The Newton’s second law usually is expressed as \({\varvec{F}}=ma\) and as if the mass remains constant as: \({\varvec{F}}=md\left({\varvec{v}}\right)/dt\) . From this we can obtain another way of expressing this law that is supremely useful in the study of vehicular collisions. Integrating it between times t1 and t2 and rearranging we obtain: \(m{{\varvec{v}}}_{1}+{\int }_{{t}_{1}}^{{t}_{2}}{\varvec{F}}dt=m{{\varvec{v}}}_{2}\) . This equation is extremely important in collision dynamics and is known as the “Principle of Impulse and amount of Movement” and in words, it simply says that the impulse of a force is equal to the change in the amount of movement of the body on which it acts. Both quantities are vectors.