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Mach's Principle and Newton's Pail Experiment?

  1. Mar 22, 2003 #1
    Can someone please explain this to me and the apparent mystery it's suppose to offer with regard to absolute acceleration.

    I don't see it, a spinning pail of water that results in a curved surface is meant to somehow reveal an insight into relative mass and absolute acceleration. As far as I can tell, the curvature of the water is completely explained by classical mechanics.

    What am I missing? Thanks.
  2. jcsd
  3. Mar 23, 2003 #2
    Someone? Please? Am I being unclear?
  4. Mar 24, 2003 #3


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    Newtonian mechanics relies heavily on inertial frames of referance. Those are the frames of referance in which Newton's laws are supposed to hold. Mach observed that inertial frames were not rotating relative to the so-called "fixed stars". This gave rise to Mach's Principle which states that the inertial property of any given object depends on the presence and the distribution of other masses.

    SO if you're accelerating with respect to an inertial frame of referance what that is supposed to mean, at least according to Mach's Principle, is that you're accelerating relative to the "fixed stars". so there is no "absolute acceleration". Only acceleration with respect to an inertial frame which in turn means that you're accelerating relative to the "fixed stars".

    A spinning pail of water is thus rotating relative to the fixed stars. So if the water is trying to climb out of the pail, i.e. the surface of the water is curved, then you're (or actually the bucket) is rotating relative to the fixed stars.

  5. Mar 24, 2003 #4
    Well, ..OK; but only because you're a friend of St. Thomas Aquinas.

    Newton recognized constant motion is relative. However, rotation is different; and one can determine which party is actually rotating and which is stationary due to the principle of inertia.

    If two bodies attached to a string appear to be rotating about each other with a string taut between them, then IF they are actually rotating then the string would have tension. If however it was the observer that was rotating around the two bodies, there would be no tension in the string.

    Likewise, he reasoned with a pail of water. If you rotated around it, you would observe no curvature of water up the sides of the bucket; but if the water were actually rotating (and you were stationary) curvature would be observed. Thus, one could distinquish which party was actually rotating (WITHOUT reference to the outside environment)- and so he concluded rotational acceleration is not relative.

    Creator :wink:

    Opps I think I crossed Pmb's post.
    Last edited: Mar 24, 2003
  6. Mar 25, 2003 #5
    Ahhh....thanks people.:smile:
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