This video takes a look at Newton’s law of gravity that is written in the form F=GMm/r² and points out that although the law is referred to as a ‘universal’ law of gravity, it does not hold for very strong gravitational forces involving very large masses (where the General Theory of Relativity needs to be used). He also points out that the law has not been tested very precisely in cases where the force is very weak, such as with small masses, but that we assume it holds true in that regime.

I would quibble with his criticisms of the use of the word universal. I always assumed that ‘universal’ did not mean that it held true for all masses and all situations but that the nature of the mass did not matter, that the force of gravity depended only on the property of mass and not on any other thing. i.e., the force between two oranges is the same as the force between two apples, as long as the masses remained unchanged.

A more serious disagreement I have is with his saying that it is ‘crazy’ to think that the law applies in regions where it has not been tested, such as for very weak gravity. It is not crazy, because that is how science works. Once a scientific law is arrived at, it is assumed to be hold in all situations until contradictory evidence turns up that requires us to place limits on its range of applicability. So, for example, parity conservation was assumed to be a universal law until a parity violating reaction was found and placed limitations on it. This does not mean that people who had earlier assumed parity conservation was always true were doing something wrong or crazy. Scientists will try to increase the range of conditions under which a law has been tested but in the absence of any hints that there is a problem, will assume that the law is always valid. (Another plug: My forthcoming book THE GREAT PARADOX OF SCIENCE: Why its conclusions can be relied upon even though they cannot be proven looks into this question of the nature of scientific practice more deeply.)