Why the water in the center of a spinning bucket rises

[bobsmith76]

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.

 

[kurtlesker]

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 :)

 

[Bob S]

Read about spinning liquid (mercury) astronomical telescope mirrors at

http://www.aeos.ulg.ac.be/LMT/

Bob S

 

[bobthemoose]

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.

 

[bobsmith76]

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.

 

[kurtlesker]

concave means that it doesn't rise but dip?

 

[bobthemoose]

Yeah concave means it dips down.

Check this out to observe it: http://demonstrations.wolfram.com/NewtonsRotatingBucketExperiment/

 

[bobsmith76]

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.

 

[kurtlesker]

thanks for the wolfram demo link, that's pretty cool