Sir, about speed limit as c, it’s because we have (v/c)^2 in that factor, so mass have to be infinite at c ,but sir what if we have something else as v’ in place of c, then will speed limit be change? Are there any reasons why won’t we take anything else than c in that factor?

Insightful!

What is rest mass energy?How could we access this energy?

A more interesting question is can you accelerate a particle of rest mass 0 to a speed beyond the speed of light?

Mass grows without bound as it approaches the speed of light.