Newton's Discovery of Mass, Force & G in SI Units

[adjacent]

What did Newton use to determine the mass of an object(During those days)
Even the SI unit for mass was established much time later.
The SI Unit for force was also established much later.
The G in F=GMm/d² was discovered in Much later.

How did Newton knew that a force of 1N would accelerate a mass of 1KG to 1m/s²?

 

[gmax137]

There are numerous threads here discussing what Newton actually wrote (try searching on "Principia" and maybe you can find these discussions).

As I understand it, Newton expressed the idea in terms of proportionality: twice the force causes twice the change in momentum.

The modern concept of units probably wasn't really developed in Newton's time.

 

[tiny-tim]

the unit was named after Newton!

(and he used pounds and feet, anyway)​

 

[technician]

You do not need to know the units to determine the laws of physical behaviour. You need to know how to 'double' the mass and to have a means of measuring force and to know how to 'double' a force.
Both of the responses above have something sensible to say and I would say the key thing is the idea of proportionality.