1. The problem statement, all variables and given/known data A long light inflexible rod is free to rotate in a vertical plane about a fixed point O. A particle of mass m is fixed to the rod at a point P a distance l from O. A second particle of mass m is free to move along the rod, and is attracted to the point O by an elastic force of strength k. The system is subject to gravity g. The constant k is given by k = mg/2l. State the degrees of freedom and choose appropriate coordinates 2. Relevant equations None 3. The attempt at a solution I believe the degree of freedom is 2 with coordinates being θ: the angle of the rod and the distance of the second particle from the origin O. Although I am stuck on which coordinate I should use for that distance Thanks!