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  #1  
Old 12-02-2002, 07:17 PM
Jim Brier Jim Brier is offline
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Default Fuel Gage Problem

An 18-wheeler has a broken fuel gage. The driver wishes to measure the level of fuel in his cylindrical tank by using a stick. The tank diameter is 20 inches and the fill hole is directly on top. He knows that 10 inches is half a tank. He would like to know how many inches on his stick is one-quarter of a tank. Obviously, it is not 5 inches because the tank is cylindrical.
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  #2  
Old 12-02-2002, 08:40 PM
Bozeman Bozeman is offline
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Default Re: Fuel Gage Problem

Are you just asking us to do trigonometry?
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  #3  
Old 12-02-2002, 08:58 PM
Ed Miller Ed Miller is offline
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Default Re: Fuel Gage Problem

I'm assuming you mean that the tank is "laying on its side" rather than standing on its base (or else 5 inches would be the answer).

If Theta is angle between the edges of the tank at gasoline level (at quarter tankness) and the center of the cylinder:

1/4 pi * R^2 = Theta/2 * R^2 - R^2 sin(Theta/2)cos(Theta/2)

Theta/2 - sin(Theta/2)cos(Theta/2) = 1/4 pi

The number of inches X for a quarter tank is then

X = R (1 - cos(Theta/2)) where R is 10 inches

Blah.. I have a meeting... [img]/forums/images/icons/smile.gif[/img]
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  #4  
Old 12-03-2002, 02:43 AM
JTG51 JTG51 is offline
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Default Re: Fuel Gage Problem

Yes, I think he is.

Jim, it's too bad that most of your posts come in the form of math problems these days. Your contributions to the mid/high stakes forum are missed.

By the way, I really liked your most recent card player article (on playing high limits). I doubt I'll ever play that high, but I found it very interesting.
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  #5  
Old 12-03-2002, 05:49 PM
Homer Homer is offline
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Default Re: Fuel Gage Problem

Does the trucker have a computer? I came up with two ways of doing the problem, but for both I ultimately needed a computer to reach the final solution.

Method 1:

I used some trig and came up with the following formula:

{5/9*pi*cos-1((10-h)/10)} - {2*(10-h)*sqrt(20*h+h^2)} = 25*pi

I used the Goal Seek tool in Excel (basically plug this formula into cell A2 and h into cell A1, then tell it to set A2 equal to 25*pi by manipulating the value of h in cell A1) to find a solution to this equation.

With this method I got an answer of 8.04.

Method 2:

A horizontal "slice" of a vertical (circular) cross-section of the cylinder has an length of sqrt(20*h + h^2). If these "slices" are integrated from zero to x (with x being the height filled with gas) and this is set equal to 25*pi, solving for x will provide the solution.

I plugged this function into "The Integrator" and got some crazy ass function spit out at me. I guess I could plug this function into Excel and solve the same way as I did in my first method, but I don't feel like it.

...in any event, there is probably a ridiculously easy way to get the answer to this problem that I didn't think of. You probably didn't want us to use a computer anyway. [img]/forums/images/icons/smirk.gif[/img]

-- Homer
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  #6  
Old 12-03-2002, 07:37 PM
Mano Mano is offline
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Default Re: Fuel Gage Problem

Assuming the trucker knows how much his tank holds, fill the tank to halfway (which can be done using his stick), and then add another quarter tank, thus making the tank 3/4 full. Now measure the height on your stick, and the height for the 1/4 full tank will be 20 - the height for the 3/4 full tank.
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  #7  
Old 12-04-2002, 09:30 AM
irchans irchans is offline
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Default Re: Fuel Gage Problem

I used majorkong's and simpson's methods to get an answer of 5.96 inches.

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  #8  
Old 12-04-2002, 11:26 AM
Homer Homer is offline
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Default Re: Fuel Gage Problem

Your answer is correct. My formula had a slight error. It should have been:

{5/9*pi*cos-1((10-h)/10)} - {2*(10-h)*sqrt(20*h-h^2)} = 25*pi

instead of:

{5/9*pi*cos-1((10-h)/10)} - {2*(10-h)*sqrt(20*h+h^2)} = 25*pi

...oops.

-- Homer
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  #9  
Old 12-04-2002, 02:00 PM
JTG51 JTG51 is offline
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Default Re: Fuel Gage Problem

There's a problem with that solution.

After filling the tank half way, you want him to add another 1/4 of a tank. If he knew how to add 1/4 of a tank, he wouldn't need to go through the process you described. He could just add 1/4 to begin with and measure that.
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  #10  
Old 12-04-2002, 02:02 PM
JTG51 JTG51 is offline
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Default How about a solution with almost no math...

I imagine every trucker in the world knows how many gallons his tank holds. If not, it's easy enough to look up.

Divide the total capacity by 4. Add that much to the tank and measure it with the stick.

Nice and easy, only one simple division. There's always an easier way. [img]/forums/images/icons/smile.gif[/img] I doubt this is the solution Jim was looking for, but I like it a lot more than the calc and trig.
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