Suppose you were to fasten a string to a ping pong ball, then fasten the string to a stationary object, and then use a vacuum cleaner with a strong vacuum to suck the ball partially into the vacuum hose. Due to the strong vacuum's pull on the ball, the string should be very taut and the ball should also be centered inside the vacuum hose.

Based on this setup and referring to the two drawings below, if you were to then push down on the string with your finger at the point indicated by the arrow in Figure A, would the tautness of the string move the ball from a centered position in the hose down until it comes to rest on the bottom of the vacuum hose?

Or, would pushing down on the string at that point cause a bend in the string, as shown in Figure B, with the result being that the ball would be pulled back towards the opening of the vacuum hose with the ball remaining centered inside the hose due to the strong airflow flowing around it?

The reason I'm asking this question is because I have been unsuccessful with keeping the string fastened to the ping pong ball, even with super glue, so I am unable to see what the ball will do.

enter image description here

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    $\begingroup$ Put the ball inside a fine mesh and join the string to that... $\endgroup$ – Solar Mike Dec 20 '18 at 5:17
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    $\begingroup$ Tie a knot in a thread, and using a needle, put it all the way through both walls - there's no problem leaving the knot on the outside of the ball as the air will already be completely turbulent back there $\endgroup$ – Jonathan R Swift Dec 20 '18 at 18:55
  • $\begingroup$ @ Jonathan R Swift, thanks, that is a good idea. I will give it a try. $\endgroup$ – user18610 Dec 20 '18 at 19:39
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    $\begingroup$ @JonathanRSwift Actually, the point of attachment matters considerably. It changes the dynamics of the system. You want the thrust on the upstream, not on the downstream side. So work the knot through the first hole. The centering force is usually pretty small, and normally you don't use it to center spheres (I don't even know if spheres will experience a centering force). It works great to center and align potatoes in a french fry water gun knife $\endgroup$ – Phil Sweet Dec 20 '18 at 20:10
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    $\begingroup$ ^this is fair comment - was just trying to make the operation as simple as possible so he could get results fast from the sphere and move on to more interesting shapes (like potatoes) so make a largish hole , post the needle through that and out the far side so the knot ends up inside the ball. $\endgroup$ – Jonathan R Swift Dec 20 '18 at 21:34

No - the sphere would likely stick to one wall, or bounce between them.

A sphere is not a very aerodynamic shape, having both a relatively blunt front, and sudden cut off at the back. This causes a lot of turbulence, such as the familiar vortex shedding shown here:

Vortex Shedding around a sphere

This shows how the air flows around a fixed sphere in free space - in your hose, the vortex forming on one side of the sphere will push it towards the wall, whereupon the flow will change as a result of the wall being there.

The key thing to note, is that the highly turbulent flow is not a stable equilibrium state - think of trying to balance a pencil vertically on your finger.

If you replaced the sphere with a different shape, imagine a Nerf football-type situation, that is designed to create a stable flow, then you would be able to get the behaviour that you're after - more like trying to keep the pencil vertically by holding the top, and dangling it downwards.

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  • $\begingroup$ @ Jonathan R Swift, thanks for the suggestion on using a football-shaped object. I think what I will try first is to find a wood dowel and use my belt sander to shape it into a football, then I will put a small screw into one end of it in order to have something to fasten the string to. $\endgroup$ – user18610 Dec 20 '18 at 19:50

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