A 20.0 kg block is connected to a 30.0 kg block by a string that passes over a light frictionless pulley. The 30.0 kg block is connected to a spring that has negligible mass and a force constant of 250 N/m, as shown in the figure. The spring is unstretched when the system is as shown in the figure, and the incline is frictionless. The 20.0 kg block is pulled 20.0 cm down the incline (so that the 30.0 kg block is 40.0 cm above the floor) and released from rest. Find the speed of each block when the 30.0 kg block is 20.0 cm above the floor (that is, when the spring is unstretched). Figure: http://uploadplus.com/quickupload/49.bmp The solution says to take the lowest point reached on the ramp by the 20.0 kg block as Ug = 0. Then, it takes Ug of the 30.0 kg block to be based on +20.0 cm... I don't understand this. Isn't the y-distance from the lowest point on the ramp of the 20.0 kg block to the highest point of the 30.0 kg block *not* 20.0 cm?