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I was reading H. Moysés Nussenzveig "A course in basic physics, Volume IV" and in chapter 8, he is introducing the basic ideas of quantum mechanics, where he states that:

In quantum mechanics, one thing that is important is the observation changes the system. In classical physics, there are examples of systems that also change with observation, but the change is so small and unimportant that we can ignore it.

This got me thinking. What are these examples of classical physics that also change with observation?

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    $\begingroup$ pretty much any one of them, like angular momentum for instance. $\endgroup$ – ZeroTheHero Oct 31 '17 at 13:42
  • $\begingroup$ Any measurement on a classical system involves some interaction with the system even if it's just shining light on it so you can see what it's doing. $\endgroup$ – John Rennie Oct 31 '17 at 13:43
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    $\begingroup$ 5 sec of thoughts. Plunging too big and too hot a thermometer in the volume of water whose temperature you want to measure. Measuring the voltage between two point of a circuit with a voltmeter that has too low a resistance. Or the current with an ammeter with too high a resistance. But this question of yours is dangerously close to a list question: beware the police! $\endgroup$ – user154997 Oct 31 '17 at 14:19
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    $\begingroup$ The important point is that in classical physics the change caused by observation can in principle be made arbitrarily small. That is the difference to quantum physics. $\endgroup$ – Diracology Oct 31 '17 at 21:10
  • $\begingroup$ @LucJ.Bourhis But if these measures change the system because of the instrument you use to measure. Isn't quantum mechanics different? Like even if you use a "perfect" apparatus to measure something, it still changes the result, because of the collapsing between two states? I was looking for something similar to this when i first formulated the question. $\endgroup$ – Vitor C Goergen Nov 1 '17 at 2:06

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