F = m a


F     is the “force”

m    is the “inertia” of “mass”

a     is the “acceleration”


The acceleration can be measured in terms of lengths and times (change in speed per unit time) such as  (meters/second)/second


Force has no independent meaning outside this equation.


Masses are determined relative to each other in collisions, hence their absolute value involves an arbitrary constant.


Thus the equation      F = m a


Could equally well have been written by Newton as


                                  (Fc2) = (mc2 ) a


                                    f     =       M a


Thus  E = mc2 is like converting an absolute temperature from Fahrenheit to Celsius degrees –

all are measures of aspects of the energy of a system.