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December 16 - January 6

Question of the Week

Name: Thomas
Status: student
Grade: 9-12
Location: CA
Country: USA
Date: Fall 2014


When using a spoon to stir a flat bottomed pot of water with denser granules (sugar, salt crystals, rice, etc.) on the bottom, I notice that the granules move to the center of the pot. Why does that happen? There is no heat applied to the pot. I thought centrifugal force would move the granules to the perimeter instead.

Answers from Our Expert Staff

Good question Thomas. This is a classic example of how a centrifuge works. In effect, you created a centripetal force by stirring in a circular motion. The centrifugal force created which is a reaction stemming from the centripetal force will push the water towards the walls of the pot, leaving the weightier solids in the center. Hope this helps.

Best regards, Richard Anarfi

It would if you were not stirring the pot. If you just rotate the pot at constant speed, and wait until the water is moving uniformly with the pot, then the grains that are not exactly at the center will move toward the perimeter.

Stirring the water makes it turbulent everywhere but at the center, so grains at the center will mostly just stay there. Grains away from the center will swirl in all directions because of the turbulence. If they swirl into the center, they will stay there, because the water is relatively still there. If the swirl to the outside, they'll still be in turbulence, so they'll still be susceptible to being swirled toward the center.

Tim Mooney

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