Where do Newton's Laws not work? I'm working on high school level project about Newton's Laws and I picked topic that describes situations, where they dont work. Can you name any practical cases where they do not work? Why do they not work in special and general relativity or quantum mechanics? Why do they not work on very light things (atoms) or super heavy (black holes) or super fast particles in accelerator?
Thanks for any advice!
 A: A quick search and it dows not look like this has been asked before.
For a high school project I suggest that you might think about atomic physics. The way the negatively charged electrons travel around the positively charged nucleus.
I suggest this for two reasons.
1) It is a clear case where Newton's laws do not work.
2) There are simple experiments that you could try to get evidence for it
Newton's laws do not work on the small scale of atoms and molecules and we need to use quantum mechanics. For example electrons, as you may already know, are organized in shells in atoms; 1s, 2s and 2p, and so on which all have different energies.
The experiments you could try would be to look at the spectrum of light from a street light or a fluorescent (strip) light. If you can observe the spectrum with a prism, for example, you should find bright lines in the spectrum. These bright lines are due to electrons moving from one energy level to another. The wavelength (or colour) of the light depends on the ammount of energy emitted when an electron drops from a higher level to a lower level.
It is possible to make a simple spectrometer using an old CD or DVD as a diffraction grating. The link has some advice about how to do this in practice and examples.
So you could build a spectrometer to observe transitions between different states, which are not predicted by newtonian mechanics.
The question of why Newtonian Mechanics does not work at small scales and why we need to use quantum mechanics is not straightforward to answer. One way of answering it is to say that experiments have found that Newtonian Mechanics don't work for atoms and it turns out that quantum mechanics makes a good job of predicting energies. Another answer could be based on the quantum mechanical example of a particle in a 1 dimensional box because in this case for an electron in a box the size of an atom there are large energy gaps between different states that the electron can fit in, but if the box is larger - for example the size of a cup - the levels are so close together that there is effectively a continuous band of states the electron can fit in so it can have almost any energy, which is like Newtonian mechanics.
A: *

*Cartoon physics. For example, after running off a cliff, Wile E. Coyote does not begin to fall until he notices. 

*The thing about many physical "laws" is that they are very good generalities, often with exceptions. For example.


*

*You can prove that the angles of a triangle always add up to 180-deg, except for the unwritten exception, that it doesn't work for spherical geometry.

*The Second law of thermodynamics is well known, but in 2002, the BBC reported that "experiment involving lasers and microscopic beads that disobeys the so-called Second Law of Thermodynamics"

*The law of conservation of energy was put into question when Henri Becquerel discovered uranium salts that emitted radiation.


A: If you are a high school student, I'd say Newton's laws do not work in non-inertial reference frames, where objects start to move without any physical force (this is already general relativity, if you want). Of course, if you are curious about quantum mechanics, you should go for a book, I would say. The short answer could be: in quantum mechanics, there is nothing called point-mass, particle or rigid-body etc.; so Newton's laws are talking about non-existant objects.
A: Also, the third law of Newton is not correct in the most rigorous sense; since information cannot travel at speed greater than the speed of light, it is impossible for two forces to act on different objects simultaneously.
