State Newton's second law for a system of particles.

(N/A) For a system of particles,Newton's second law states that the total external force acting on the system is equal to the rate of change of the total linear momentum of the system.
Mathematically,it is expressed as: $\vec{F}_{ext} = \frac{d\vec{P}}{dt}$,where $\vec{F}_{ext}$ is the sum of all external forces acting on the system and $\vec{P}$ is the total linear momentum of the system.
Since the total linear momentum $\vec{P}$ is equal to $M\vec{v}_{cm}$ (where $M$ is the total mass and $\vec{v}_{cm}$ is the velocity of the center of mass),the law can also be written as: $\vec{F}_{ext} = M\vec{a}_{cm}$,where $\vec{a}_{cm}$ is the acceleration of the center of mass of the system.