Spinning Rim Centrifuge Experiment

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I used stiff wire to tie a clear Aquafina bottle to my hubcap. The bottle would be subjected to substantial forces, but I was confident that my rigging would hold.

I was concerned that adding too much weight would unbalance the wheel and affect my ability to drive safely. I gave it a short test run around the block, with a few ounces of water inside the bottle. There was only a tiny affect on the steering, so I made arrangements for a full-scale test.

The next day after work, I prepared my rolling laboratory for the commute home. I stopped at am/pm, being sure to park with my wheel-bottle in the upright position. 

In Am/Pm, I scanned the aisles for something worthy of fluid separation. 

I chose a bottle of Ben & Jerry's Chunky Monkey milkshake

Lately, whenever I find myself doing anything like this in public, I'm aware that the general public is on a vigilant lookout for terrorists. Except for the delicious odor of Chunky Monkey milkshake, this probably looked like the automotive version of a shoe bomber setup. 

I twisted on the cap, and began my journey. I headed south down White Rock Road, towards the Zinfandel entrance to Highway 50. 

The off-center weight gave my wheel a little eccentricity. It was OK on the city streets, but at freeway speeds, my steering wheel was bouncing around, the car was shaking, and the OnStar co-pilot threw up in my lap.



I got off the road as soon as possible and checked my results. 
Luckily, I had parked with the bottle right-side up. Unfortunately, the results were unimpressive: I couldn't see any separation between the fluids in the bottle. It just looked like a homogeneous brown sludge. At the very least I had expected the chunks of monkey to be collected at the bottom.

I had traveled about two miles, at a top speed of about 50 mph. Fifty mph is about .83 miles per minute, and my tire spun about 1,287 times covering that 83-hundredths of a mile, so the milk was spinning at 1,287 RPMs.

1,287 RPMs is 67 radians per second. The radius of the circle made by the rotating milk was about 6 inches (0.15 meters). To figure the centripetal acceleration, I read that I needed to multiply this radius with the square of the radians per second, so (672 x 0.15), or (4489 x 0.15) = 673 m/s2.

673 m/s2 is equal to 68 times the force of gravity, better known as 68 Gs

A lot of force, but probably not applied gently enough to separate the fractions of this mixture. I knew those bleeding hearts down at Ben & Jerry's were against segregation, but this was absurd.

On the other hand, homogeneity tasted delicious!

Using my car as a centrifuge had failed, but I'm not giving up on finding a new use for these wheels. Maybe I can use them to aerate your lawn.


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March 28th, 2007

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