In Theory or Why I Would Make a Crummy Engineer


Article copied from Microsoft OneNote

Article copied from Microsoft OneNote









?? to.


v Y





We know the shape of the cap and we know the area of the base, and the volume so we can find the unique height, , unique because the volume will be some multiple of the height times the area of the base.  If we know the height we can find the radius,  of the ball with a cap with dimensions  and find the angle with a little geometry as in this diagram.




↳7¢  R’









Personally I want to stop here.  The interesting part is over.  Yet the problem is to actually find  so let’s press on. From the diagram,   So it remains to find the relationship between  We could look up the formula.  We could use a CAS (computer algebra system), or we could derive it.  I will use the CAS when I really need it, so for now let’s find a formula for the volume of a cap given the radius of the ball and the radius of the cap.  I will use the calculus method of volume by disks with the setup as in this diagram.








dry “inn”

















We form the cap by rotating the yellow section about the y-axis.  Think of it as composed of disks formed by rotating the blue-shaded rectangle about the y-axis.  Solving the equation of the semicircle for  the volume integral is




So now we have an equation that relates   Given  and  we could use numerical techniques to get  or we could attempt to solve for  by isolating the square root, squaring both sides to arrive at a sixth degree equation. I give. Time for a CAS.  This is what Wolfram Alpha gives for the solution.



I have omitted the two other solutions which were complex number expressions since we are looking for a real number answer. Briefly I wonder if there isn’t a more elegant way to find an expression for  that gets directly to the real number. Anyway we now have an expression for  in terms of  and  and I’ve had it.  Nothing particularly beautiful or surprising here.


Wait.  We wanted to know the angle .  This is a practical problem.  All those pesky engineering questions intrude.  We need to check if the formula is correct by using  Wolfram Alpha or preferably another CAS. We need to check if typical values for  and  give the anticipated values for .  We also need to know the sensitivity of  to the uncertainty of the measurements for  and .  This is engineering – turning a math model into a real-life solutions generator, tedious and often ill-conditioned requiring patience and attention to detail – not my strongest trait.  That’s why I would make a crummy engineer.


About jrh794

I am a sixty-five year old math instructor at Southern Oregon University. I taught at the College of the Siskiyous in Weed California for twenty-six years. Prior to that I worked as a computer programmer, carpenter and in various other jobs. I graduated from Rice University in 1967 and have a MS in Operations Research from Stanford. In the past I have hand-built a stone house and taken long solo bicycle tours. Now I ride my mountain bike and play golf for recreation.
This entry was posted in Math Explorations, Rants and tagged , . Bookmark the permalink.

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