[ME statics] Papus' theorem - find required height of cup

In summary, the problem is to determine the height h at which liquid should be poured into a cup with a radius of 11mm and a length of 60mm, so that it contacts half of the surface area on the inside of the cup. The surface area equation used is 2πXL, where X is the distance to the center of mass from the axis of rotation and L is the length of the line that is rotating. The solution involves adding the area of the bottom to the area of the side, then dividing the total area by half. However, to account for the area of the bottom, it is necessary to subtract it from the target area.
  • #1
Feodalherren
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Homework Statement


Hibbler_ch9_p111.jpg
Determine the height h to which liquid should be poured into the cup so that it contacts half the surface area on the inside of the cup. Assume that r=11mm and l= 60mm . Neglect the cup's thickness for the calculation.

Homework Equations


Surface Area = 2πXL

where X is the distance to the center of mass from the axis of rotation and L is the length of the line that is rotating.

The Attempt at a Solution


IMG_20141023_013835.jpg


Link to my solution:

https://www.wolframalpha.com/input/?i=4243.57=2pi(11+((.317h)/2))sqrt(h^2+++(.317h)^2)
 
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  • #2
Is there a question or something to go with your working?
 
  • #3
It's in the problem statement. You can't see it?
 
  • #4
I followed it up to the "want" (4243.57). Then I expected you to subtract the area of the bottom. I didn't try to follow your math passed that point, but I'm wondering if you took that "free bottom" into account.
 
  • #5
I added the area of the bottom to the area of the side, then divided that total area by half. The question is kind of ambiguous and I don't know if they want me to account for the area of the bottom or not. I'm assuming yes since they only state "surface area" and the bottom certainly has some surface area.
 
  • #6
That gave you the correct target area. ("wanted")
But as soon as you put half a drop into the container, you cover the bottom - and that contributes to your "wanted" area. So, to compute how much you need to take from the sides, subtract the surface area of the bottom from your "wanted" number.
 
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Related to [ME statics] Papus' theorem - find required height of cup

1. What is Papus' theorem?

Papus' theorem is a mathematical principle that states that the volume of a solid of revolution is equal to the product of its cross-sectional area and the distance traveled by the centroid of the cross-section. In simpler terms, it allows us to calculate the volume of a three-dimensional object by using the area of its cross-section and the distance traveled by its center of mass.

2. How does Papus' theorem relate to statics?

Papus' theorem is commonly used in statics to determine the volume of irregularly-shaped objects, such as the required height of a cup. By using this theorem, we can accurately calculate the volume of an object without having to measure its entire surface.

3. What is the formula for using Papus' theorem to find the required height of a cup?

The formula for using Papus' theorem to find the required height of a cup is V = 2πrA, where V is the volume, r is the distance between the center of the cross-section and the axis of revolution, and A is the area of the cross-section.

4. Can Papus' theorem be applied to any shape?

Yes, Papus' theorem can be applied to any shape as long as it is a solid of revolution, meaning that it can be rotated around an axis to create a three-dimensional object.

5. Are there any limitations to using Papus' theorem?

One limitation of Papus' theorem is that it only applies to solids of revolution, so it cannot be used to calculate the volume of irregularly-shaped objects that cannot be rotated around an axis. Additionally, it assumes that the object is uniformly dense, which may not always be the case in real-world scenarios.

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