Question

where Newtonian mechanics get failed and why?​

Answer 1

Answer:

The main failures are two:

In reality momentum is not proportional to velocity, but goes as p=γm0v , where m0 is the (rest) mass and γ(v)=1/1−v2/c2−−−−−−−−√ is the Lorentz factor.

In reality, kinetic energy is not proportional to velocity squared, but goes as E=(γ−1)m0c2 .