why value of k in f=kma is considered as 1 or rigid why it not changes

Newton's second law is given by,

 

F = kma

 

Here, k is a praportionality constant.

 

Now, the SI unit of force is defined as,

 

1N Force = Force that causes an acceleration of 1 m/s² to a mass 1 kg

 

1 N = 1 kg.m/s²

 

This defination of 1 N force gives k =1 such that,

 

F = ma

 

Thus, for simplysity k is defined as 1.

 

Physicits consider that, system of units in which k has to be taken differnt form 1 is not consistant and it makes the calculations very difficult.