This isn't actually a problem. It is a doubt which I have about wedges according to Newton's Laws Consider a friction less wedge, of mass 'M', inclined at angle θ, placed on a smooth horizontal surface, on which a block of mass 'm' is placed. The block exerts some normal force perpendicular to the surface of the wedge, which can be resolved into two axes such that one is parallel to the ground. Now, after resolving we will have an horizontal component of force F=mgsinθ, which makes the wedge to move in that direction with an acceleration of a=mgsinθ/M. And since the wedge is friction less, the block moves down too. This is what I've been taught by my teacher. And My doubt here is that, the normal force is actually due to gravity(in this case), which has a vertical component of mg, then if we resolve the normal force according to the above method, then we will only have null vector in the horizontal direction, that is mgsinθ=0. Then, how does the wedge move?(I can certainly understand that if we actually apply a force on the block, besides gravity, then we may induce a force with a horizontal component, in which case the block will move) In a nutshell, if the normal force itself is caused by only gravity, which has only a vertical component, then how is it possible to have a horizontal component, and alas, if there isn't no horizontal component, why should the wedge move?