Question

under what condition does Newton's second law of motion take the firm F=ma

Answer 1

on the following conditions-
1)when mass of the particle remains constant.
2)when the velocity of the particle is of order 10^6 or less than that because at such velocity the change of mass to energy according to equation E=mc^2 is very very negligible.

Answer 2

When mass is constant then newtoms second law of motiom is applied.Nowadays mass is not considered as constant , it can be converted in to energy by einstein equation E= mc2