# Page:The Meaning of Relativity - Albert Einstein (1922).djvu/75

 ${\displaystyle {\text{(Inert mass)}}\cdot {\text{(Acceleration)}}={\text{(Intensity of the gravitational field)}}\cdot {\text{(Gravitational mass)}}.}$
It is only when there is numerical equality between the inert and gravitational mass that the acceleration is independent of the nature of the body. Let now ${\displaystyle K}$ be an inertial system. Masses which are sufficiently far from each other and from other bodies are then, with respect to ${\displaystyle K}$, free from acceleration. We shall also refer these masses to a system of co-ordinates ${\displaystyle K'}$, uniformly accelerated with respect to ${\displaystyle K}$. Relatively to ${\displaystyle K'}$ all the masses have equal and parallel accelerations; with respect to ${\displaystyle K'}$ they behave just as if a gravitational field were present and