I think it important to keep in mind that the measure of energy is always
defined in terms of the CHANGE in the measure of energy.
(This leads to the possibility of the actual measure being negative as well as
positive. )
The law of conservation of energy takes the form:
The CHANGE of total energy of an isolated system = 0
Also written as:
Total energy of an isolated system at time 1 =
Total energy of an isolated system at time 2
To be avoided: PE = KE
Use instead: PE(1) + KE(1) = PE(2) + KE(2)
And if some of these are 0, WRITE THEM IN. eg if the reference configuration
for PE is 0 at position 2 , and KE at position 1 is 0, then write
PE(1) + 0 = 0 + KE(2)
(isolated = no energy exchanges across system boundary)