Newton's Third Law equal and opposite forces

Thread starter protonman
Start date Mar 12, 2004
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Forces Law Newton's third law Third law

AI Thread Summary

Newton's Third Law states that for every action, there is an equal and opposite reaction, but this does not imply a cause-and-effect relationship; the forces arise and disappear simultaneously. In a scenario where a washer is spun by a string, the forces acting on the washer and string are equal and opposite, yet when the string is cut, the forces cease to exist at the same moment. Discussions highlight that information about the string being cut does not propagate instantaneously, as it travels at the speed of sound through the medium. The debate also touches on the limitations of Newton's laws in real-world applications, emphasizing that forces do not act instantaneously and that momentum conservation is upheld. Ultimately, the conversation reveals a misunderstanding of the nature of force interactions as described by Newton's laws in the context of relativity.

Mar 25, 2004

JohnDubYa

There is probably a better example of Newton's Third Law being violated by classical particles.

Consider two protons. One is traveling North, the other traveling West. Both travel along the same plane.

At some point their paths will cross. Let us suppose that Proton A reaches the crossing point first. When Proton B reaches the crossing point, it will exert a magnetic force on Proton A. But Proton A will not exert a force on Proton B, because Proton B is positioned along the path Proton A is travellling.

Of course, the problem here is the time delay in which the electromagnetic force propagates. This problem is similar to the whirled string problem, I suppose.