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How does a ball rotate in a waterfall?

  1. May 4, 2005 #1
    I needed to check if I was correct in my thoughts about how a ball rotates under a waterfall.

    Let me describe the situation: there is a mini-waterfall in my pool that has water that flows like a sheet (smooth water without any gaps, if that makes sense). Whenever a ball is placed near the waterfall, it glides over and starts rotating at a constant velocity and direction directly underneath and inbetween the water's flow.

    Why does this happen?

    So far, I have found out that it is because the presence of torque force. If you use a vector to represent the force of the water hitting the ball's surface, the vector can be extended to the ball's center of mass. From there, an x component and a y component can be created; the x component is the force applied to the surface of the ball, while the y component is the resultant torque force.

    Also involved in this phenomenon is the force of the ball's mass or gravity which pulls the ball downwards, and buoyancy of the water's surface due to surface tension that pushes the ball upwards; these cancel each other out to balance the ball.

    I don't know if I am missing anything - of course there are probably a lot of other factors influencing the ball to rotate (such as velocity of the water, etc), but I'm not sure exactly. Please help with what you can offer! :confused:
  2. jcsd
  3. May 4, 2005 #2


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    I think that you're dealing primarily with Bernoulli here. Don't take it as a fact, but I suspect that the water flowing around the ball acts like air over a wing; the pressure drops because it has farther to go. The ball is then kept centred by external air pressure pushing it back into the stream.
    The reason that I think this is that I used to have a toy called 'Johnny Astro' that used a joystick-controlled fan to manoeuvre a balloon done up like the hot air variety. Varying the speed of the fan brought the balloon closer or sent it farther out, and it always stayed centred in the airflow. It was accurate enough that I could drop it six feet away, snag something with the built-in hook, and take off again. (By the way, I was about 12 at the time.)
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