When we first began formulating physical laws, we did so empirically: through experiments. Drop a ball off a tower, like Galileo may have done, and you can measure both how far it falls and how long it takes to hit the ground. Release a pendulum, and you can find a relationship between the pendulum’s length and the amount of time it takes to oscillate. If you do this for a number of distances, lengths, and times, you’ll see a relationship emerge: the distance of a falling object is proportional to the time squared; the period of a pendulum is proportional to the square root of the pendulum’s length.
But to turn those proportionalities into an equal sign, you need to get that constant right.