For a photon, it is the energy divided by the speed of light, while Newton's second law states that force equals its time derivative.  
The canonical form of this is the partial derivative of the Lagrangian with respect to the generalized velocity.  For a photon, it is the energy divided by the speed of light, while Newton's second law states that force equals its time derivative.  For ten points, what quantity is sometimes written as the product of the mass times the velocity?
The canonical form of this is the partial derivative of the Lagrangian with respect to the generalized velocity.  
momentum