As with classical mechanics, the subject can be divided into " kinematics "; the description of motion by specifying positions, velocities and accelerations, and " dynamics "; a full description by considering energies, momenta, and angular momenta and their conservation laws, and forces acting on particles or exerted by particles. There is however a subtlety; what appears to be "moving" and what is "at rest"—which is termed by " statics " in classical mechanics—depends on the relative motion of observers who measure in frames of reference.