(i) A particle continues to be at rest or moving with constant velocity if the total force acting on it is zero.

(ii) The rate of change of linear momentum of a particle is proportional to the total force acting on the particle. The linear momentum p of a particle is p=mv, where m is the mass and v is the velocity. So dp/dt =  m(dv/dt) =  ma, where a is the acceleration. Thus, the second law is usually stated as F =  ma, where F is the total force acting on the particle.

(iii) Whenever a particle exerts a force on a second particle, the second particle exerts an equal and opposite force on the first. This is often phrased as ‘To every action there is an equal and opposite reaction.