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#1045: Pi Over Two Dopes

Original Air Date: 11.06.2010

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This week on Car Talk, trucker Richard has a busted fuel gauge in his 18-wheeler. He's got a trusty stick to help measure the level of fuel in his cylindrical tank, but needs a formula to tell him whether that level means he's got 3/4 of a tank, or is running on fumes. You'd think two MIT grads would be able to help, but, well, you'd be wrong. Happily, Tom and Ray are more on the ball in recommending a snow-worthy vehicle for Gail, her husband, and their 8 kids, and in helping Louann figure out why her Accord smells like burnt toast. Also, why "oil replenishment" is like vacuuming your living room without a bag, and Tommy goes on the rampage against--Gomer Pyle? All this and lots more, this week on Car Talk.

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    Then there's the stupid approach

    by doctroid

    If you don't feel like looking up the formula for the area of a section of a circle, or doing trigonometry or integrals to derive it, there are much easier ways to get the answer. You can draw a circle on graph paper, count up squares, and get an answer close enough for any trucker's purposes. Or you can use an experimental model: get a can of soup, punch a hole in the side to emulate the gas tank fill hole. Empty out the soup (have it for lunch), fill the can with water, and empty it into a measuring cup to determine the volume (I'm not assuming here the soup filled the volume completely). Now put 1/4 of the water back, stick a toothpick or something in to act as a dipstick, and measure the height of the water. Scale up to the radius of the gas tank and you're all set, not a soh-cah-toa in sight.

    Try Excel Solver

    by RuKaz

    My husband and I are an engineer and a rusty mathematician, but we independently came up with roughly the same answer (mine was the more precise 5.9602724...) for the distance from the bottom of the barrel to the fuel level, such that the area of the cross-section of the barrel content is 1/4 that of the cross-section of the full barrel. The length of the cylinder is irrelevant, so we can work with area instead of volume. My husband derived his own equations, and used trial and error to obtain the approximate answer of 5.96, while I used Excel Solver add-in to get the precise answer above. We each ended up with one trigonometric/transcendental equation, and several algebraic ones -- NO calculus!

    gas tank problem

    by kpederso

    In order to figure out the 1/4 and 3/4 full heights of the gas tank, it is useful to consider only the circular cross section of the tank and first plot an estimate of what the volume (or area in this simplified case) of the tank would look like with height of the liquid in the tank. Once you have this plot, you can see that it resembles an inverted cosine function. Using the rules of adjusting cosine functions to fit values at certain points, the resulting function is: -((Pi*r^2)/2)cos(h*Pi/2*r)+(Pi*r^2)/2. By setting this function equal to (Pi*r^2)/2, you can solve for h and check that you get r. Once you are satisfied that the equation is correct, you can set it equal to (Pi*r^2)/4 (or 1/4th the total volume) and again solve for h, which gives a solution of 9.2 inches.

    Fuel Tank Solution

    by bhartman117

    If you want to use calculus, here is the solution: Come up with a 2-D section of a circle: x^2 + y^2 = r^2, or x^2 + y^2 = 100. Set y = sqrt(100-x^2). Integrate from t to 10, t being the distance from the center of the fuel tank down to the surface of the fuel at 1/4 full. Set this integral equal to 1/8 the the Area of the full circle (it's one-eight since you're only calculating for positive values of the y-axis). 1/8 of the area is 39.27 in^2. t comes out to be about 4.04. Thus, the distance from the top of the fuel tank is 14.04 inches. (Note, integrating sqrt(100-x^2) is difficult and I had to use an online integrator calculator)

    14 inches...

    by jackparker

    The necessary calculation can be found from http://mathworld.wolfram.com/CircularSegment.html mid page it says: To find the value of h such that the circular segment has area equal to 1/4 that of the circle, plug A=pi*R^2/4 into equation (18) and divide both sides by R^2 to get (23) where x=h/R. This cannot be solved analytically, but the solution can be found numerically to be approximately h=0.596027 or r=0.403973 So to measure a quarter tank, mark your dipstick at 14 inches from the top or 6 inches from the bottom.

    GaaaaLeeee

    by craigaddis

    I take offense to your views of Gomer Pyle! What American hates Gomer Pyle???

    Solving the Cylinder Problem

    by tads

    My brother told me about the problem you guys were trying to figure out with the truck driver's gas tank. I'm a math teacher and I tried my best to figure it out. If the tank measured 20in across in diameter, the quarter tank mark, (and 3/4 tank mark), should be 4 inches above or below the middle of the stick used to measure the diameter. I solved it by taking a formula for the area of the quarter of a circle, and set it equal to the formula of a circle's segment: (pi*r^2)/4 = ((A - SinA)r^2)/2 Then I solved for A, which is the central angle for the segment, and then I used the central angle to find the length. I hope this is helpful.

    The answer: 4 inches!

    by Anonymous

    Your e-mail reviews have 2 answers so far: 6 inches, and 4 inches. The math teacher is correct that (about) 4 inches in depth equals one-quarter tank. By the way, the solution uses Trigonometry, and not Calculus (as mentioned on the show). So much for MIT.

    teach your girl to drive stick

    by eborshard

    I usually love your show, but as a former 16-year old girl (roughly 16 years later) I took offense at the recommendation to teach a teenage girl to drive an automatic first, and a stickshift later. Thanks to my Dad's patience and my parents only having stickshift cars, I begrudgingly learned how to drive a manual transmission while all my friends had instant success in automatic cars. I have to say it was the best decision my parents made; since then I have been able to drive anything, from a 1969 VW bug in college to a fiesty pick-up truck for an on-campus recycling crew that only went into 3 gears. Teach her a stick and a stick only now - any idiot can drive an automatic. If we assume our teenage girls are so distracted by their cell phones and getting used to the steering wheel they will sink to that expectation. Teach them to handle a stick, check and change their own oil, and change a flat. They will feel confident for the rest of their lives, and can always fall back on an automatic and AAA later.

    1/4 full at 6 inches on stick

    by MikeVV

    You had a great opportunity to show off your calculus skills! I used a spreadsheet to estimate the dipstick level and then checked it with a google.com search for "horizontal tank volume" The spreadsheet took an hour and the google search took 5 minutes! The fuel dip stick level for 1/4 tank remaining if the tank depth is 20 inches is 6 inches. (See http://www.1728.com/cyltank.htm)

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