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 independent variable, the one that is intentionally manipulated, is whether or not the car hits the barrier.

the dependent variable, the one that you measure the response in, is the distance that the washers travel.

would be false hope this out