your physics book has it right, that website has it wrong.
as to the every living thing made of cell thoery--we didn't see cells until the microscope was invented, and then, it was a little while after that. thirty, fourty years, I think? anyrate, microscope in 1500s, cells found early 1600s.
the theory was made a little bit later (1700s or late 1600s) that all plants, and all animals were made of cells, based off of the fact that everything observed so far had cells.
It was two theories at first, (one for animal, one for plant) but got merged with a couple of other theories into the definition of Life. which today is challenged by the existance of viruses, and what the first life was like. So the definition is still a theory.
A theory never does become a law--in fact, laws are explanations of the facts, and explain them--just like a theory. The laws of motion are also theories of motion.
The confusion stems from the fact that at one point, Law was used to explain phenomenon, and the convention stuck. What was a law, like the law of gravity, is still called a law, even though it's technically a scientific theory.
The round earth theory still only explains why we see the earth as round. It is an interpretation based off of the evidence, and if a new interpretation that was better, or if there appeared evidence that suggests a nonround earth, it will replace the round earth theory. Yep, it's still not a fact that the earth is round. wierd, huh?
All a man's knowledge comes from his experiences