In an oblique collision my understanding is that linear momentum is conserved in all directions (x, y, normal, tangential). But in a constrained oblique collision, does this change? For example if we had a block lying between two frictionless surfaces with an angled face ( a slope on one face) which a ball going horizontally hits and bounces, the block can only move horizontally but given the normal tangential components, it technically has a velocity component in the normal and tangential directions. However, since the ball hitting the surface shouldn't have an affected tangential velocity component because there is no force of deformation or restoration in the tangential direction, doesn't this mean tangential momentum is not conserved? The block suddenly gained tangential velocity while the ball preserved its own? It would seem that linear momentum horizontally is conserved however. Edit: After going through this, I see that there must be a force acting on the system if my assumptions are indeed correct. Does this mean that because the sides of the constraint act on the block to prevent movement, linear momentum in the tangential direction is not conserved, and if so, how can one actually explain what happens?