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I'm sure momentum is conserved and could be used to construct an
explanation of that funnel, but it's beyond me, too.  We have this as an
exhibit and it is easily explained directly by Bernoulli -in this case
the air on the bottom of the ball is hardly moving at all and so exerts
normal atmospheric pressure or close to it.  The flow is probably not
conforming to the ball, but likely separates from it, leaving eddies
below the ball with slower flow.  There's a classic classroom demo of
this with a spool and an index card.  Blow into the spool, facing down,
with the card on the other end and the card will be suspended just below
the spool.

Dave

David L. Smith, Ph.D.
Director of Professional Development
Da Vinci Science Center, Allentown, PA 
http://www.davinci-center.org

Engage, Explore, Share

> 
> Hey, Dave, the momentum explanation works great for aircraft, BUT....
> 
> There is a classic classroom demonstration in which the demonstrator  
> connects a short length of plastic tubing to the stem of a funnel,  
> drops a ping pong into the open end, and then asks the 
> biggest kid in  
> the class to blow through the tube to launch the ping pong 
> ball.  The  
> kid blows, but nothing happens - the ball stays put.  The 
> demonstrator  
> then turns the funnel upside down and has the kid try again - same  
> thing.  As long as the kid continues to blow, the ping pong 
> ball hangs  
> unexpectedly in the funnel, defying gravity.  As soon as the 
> kid stops  
> blowing, it drops out.
> 
> I am no physicist, but I am having a real hard time explaining this  
> effect in terms of momentum.  The force of gravity is 
> downward, the air  
> flow is downward, yet the ball doesn't fall.  The conventional  
> Bernoulli explanation would be that air coming through the funnel is  
> accelerated around the ball, resulting in faster moving air 
> above and  
> slower moving below, and since slower moving air exerts 
> higher pressure  
> the ball is pushed up.
> 
> As Natasha  says, Bernoulli doesn't make planes fly, flying 
> planes make  
> Bernoulli.  So what exactly is it that keeps that ball in the funnel?
> 
> Chuck
>

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