Newton's second law says that :
The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts. Mathematically it says,
(∆p/∆t) ∝ F
$$\bbox[5px,border:1px solid black] { \frac{\Delta p}{\Delta t}\propto F } $$
Before I ask my question let me mention something:
One quantity A is said to proportional to another B, when the two quantities depend upon each other in such a manner that if B is changed, A is changed in the same ratio. Therefore, establishment of a proportionality relation beforehand requires the quantitative measurements of both A and B.
here comes my question i.e. in order to establish such a relation (∆p/∆t) ∝ F it requires quantitative measurements of both force and rate of change of momentum and as for the latter one can easily calculate the numbers but how come can someone calculate force ? Hence, how can one establish such a relation without having numbers for the force?
NOTE 1 : I'm considering the motion to be in right line(straight line) since for other cases one only requires to introduce vectors and the basic framework of the law remains the same.
NOTE 2 : For those who might think that how will I measure mass? It's quite simple. One can use a balance to measure the relative mass(as it is done in actual) but you might say hey,wait a minute we weigh stuff with a balance! so aren't we calculating the weight rather than mass and my answer would be NO!. Think about it once again. By using a balance you are making a relative measurement which is independent of the strength of the gravity so weather you are on moon, mars etc the balance weights will change by the same factor as the object you are measuring and so if you have some stuff in pan on one side of the balance and it requires(on earth) say 5 pieces of 1 kg(in other pan) to make pointer come to middle the same is true if u are on moon,mars etc.
