Newton's laws of motion are based on a theory. In the case of the 2nd kinetic formula, the theory can be summarized as a formula. The formula is - the rate of change of momentum of an object is proportional to the force applied to it and the force of action of the force wherever the force changes. The mathematical form of this formula is , F = ma . That is, in this case, the relevant theory can be expressed in a concise and mathematical form, so F = ma is the formula.