# Strange Physics

Hey, here’s an unusual phenomena my roomate and I stumbled upon. A balloon over an upright fan stays above the fan and doesn’t fall!

Interestingly enough this also works if the fan is at a 45 degree angle, the ballon just floats in midair!

I recorded a video and uploaded it
http://johnpeb.dyndns.org/~joelm/100_0274.MOV

Can anyone try to explain this in terms of physics?. I think i’ve got a pretty good reason I’ll explain it if you want

I think that the grill on the fan seperates the air. And the shape of the fan, it looks as if it’s like a cup. So the air flows to the edges. When the ballon glides to the edge of the fan, it gets blown to the other side. The reason I think this is because every time it goes to the edge, it starts rotating, because only the edge is getting hit.

It’s called Bernoulli’s Principle, and the Wikipedia article can explain it much better than I can.

I don’t think this is so complicated bernoullis principle. I think it may just be Newtons Laws at work.

From observing the motion of the balloon, you see that the greatest force upward is at the edges, while the least force upward is in the center. The balloon is more dense than air and the bouyant force is not enough to keep it suspended so it falls normally. If we were to use net force to find the required force upward to keep the balloon suspended it would be equal to the weight of the balloon downward. Therefore, towards the center of the fan, the upward force is nearly equal to the weight.

However, towards the outside of the fan the force upward is greater than the weight and movew the balloon upward. This explains why it rises and falls.

However, the spin is created by Bernoulli’s Principle and Efflux flow. The balloon, a round object, gets angular momentum from the force exerted upward because of the efflux flow around the balloon. The linear and angular momentums cause it to spin and then go the opposite direction, in short. I’m sure you could probably work out the actual equations if you wanted. :o

Believe me, my dad is a physicist, so I think he’d know. He uses this demonstration all the time for demos at elementary schools.

oh, ok, then I’m probably wrong, but in my mind it made sense! I’m used to the more problem solving stuff (i.e. solving problems with given information, deriving formulas, etc.) not the real world application.

Yea. this is a classic example. Like putting a pingpong ball over a hair dryer. Try it it’s fun…

Yes, it is indeed due to the Bernoulli’s principle

At first i thought it was due to interia, becuase the least amount of movement occurs in the centre of the vortex. I thought of comparing it do an object in a whirlpool

Two more nice and interesting physics phenomena:

1. Water coming out of a tube (like a hose, pipe, something with no nozzle) will start spreading outwards (think about why - it doesn’t seem intuitive), then re-form into a stream due to the surface tension. Increase the pressure and it will fail to reform. Have a look.

2. In a sink where the water hits the basin (best in a flat kitchen sink, or a chopping board or something) if the water pressure is high enough (turn the tap reasonably strongly on) you will notice a ridge in the water at a certain distance from where it hits the board and starts spreading. This is the shockwave where the water goes from supersonic (as in the speed of sound in water, NOT air) to subsonic. Upstream from this ridge ripples created in the water will never be able to propagate upstream.

Alex

I guess this is why I did not understand fluids in high school physics. Oh well, whats sad is that in a college level high school class of 20 people, I understood it the best, and thats not saying much. :o

So, was what I said right? I have not taken anything physics related, other than science class. I read that wikipedia article, and can see how it’s related.