Drinking cup

  • Thread starter Winzer
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  • #1
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Homework Statement


A cone-shape drinking cup is made from a circular piece of paper of radius R by cutting out a sector and joining the edges CA and CB. Find the maximum capacityof such a cup. This is page#312 In James stewarts Calc book, 3rd edition by the way.

Homework Equations


[tex]V=\frac{1}{3}\pi r^2 h[/tex]

The Attempt at a Solution


So realize this will form a cone with an inner triangle of hypotnue R. The base will have a radius r and a hieght h. So I will have to make a relationship between r and h to get R to plug into [tex]V=\frac{1}{3}\pi h r^2[/tex] and differeniate?
 
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Answers and Replies

  • #2
Dick
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(1/3)*pi*r^2 doesn't even have the units of a volume. (1/3)*pi*r^2*h does. I think that's what you want. You will want to parametrize r and h by the angle cut out of R. And then differentiate and maximize wrt to the angle.
 
  • #3
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sorry clumsy mistake, I forgot the H, lol.
 
  • #4
Dick
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sorry clumsy mistake, I forgot the H, lol.

I thought so. But just write r and h as functions of the cut out angle. That's just geometry.
 
  • #5
HallsofIvy
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Notice that the entire circle of paper has circumference [itex]2\pi R[/itex] but the circle at the top of the cone has circumference [itex]2\pi r[/itex]. The arc length of the cut out wedge is [itex]R \theta/\2\pi[/itex] where [itex]\theta[/itex] is the angle in radians. That must be the difference between [itex]2\pi R[/itex] and [itex]2\pi r[/itex].
 
  • #6
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mmmm.......
 
  • #7
Dick
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Ok, then let's try it another way. Draw the right triangle joining a point on the circle of the base of the cone to the apex of the cone. Height is h and base is r, right? What's the length of the hypotenuse? Use Pythagoras to eliminate one of the variables (r or h) from the formula for the volume and maximize.
 

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