Newton's second law is Newton's second law of motion.
The magnitude of the acceleration of an object is directly proportional to the force acting on the object and inversely proportional to the mass of the object, and the direction of the acceleration is the same as that of the resultant force. From the physical point of view, Newton's second law of motion can also be expressed as "the rate of momentum change with time is proportional to the sum of external forces", that is, the first derivative of momentum with time is equal to the sum of external forces. Newton's second law shows that at macro low speed, the proportional expressions are: a∝F/m, f ∝ ma; The mathematical expression can be written as F=kma, where k is a proportional coefficient and a constant. However, since the force was not designated as the unit of force at that time, the selection of the proportional coefficient k was arbitrary. If k= 1, there will be F=ma, which is the mathematical expression of Newton's second law we are familiar with today.