Why the water in the center of a spinning bucket rises

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Discussion Overview

The discussion centers around the behavior of water in a spinning bucket, specifically addressing whether the water in the center rises or dips. Participants explore concepts related to forces acting on the water, historical experiments, and interpretations of physical principles.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Historical

Main Points Raised

  • One participant explains that adhesive forces between water and the bucket's walls cause water to cling to the sides when at rest, suggesting a connection to centrifugal force affecting cohesive forces when spinning.
  • Another participant proposes using a free body diagram to analyze the forces acting on the water, suggesting that the bucket compresses the water inward, causing it to deform upwards in the center.
  • A participant references spinning liquid mirrors in astronomical telescopes, implying a connection to the discussion.
  • Contrarily, another participant asserts that the water does not rise in the center but instead vacates it, bunching up at the edges due to the forces at play, downplaying the role of cohesive forces.
  • Historical context is provided by referencing Sir Isaac Newton's experiment with a spinning bucket, noting that the water's surface becomes concave, higher at the edges and lower in the center.
  • Clarification is sought regarding the term "concave," with participants confirming it means the surface dips down rather than rises.
  • One participant admits to confusion over a reference due to misinterpreting a photo of a page in a book.

Areas of Agreement / Disagreement

Participants express disagreement regarding whether the water in the center of the spinning bucket rises or dips, with multiple competing views presented without consensus.

Contextual Notes

Participants reference historical experiments and concepts that may not be fully resolved or universally accepted, indicating a reliance on interpretations of physical principles and definitions.

bobsmith76
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When a bucket of water is at rest the water on the sides will rise for the following reason taken from my textbook:

Water tends to cling to the walls of the glass because the adhesive forces between the molecules of water and the glass molecules are greater than the cohesive forces between the water molecules. In effect, the water molecules cling to the surface of the glass rather than fall back into the bulk of the liquid.

In Brian Greene's book the Fabric of the Cosmos he says that when a bucket of water spins the center will rise. He never said why. It has to be something about the cohesive forces begin to become stronger when centrifugal force is applied but I can't think why.
 
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I guess one can think of it in terms a free body diagram of forces just acting on the water as a whole. the bucket would be supplying a force radially inwards towards the center to give it centripetal force to sustain the circular motion of the water, so that the water as a whole is being compressed inwards by the bucket. since the water cannot deform downwards as the bottom of the bucket is inflexible, the water deforms upwards in the center in response...

i might be wrong :)
 
The water in the center of a spinning bucket does not rise. What page do you read that on? When a bucket spins, the water gains speed and goes against the edge of the bucket, the edge of the bucket pushes back. As that happens, the water vacates the center and bunches up on the edge. The cohesive forces you mention are minimal in comparison.

Check out Bob's link, it should be informative.
 
Bobthemoose, page 24

It’s not often that a bucket of water is the central character in a three-hundred-year-long debate. But if you spin a bucket of water, the water will eventually become concave
a bucket that belonged to Sir Isaac Newton is no ordinary bucket, and a little experiment he described in 1689 has deeply influenced some of the world’s greatest physicists ever since. The experiment is this: Take a bucket filled with water, hang it by a rope, twist the rope tightly so that it’s ready to unwind, and let it go. At first, the bucket starts to spin but the water inside remains fairly stationary; the surface of the stationary water stays nice and flat. As the bucket picks up speed, little by little its motion is communicated to the water by friction, and the water starts to spin too. As it does, the water’s surface takes on a concave shape, higher at the rim and lower in the center, as in Figure 2.1.
 
concave means that it doesn't rise but dip?
 
Bobthemoose,

I got confused because I was looking at a photo of the page not the page itself (I convert all my books into digital copies using a camera, that's how I used the long cut and paste) and I mistook the far edge of the bucket for the water.
 
thanks for the wolfram demo link, that's pretty cool
 

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