Einstein’s postulates also force us to reconsider meaning of mass
Newtonian (Classical) Physics: Mass is an absolute quantity for each object in Newton’s laws (i.e. it is conserved and it never changes for each object). This is a central idea for Newton used in 2nd law: F = ma In Newton’s time (and in our everyday experience), it seems to be verified that mass never changes.
Einstein: Mass is what we measure it to be. We must define mass by an operational measurement. Einstein did “Gedanken” experiments (which were later supported by real experiments) that show that the apparent mass of of an object depends on how fast the object is moving with respect to us.