August 15, 2006

So what do YOU do when you're driving?

Many people have bad driving habits. Giving the finger, blabbing on the cellphone, fiddling for a station on the stereo, etc. Aside from the obvious vehicular dangers created, this doesn't seem like very worthwhile behavior. Dad was very good driver but one day he became intrigued by the low pitch he always heard when passing over the bridge. It was at a definite pitch and it was created by the tires passing over those little spaced rivets found on many bridges. Given his fascination for physics he set out to figure out how far apart these rivets were in order to produce the pitch he was hearing at the speed he was traveling. I don't remember the exact speed or the pitch he said it was but I do rememember that he told me after humming and figuring out the pitch, observing his constant speed and going home to calculate, he determined that the rivets were 5 and half inches apart.

This afternoon I figured it out with a spreadsheet and a hypothetical pitch. A440= 440 Cycles per second. The low pitch hum he was hearing was most likely two octaves below so let's say that he heard a low "A" at 110 cycles per second. In order to deterimine the rivet spacing distance you would first have to translate your car speed in miles per hour to inches per second. Then you would determine that unit distance in inches-based on the car speed you observed that produced the pitch you heard-that would have to be traversed 110 times in a second to produce that pitch. This is how I calculated the math:

cycles/sec to produce low "A" =110
speed: miles/hour=~42mph
speed: inches/sec=~605in/sec
unit inches to produce low"A" = 605/110=5.5

In other words if hypothetically, your constant speed produced a low A, then the speed in inches per sec to produce that would be ~605 in/sec (42mph)and it would have to pass over at least 110 rivets a second spaced 5.5 inches apart to do that.(605 inches/110=5.5 inches)

Nifty, huh?

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