   Collision on an inclined plane
  I'm having trouble getting my head around this problem in a 2D side scroller:I have a wheel moving across a flat surface and it suddenly comes to an incline ( could be constant or curved like a hill).What happens to the forces acting on the wheel so that it actually begins to move up? I have a decent understand on why it slows down as you get further up the hill with the angle increasing and the vertical component in the opposite direction increasing. but i still dont understand how it moves up in the first place haha   
  There is a continuously moving kinematic constraint at the contact pt.A kinematic constraint means it moves in a completely defined way. often with a joint of some kind. You can imagine that there's a hinge joint between the circle and the incline at the contact point. As the object moves it can only rotate around that point. But as it rotates, the point moves, if you break that down into infinitely small rotation and movements, the circle will follow the incline.In a game world with discreet simulation  you can't break it into infinitely small steps. This often results in penetration. So the first step is to remove the penetration and make the circle rest on the incline again. The next step would be to modify the velocity of the circle so that it satisfies the incline constraint. That is, you must project the velocity of the circle to be along the incline plane.