The statement that a property of a system does not change if the system is isolated.

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Is the giant Newton's cradle in the Kit-Kat ad feasible?

Apologies in advance if this is too basic a question for Phys.SE. I don't want to dumb down this venerable institution. :) My wife and I just watched this TV ad for Kit-Kat where a crew of crane ...
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Why does a ballerina speed up when she pulls in her arms?

My friend thinks it's because she has less air resistance but I'm not sure.
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What is the symmetry which is responsible for preservation/conservation of electrical charges?

Another Noether's theorem question, this time about electrical charge. According to Noether's theorem, all conservation laws originate from invariance of a system to shifts in a certain space. For ...
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What is the symmetry which is responsible for conservation of mass?

According to Noether's theorem, all conservation laws originate from invariance of a system to shifts in a certain space. For example conservation of energy stems from invariance to time translation. ...
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What is the conserved quantity of a scale-invariant universe?

Consider that we have a system described by a wavefunction psi(x). We then make an exact copy of the system, and anything associated with it, (including the inner cogs and gears of the elementary ...