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I was wondering if anyone could offer an explanation as to why the balls described in this video spin so fast.

Here's the setup: Two metal balls are wielded together. When spun with air, they acquire a massive amount of rpm.

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have anybody tried it in real? –  Yrogirg Sep 2 '12 at 15:39

4 Answers 4

up vote 4 down vote accepted

Look at around 0:34 in the video. I want to point out something relevant to the question here. The end of the tube is narrowed. That is, in technical terms, a nozzle. Nozzles are extremely common in engineering and they work as a form of mechanical leverage just like a lever. I should also note that the straw itself is already a form of nozzle and allows (I think) a more potent blow than would otherwise be possible with the lips.

A human mouth has limitations. The most accurate way to frame this would be to say that one's mouth can only produce a given flow rate, $\dot{m}$, at a certain pressure above atmosphere, $\Delta P$. Combined, these give an energy rate, or power, that can be produced by the mouth.

The comment by Steve Melvin does apply - that the balls can not move faster than the fluid that is passing by it. However, the small outlet of the straw he uses is a way to make a tradeoff, getting high fluid velocity by sacrificing volume of flow. This would, in fact, be rather more difficult to do as accurately and gracefully with mechanical forces. This type of easy conversion ability of forms of fluid mechanical work is a major reason that hydraulics is such a useful science.

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The balls never spin faster than the velocity of the air being blown on them. Notice that when the presenter blows on the hurricane balls, he angles the mirror towards himself. He does this so the balls aren't blown off the mirror (he's blowing pretty hard). Also, it seems they are spinning ridiculously fast due to the frame rate of the video.

Edit: When I referred to the velocity of the balls, I was referring to their tangential velocity.

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Also, i think The Magnus Effect operates here. [1]: en.wikipedia.org/wiki/Magnus_effect –  Martin Gales Sep 5 '11 at 6:41
    
and any idea about the significance of electrical weld for ball bearing!!! –  Vineet Menon Sep 5 '11 at 10:55
    
So the Magnus effect would push it to the balls to the left, looking at it from the direction of the person blowing, assuming he is blowing the left ball (and it spins clockwise). Is this correct? –  AlanSE Sep 5 '11 at 14:17
    
@Vineet: No significance other than it is a simple way to weld a pair of ball bearings together (!!!). –  Colin K Sep 5 '11 at 15:16

He says that the balls are spinning at 2000 Hz, and that they are two half inch balls welded together. That says that their outer surface is moving at most something like $2\pi 0.5$ inches in 0.0005 seconds, for a speed of 6300 inches per second or 523 feet per second or 356 miles per hour. This is a difficult to believe speed, but blowguns achieve 350 feet per second or 240 mph and it's tougher to accelerate down a blowgun because of the long length one must blow through.

Two half inch steel balls have a lot of weight and that means they have a lot of angular momentum even at low speeds. They have a low coefficient of friction and so they keep spinning for a long time. In terms of getting the object to spin at high speed, the primary advantage having two balls welded together is that it makes it easier to get them moving. If it were just a single sphere, blowing on it wouldn't make it spin much.

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I think the direction of the answers are a bit off. With conversation of energy, you can get some how estimate the "initial rotational speed" of the hurricane balls. It must less than or equation to the amount of energy of the guy blow out plus the starting rotational energy by hand.

However, the key question is why the rotation could keep at the speed for so long? The flat mirror surface would help. But I believe "THE RPM IS INCREASING". It just likes a Euler Disc which trades the potential energy to rotational energy

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