Let's say an object is sliding on a slope and is the object has a velocity of (0,0,5). The friction would be acting in the opposite direction of motiong, being (0,0,-1).
However, gravity is also affecting the object on the slope. The gravity is doing an impulse of (-1,-1,0) every frame in the direction of the slope tangent. Should the object also experience friction in the direction of (1,1,0)? Or would it only experience friction in the direction of (0,0,-1)?
I find the notion that friction always opposes the direction of motion strange, because this means that when an object is sliding downwards slightly on a slope, but not much, due to friction acting in opposite direction of gravity, when you add an impulse in a perpendicular direction, suddenly the object will be sliding downwards much more because friction is no longer resisting gravity. Is that correct?