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Car Talk Math Puzzler Solution Wrong!

On the NPR program Car Talk, hosts Tom and Ray presented a puzzler in which a truck has a broken fuel gauge. The trucker wishes to determine when his fuel tank is one-quarter full. The tank is in the shape of a circular cylinder on its side. The trucker can put a stick in the filling opening at the top of the tank, and use it as a dipstick to mark the height of the fuel. Obviously, the half-full mark is the radius of the circle from the end of the stick. Where is the quarter-full mark? A solution was requested that would use a minimum of advanced math. Solvers could use a pizza box, string, a pencil and a knife.
I loved the problem, and their method of solution which involved using the string and pencil to draw a circle the size of the tank cross section on the top of the pizza box, then using the bottom of the pizza box as a to draw a diameter of the box, and using the box as a square to draw a radius perpendicular to the diameter. Finally, they cut out the resulting semi-circular region, and balanced the semicircle on the pencilpoint to determine the center of mass of the region. They claimed that the quarter-full horizontal line would pass through the center of mass.
Not so! Look at the fuel tank "head on", so you see a circular region. Imagine drawing 3 horizontal lines on the region. The first line is the diameter of the circle, or the half-full line. The second line is the one that passes through the centroid of the lower semicircular region and is parallel to the first line. Let's call this the TARL (Tom and Ray line). If Tom and Ray are right, the TARL is the quarter-full line. However, they're wrong so there is a third line, the BL (bisector line) which is parallel to the previous two lines and divides the semicircular region into two pieces of equal area. If the circle involved has a radius of 10 inches, the TARL is approximately 4.24 inches from the half-full line, while the BL is approximately 4.04 inches from the half-full line.
The mathematical details are interesting, and can be shown without any explicit reference to integral calculus. See them at Scribd.


w.c.sessums said...
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w.c.sessums said...

When I heard the solution on the show, I too went slack-jawed. I knew instantly that the answer was incorrect. I'm glad someone else was paying attention. You would think that someone who uses a wrench for a living would know that distance from the pivot point makes a difference.