Newton's Second Law or the Law of Dynamics states that the force acting on an object is equal to the mass of the object multiplied by the acceleration it undergoes:
$$\vec{F}=m\vec{a}$$
where,
m— mass of the object
a— acceleration.
In the International System of Units (SI), the unit of force is the newton (N). One newton is the force that gives a mass of 1 kg an acceleration of 1 m/s2.
Newton's Second Law can also be expressed as:
$$\vec{F}=\frac {d\vec{p}}{dt}$$
where,
dp— change in momentum
dt— time interval for the change in momentum.
In this more general form, the law remains valid in special relativity.