The acceleration magnitude of the object is proportional to the force on the object and inversely proportional to the mass of the object, and the acceleration direction is the same as the direction of the external force. From the point of view of physics, Newton's second law of motion can also be expressed as "the rate of change of momentum of a body with time is proportional to the sum of the external forces", that is, the number of the first derivative of momentum to time is tantamount to the sum of the external forces.

Newton's second law explained that at a low macro speed, the proportional expression is: a∝F/m, F∝ma; The math expression can be written as F=kma, where k is the proportional number and is one constant number. However, since there was no rule on how much force to act as a unit, the proportional system number k was arbitrarily chosen, and if k=1, F=ma, which is known today as Newton's second law math expression.