Page 12 - GALIET PHYSICS BLOSSOMS II+

a bucket half-filled with water from a rope. The fact that the water's surface gradually assumed a concave shape showed that it was spinning with respect to “something,” for how else would the water behave this way? For Newton, this “something” was absolute space. Proof of the reality of space could be found in the inertia or the inherent force of matter (when an object keeps moving at the same speed and in the same direction unless acted upon by a force).6
Newton’s rotating bucket experiment consisted of suspending with a rope at a fixed place, a bucket half-filled with water. After rotating the bucket by twisting the rope to its maximum twisting point, he held the bucket at rest until the water became still, at which point he let go. Newton noticed, at first, that as the rope began to untwist, the bucket began to rotate, however, its water did not rotate right away. Curiously, the water remained still and its surface flat. Then Newton noticed that as the rope continued to untwist, the water begun to whirl about in a contrary way, receding little by little from the middle, and ascending to the sides of the bucket, forming a concave figure. Perplexed, Newton discovered that the swifter the motion became, the higher the water rose, until at last, the water in the bucket became still or at rest once more.7
In his Rotating Bucket experiment, Newton observed two things:
1. When the water spun fast, it remained still or flat on the surface. That is, the water neither receded from the axis, nor tended to create a circumference, nor ascended towards the bucket’s sides. Newton argued at this point that the water’s true circular motion had not yet begun. Because this instance exemplified relative motion, as shown in ii) below, Newton related it to his notion of relative space.
2. When the water spun slower, it begun to recede from the axis, and to ascend towards the bucket’s sides. This demonstrated that the water’s true circular motion continued to increase until it acquired its greatest circularity when it rested in the bucket. Because this instance exemplified true motion, as shown in i) below, Newton related it to his notion of absolute space.
Given these observations, Newton concluded that:
1) The receding forces from the axis of circular motion distinguish absolute from relative motion. Newton argues that in circular true motion, these receding forces are not purely relative given that they are greater or lesser in accordance to how much motion there is.
i) Of any one revolving body, there is only one true, absolute circular motion, which is the one that recedes from the axis of motion for this is the proper and adequate effect of the water.
ii) Of one and the same revolving body, there are innumerable, apparent relative motions because they relate to external bodies. This is why the apparent relative motions are destitute of any real effect, other than they may possibly partake in the only one true motion.
6 http://einstein.stanford.edu/SPACETIME/spacetime1.html 7 Summarized from Stamp, Phillip. Newton. Mechanics. 10-12.
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