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How does buoyancy cause this wheel to spin?

  1. May 12, 2010 #1
    Hi all,

    In the attached image I have a cylinder shaped wheel with a air pocket embedded near its edge. The wheel has the same density as water and is fully submerged in a tank of water. Also the wheel is only free to spin about its center axis. If the wheel is rotated down the air pocket will cause the wheel to spin upward but how?

    Buoyancy is defined as an upward acting force, caused by fluid pressure.

    The wheel is under pressure from all side with a greater pressure from the bottom caused by gravity but I'm not understanding how fluid pressure can cause the wheel to spin upward?

    If a ping pong ball is fully submerged in a tank of water the water is able to flow under the ball to give it lift. but in this wheel example the wheel is not lifting but spinning so water is not flowing at all.

    Maybe someone could help me understand this a little better.


    Attached Files:

  2. jcsd
  3. May 12, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    It doesn't. What would cause the wheel to turn is its unbalanced weight: The solid side weighs more than the side with the air pocket.
  4. May 12, 2010 #3
    Thanks Doc AI

    That was the answer I was looking for. I assumed the solid side would have a neutral buoyancy (the wheel having the same density as water) but if buoyancy does not effect the side with the air pocket it will not effect the solid side ether. So the wheel behaves the same whether it is fully submerged underwater or not.

    Thanks very much!
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