Question

Newton’s second law cannot be used when the particle’s speed approaches the speed of light, or if the size of the particle is extremely small (~ size of an atom). Why?

Answer 1

Newton's second law is only applicable if the mass of the object is constant. Since the mass changes when an object approaches the speed of light, Newton's second law can not be applied.

In the case of object size, Newton's second law has no restriction, since a particle is defined as a pointless object without size. But if we want to apply Newton's second law to an atom (even to an electron or a proton) it could not, since these particles have masses that tend to zero, and in this case Newton's second law would not be able to describe the physics involved.

F = m.a

if m tends to zero, the force F tends to zero. This behavior is not observed in nature.