Bullets and Bananas

A Bullet and a Banana

Almost 30 years ago a banana was growing in the tropical regions of the Americas.  It made its way into the lunch bag of a Physicist working for the United States Air Force. The vast majority of bananas that make it into a lunch bag are eaten. But this particular banana sat on a desk patiently waiting to be consumed by someone, if not the one who brought it to work in the bag - but no one did.  And so it sat.

In the airconditioned offices there was little moisture and the banana was far from the door where fruit flies might have entered.  The banana simply waited, and waited, and waited. As it waited the dry air slowly evaporated its moisture.

The Physicist was intrigued.  How long would the banana last? He named the banana Bob and left it atop a file cabinet.  Office tomfoolery gave Bob a tombstone.  Bob, the banana, became an office topic of discussion long before artist Mauizio Cattelan taped one to a wall and sold it for $120,000.

When people get together socially, in families, in work environments, play happens.  Whether it is Puma with his landscaping pals or the Physicist and me while waiting on weapon analysis computations to finish, it happens.

Six or seven years before the banana, the Physicist was playing with the base of a 20mm projectile while standing beside a filing cabinet. He often spun it like a top, while we discussed approaches to our next project. Over time we developed various ways to spin the top.  First there was the simple one handed two finger twist.  Then came the two-handed opposing thumbs spin (my favorite). Finally, we attached a string to it and spun it like the wooden tops of his childhood.

Then came the question.  How fast is it spinning? We guessed at first, but we were not going to be satisfied with a guess.  How could we find out?  We threw out several suggestions then he asked - how would Newton have done it? I suggested dropping a drop of water on the top and measuring the maximum distance it traveled.  We derived the formula for such an experiment.

The Formula

We carefully dropped the water on top then measured the diameter of the circle the water formed on the top of the file cabinet. We took several measurements and averaged them (both of the circle and of different spins). We came up with speeds ranging over 6,000 rpms with our fingers and up to 10,000 rpms with the string technique. He didn't believe it. What error could there be that would make it err toward being too big?

I proposed a few.  There was a lip on the top of the top that could provide some lift to the water.  The top does travel so maybe it is traveling too much during the period of interaction; but it seemed like those events would be negligible since the interaction was occurring in milliseconds.

Finally, we found someone with a strobe meter.  We put a white strip of paper on one side of the top, spun it and measured it with the strobe while simultaneously applying the water method. The differences were within 400 rpms. The technique was a complete success.

The Physicist retired from the USAF around 2006 and then started working for a Defense contractor.  He put in another 20 years there.  I started some part-time work with the same company. Just before he retired again, he handed me the banana and the bullet.