1. The problem statement, all variables and given/known data What is the definition of p_Θ? 2. Relevant equations I search all over the web using the keyword linear "momentum polar coordinates", "lagrangian in polar coordinates", "hamiltonian in polar coordinates", "momentum in 2D polar coordinates" for about half an hour. but i fail to find it. 3. The attempt at a solution I think it is either: 1. p_Θ = m*(r*d/dt(θ))^2 2. p_Θ = m*(d/dt(θ))^2 which one is correct? Thank you!