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Another Table Tennis Paradox

  1. Oct 16, 2007 #1
    Well, I have read a thread regarding a table tennis paradox and I wondered why a top spin ball will dive faster. I have a question here: Does the direction of air flow will affect the ball whether it will float more or dive faster? I mean if the ball spins in the opposite direction of air flow(counter air resistance), will it float more?? And vice versa, if the ball spins in the direction of air flow,will it be diving faster?

    If so,by referring the uploaded figure below, can someone tell me, in between which region the air pressure will be lower and where the ball would curve...Thanks a lot
     

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  2. jcsd
  3. Oct 16, 2007 #2

    rcgldr

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    Homework Helper

    The diagram at this web site explains the seperation of air flow component of Magnus Effect with a backspin example: the air is accelerated downwards, and the ball is accelerated upwards.

    Magnus Effect .htm

    Wikipedia has a similar diagram, but refers to Bernoulli effect, which I personally don't like when describing open systems (the air isn't contained). It also refers to the air stream detachment as described above. This diagram shows topspin, air is accelerated upwards and the ball is accelerated downwards.

    http://en.wikipedia.org/wiki/Magnus_effect

    What's common to all of these explanations is that a thin layer of air spins with the ball because of surface friction and viscosity (friction within the air itself).

    In addition to air stream detachment, the air that is spinning "forwards" (in the same direction as the ball lateral movment), causes more forwards acceleration of the nearby air than the air that is spinning backwards, also due to viscosity. This differential in acceleration of air also contributes to a pressure differential, which accelerates the air in one direction and the ball in the other direction.
     
    Last edited: Oct 16, 2007
  4. Oct 17, 2007 #3

    russ_watters

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    Staff: Mentor

    Did you read the previous thread on exactly the same subject, beatenbob? It has all the answers in it...
     
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