1. The problem statement, all variables and given/known data There is a frictionless incline with a "block" mass on it. The incline and the mass are able to move. The mass of the block is m and the mass of the incline is M. I am supposed to find a suitable set of generalized coordinates and write the Lagrangian of the system, then from that, determine equations of motion. 2. The attempt at a solution I chose coordinates in a cartesian frame to describe the position of the block mass. (x, y) From this, I constrained the incline so that it is "in contact" with the block mass at all times (assuming the block to be a point mass): (x2,y2) = (x1 + y1/tan(θ), 0) where this is measured from the end of the inclination at some x and y = 0. θ is the angle from the horizontal. I then constructed the Lagrange function: L = (1/2)m((d/dt x)^2 + (d/dt y)^2) + (1/2)M(d/dt(x1 + y1/tan(θ)))^2 - mgy where the last term is the potential of the block measured from the zero potential at y = 0. The inclination should have no potential term as it never moves along the direction of the conservative field, gravity (its height is constant.) I computed the equations of motion, and they are incorrect. What am I doing wrong? Given (x, y) of the block mass, the position of the incline is fully defined and so it seems I have fully described the system. Any help is appreciated, this problem is driving me nuts!