The assumptions with observations are very tricky in not being recognized as assumptions at all. Logic over Perception. There is a tendency to put perception above logic, as if perception were proven fact. Supposedly, seeing is believing. But perception is the basis of deception and illusion.
Logic is the basis of knowledge.
The purpose of science is to add logic to a subject. There are numerous problems in the logic of mv being energy, as explained later. Contradictions often stem from superficial assumptions being in error, such as levers not conserving energy as momentum. To correct the assumptions or errors, the more basic points must take precedent over less basic points. For example, force amplification is not understood well enough to draw conclusions about energy being transferred by levers.