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- How did Newton discover his second law? 10 answers
I have been told always that $F$ is directly proportional to acceleration.
My question is that for finding such a relationship there should be source that produces desired force and in which the force can be adjusted i.e. Twice, thrice and more. But the problem is initially before the discovery of laws of motion how can one say that a force is twice the other, how can he even judge the relationship between two forces without knowing the quantitative definition of force?