1. The problem statement, all variables and given/known data Here, have a diagram I made! The problem is to find the moment about the midpoint of the bar when it is horizontal, given that the right end is moving in towards the frictionless slot at a velocity v. Gravity and friction are neglected in this system. If you want, assume you're looking down on this problem from above, and it is happening on a table. 2. Relevant equations Force is the time derivative of momentum. Moment is time derivative of angular momentum. 3. The attempt at a solution My problem stems from trying to figure out the geometry of this. I am not sure how to even approach this. See, if the slot was vertical instead of inclined, this would feel like a much simpler problem to me. I know that what I want to do is find an equation for angular momentum about the midpoint. This requires finding the x and y velocity of the midpoint, as well as an equation for the rate of change of the angle of the bar. I just do not know what to do about the geometry. Do I use the law of sines? Something is not clicking here.