does the friction of the plane affects the y axis? Thanks!

Since you are on a sloping plane, sliding along the plane would modify both your x and y velocities.

Ok then, yeah, I think that makes sense! Thanks a lot!