Question

What are inertial frames? How has it been established that Earth is not an inertial frame?

Answer 1

An inertial frame is a frame relative to which all velocities and accelerations are considered, i.e, the velocity and acceleration of the inertial frame is taken as zero.

But it's all a relative matter, even the Earth which is considered as the inertial frame as far as Mechanics problems are involved, is revolving the sun, which in turn supposedly revolves around the massive black hole at the center of our Galaxy, which in turn might be revolving an object we don't yet know of.