The acceleration of an object is proportional to the force exerted on it and inversely proportional to the mass of the object. The direction of acceleration is the same as that of the external force. From the point of view of physics, the second law of Newton's motion can also be expressed as "the rate of change of momentum of an object with time is proportional to the sum of external forces", that is, the first derivative of momentum to time is equal to the sum of external forces.
Newton's second law shows that under macroscopic low speed, the proportional expression is: a∝F/m, F∝ma; F = kma can be written by mathematical expression, where k is the proportional coefficient and is a constant. But because there was no stipulation on how much force was the unit of force at that time, the selection of the proportional coefficient K would be arbitrary. If k = 1, there would be F = ma. This is the mathematical expression of Newton's second law, which we are familiar with today.
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