Hi, my question in full; Do DIFFERENT observers (in different INERTIAL reference frames) see a DIFFERENT force (Magnitude specifically) acting on an accelerating body? I beleive yes . Because... : ƩF = d(p)/d(t) = m*a + v * d(m)/d(t) where m = m0/√(1-v^2/c^2) where v is different for every observer in a different inertial reference frame? Hence, people in different reference frames 'see' a different force Hence they also record different measurements for acceleration of the body? or does the acceleartion always agree and they record different measurements for the mass of the body w.r. to their own reference frame? How correct is all this? Thank you in advance.