What scientific law or principle allows us to
say that all the PE is converted to KE?

The law of conservation of energy

Energy cannot be created or destroyed, but it can be changed from one form to another.

KE + PE = constant

the order for strong to weak forces such as

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force

the fundamental forces are those forces that act between to objects or element to make interactions. there are four types of fundamental forces such as strong and weak nuclear forces (due to protons and electrons), electromagnetic force (due to the motion of electrons) and gravitational force (due to orbital movement of the planet). if these forces are arranged on the basis of their maximum strength then the order is

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force

continuous conducting path for current, power supply