We propose that inertia is the result of the gravitational force of induction of the entire Universe; that is, we understand that the force of inertia is the induction force exerted on a body by its accelerated movement with respect to the Universe, a statement that we call Mach's principle. We calculate this force by applying the weak field theory of General Relativity and using the results of electromagnetic theory. To avoid the divergence of the integral in calculating the induction force, we assume that in the Universe's early stages there was a strong gravitational absorption, which weakened as the cosmic density decreased. Admitting an ad hoc absorption function, we demonstrate Mach's principle, verifying that the inertial mass of a body is proportional to the gravitational mass with a proportionality coefficient dependent on cosmic time. Applying this result, we calculate the relative variation of the universal gravitation constant.