Can Cost Optimization: compute optimal dimensions

heartshapes

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1) Find a can
2) Estimate cost ratios between the top, bottom and sides of can.
3) Measure the can dimensions
4) Finally, compute the optimal dimensions of the cylindrical can holding your given volume which will minimize the cost of material to produce that can. Notice we can do this without knowing the actual cost of the material to produce the can. So, you DO NOT need to know the actual cost per square centimeter of aluminum alloy 3104-H19 used to make your can. It is a constant number anyway so treat is as a constant C.

Okay so..
Using a soup can, I found the radius to be 3.75cm and the height to be 11.2cm, the volume in 480cm[sup:itza6xj7]3[/sup:itza6xj7].
I am saying that the cost to produce the top is 2 times the cost to produce the sides by area of material (aluminum)
I don't understand how to tie the cost in it.. if you don't have to know it.. (basically I get lost on number 4)

This is what I did..

\(\displaystyle A_{min}\, =\, 2{\pi}r^2\, +\, 2{\pi}rh\, \mbox{ and }\, h\, =\, \frac{480}{{\pi}r^2}\)

\(\displaystyle A_{min}\, =\, 2{\pi}r^2\, +\, 2{\pi}r\left(\frac{480}{{\pi}r^2}\right)\)

\(\displaystyle A_{min}\, =\, 2{\pi}r^2\, +\, \frac{960}{r}\)

\(\displaystyle A_{min}^{'} \, =\,4{\pi}r\, +\, \frac{960}{r^2}\)

\(\displaystyle 0\, =\, 4{\pi}r^3\, -\, 960\)

\(\displaystyle 960\4{\pi}\, =\, r^3\)

\(\displaystyle r\, =\, 4.24314\)

\(\displaystyle h\, =\, 8.48826\)

Then using a numberline I found that 4.24 is a minimum.. But I don't see how this includes the cost.
Any help would be greatly appreciated!
 
Re: Can Cost Optimization

My calculator says that (3.14156)*3.75[sup:1yzsjt1w]2[/sup:1yzsjt1w]*11.2 = 494.8...
 
Re: Can Cost Optimization

Loren said:
My calculator says that (3.14156)*3.75[sup:1taeosuu]2[/sup:1taeosuu]*11.2 = 494.8...

Yeah but my professor said to use what was on the label instead.. it says it has 480ml=480cm[sup:1taeosuu]3[/sup:1taeosuu]
 
heartshapes said:
... I don't see how this includes the cost. I do not think that it does.


If you are supposed to take the volume directly off of the can's label, then I do not understand why you need to measure the radius and height of the same can. :?

Regardless, you want to find the dimensions of a can of volume 480 ml that minimize material costs, and you are free to pick an arbitrary ratio of material costs.

Okay -- let's go with this. You chose the side material to be half as expensive as the top and bottom material.

Let C = the cost of the side material

Then 2C = the cost of the other material

2 * Pi * r^2 is the combined area of the can's top and bottom.

960/r is the area of the side.

Let's write a function for the total cost in terms of the radius.

P(r) = 2C * 2 * Pi * r^2 + C * 960/r

P(r) = 4 * C * Pi * r^2 + 960 * C * r^(-1)

Take the derivative of P(r), and then find the critical value.

I found the dimensions to be as follows (rounded to the nearest hundredth of a centimeter).

The radius is 3.37 cm and the height is 13.47 cm.

Do you agree?

Cheers,

~ Mark :)

MY EDITS: Changed sloppy name choice of cost function from C to P because I already used the symbol C as a constant; corrected some spelling errors
 
Re:

mmm4444bot said:
I found the dimensions to be as follows (rounded to the nearest hundredth of a centimeter).

The radius is 3.37 cm and the height is 13.47 cm.

Do you agree?

Yes, thank you! I don't know why we had to measure the can either but whatever.
I see now how I needed to add the cost without actually picking a cost value.
Thanks!
 
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