My professor said that a law was stated and announced as a law because it happens in our everyday life. He gave us an example of Newton's 3 laws. He said that walking possess 3 laws of Newton's. Is it true that all laws really happen in our daily motion?
closed as not constructive by Brandon Enright, Ben Crowell, dmckee♦ May 31 '13 at 4:44
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There is no agreed upon definition of "law," or even "theory" in the sciences - including in physics. A better term which describes what scientists actually come up with is "model." A model is a description, usually mathematical, that relates quantities or behaviours that are seen in the world. For example, you will see mathematical formulas relating the acceleration of bodies to the forces impressed upon them, and formulas relating the forces to other quantities like mass or charge and so on. In principle these formulas allow you to compute what is going to happen in any given situation, and if the prediction is right then your model survives. The formulas are not "laws" in the sense that they have some independent existence: they only make sense within the larger model of which they are a part - in this case Newtonian mechanics. Nature doesn't sit down and ask "now how can I obey the laws now" and work out what to do next with a pen and paper like we physicists do. No, nature just does what it does. We try to describe and predict it using models.
Often a model starts with a simple observation, but as more and more observations are brought together the model becomes more general and more powerful, capable of describing more phenomenena. When new things are observed which cannot be incorporated into the model a new model is formed. The hard part is that the new model must still be capable of explaining the old things. It has to explain why the old model worked so well for the phenomena that were seen before. For example, we now know that Newtonian mechanics is an incorrect picture of the world. It works very well in everyday life, but it cannot describe things which are moving very fast or which are very small. For these you need the theory of relativity (an unfortunate name that stuck) and quantum mechanics, respectively. Or a combination of the two (quantum field theory) if things are both very fast and very small. At the moment quantum field theory is the most general framework in which we know how to do physics. But you can recover Newtonian mechanics as an excellent approximation in ordinary situations that you will face every day.
This is the sense in which "laws" apply to the real world. All scientific knowledge is provisional. Nature just is what it is, and does what it does, but we try to describe it. The remarkable thing about physics is that we can describe nature quantitatively to great accuracy. New discoveries may be made which require a new understanding of the world, but the new understanding will always be one that subsumes, or explains, why the old understanding worked well in the past. Newtonian mechanics is still very useful. I wouldn't want my bridge designers to spend a lot of time learning quantum field theory - I'd want them to spend a lot of time mastering the practical application of Newtonian mechanics (that is, applying the formulas well and speedily). But in the end all of our laws, theories or models are at best approximate quantitative and provisional descriptions of nature.
Opinion: A true "law" in physics is roughly equivalent to a theorem in Mathematics. Usually to become a law there needs to be some sort of theoretical or mathematical basis for it as well as be validated experimentally.
Answer (counter example): A physical law like "nothing can travel faster than the speed of light" is not something we encounter at all in our daily lives.