If you are referring to the equivalence principle, then the proportionality is implied when it is not stated. Also, the equivalence principle does not involve force. That's because [itex]F = m_i a[/itex]. However, [itex]F \neq m_g a[/itex].sodium.dioxid said:This is what I don't get. Why are we literally equating [a measurement of mass] to the object's [resistance in motion]? They are proportional. But why equal?