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Not shaken, not stirred, but stable. (Posted on 2005-09-10) Difficulty: 3 of 5
A cylindrical glass has a mass of 100 grams. It is partially filled with water (density = 1 gram/cubic centimeter.)

The glass has an inside diameter of 8cm and an internal depth of 15cm. When empty the center of mass is 8cm from the top of the glass.

The glass is most stable when its center of gravity is as low as possible. How much water is then in the glass?

  Submitted by Jer    
Rating: 2.0000 (3 votes)
Solution: (Hide)
The glass is most stable with about 183.5 grams of water.

The center of gravity of a system is the weighted mean center of gravity of its components.
The first component is the glass, with mass 100 g at a distance 8 cm
The second component is the water of depth x. Its mass is pi*4^2*x = 50.265x cm. The distance from the rim is 15 - x/2.

The weighted mean (center of gravity) is given by the formula:

C.G.= [100*8 + 50.625x(15 - x/2)]/[100 + 50.265x]

Since this is measured from the rim, we want to maximize this. My TI-83 gives x=3.651 as the depth corresponding to a mass of 183.5

Interesting to note is that the center of mass is right at the top of the water line in this situation.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
re: Some ThoughtsHugo2005-10-09 22:02:17
Some ThoughtsSome ThoughtsCeeAnne2005-10-09 21:15:35
Hints/Tipsre: No SubjectgoFish2005-09-14 12:43:04
No Subjectjames2005-09-13 11:00:19
No SubjectDAVID2005-09-11 12:48:22
SolutionSolutionBractals2005-09-10 04:42:07
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