Newton’s Second Law: The rate of change of linear momentum of a body is directly proportional to the external force applied on the body and this change takes place always in the direction of the applied force.

                                (K = 1 in C.G.S. and S.I. units)

∴

      Force = mass ´ acceleration

Important points about Second Law

(a)The Second Law is obviously consistent with the First Law as F = 0 Implies a = 0.

(b)The Second Law of motion is a vector law. It is actually a combination of three equations, one for each

component of the vectors :

This means that if a force is not parallel to the velocity of the body, but makes some angle with it, it changes only the component of velocity along the direction of force. The component of velocity normal to the force remains unchanged.

(b)The Second Law of motion given above is strictly applicable to a single point mass. The force F in the law stand for the net external force on the particle and a stands for the acceleration of the particle. Any internal forces in the system are not to be included in F.

c)The Second Law of motion is a local relation. What this means is that the force F at a point in space (location of the particle) at a certain instant of time is related to a at the same point at the same instant.

That is acceleration here and now is determined by the force here and now not by any history of the motion of the particle.