 Further problems can arise, however, because of another \(T\)-theoretical term that is invoked implicitly when \(\bF=m\ba\) is asserted. The accelerations \(\ba\) are tacitly assumed to be measured in relation to an inertial system. If the acceleration is measured in relation to a different reference system, a different result is obtained. For example, if it is measured in relation to a system moving with uniform acceleration \(\ba\), then the measured acceleration will be \(\ba' = (\ba - \ba)\). A body not acted on by gravitational forces in an inertial frame will obey \(0=m\ba\) so that \(\ba=0\). The same body in the accelerated frame will have acceleration \(\ba' = -\ba\) and be governed by \(-m\ba = m\ba'\). The problem is that the term \(-m\ba\) behaves just like a gravitational force; its magnitude is directly proportional to the mass \(m\) of the body. So the case of a gravitation free body in a uniformly accelerated reference system is indistinguishable from a body in free fall in a homogeneous gravitational field. A theoretical underdetermination threatens once again. Given just the motions how are we to know which case is presented to  us?[1]  Resolving these problems requires a systematic study of the relations between the various \(T\)-theoretical concepts, inertial mass, gravitational mass, inertial force, gravitational force, inertial systems and accelerated systems and how they figure in the relevant laws of the theory \(T\).