Find the center of mass of a semi-circular plate

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SUMMARY

The discussion focuses on calculating the center of mass of a semi-circular plate with radius r and the volume generated when this plate is rotated around its straight edge. The x-coordinate of the center of mass is confirmed to be zero due to symmetry, while the y-coordinate is derived using the formula ycentroid = ∫∫R y dA / A, assuming uniform mass distribution. The volume of the solid formed by rotation is calculated using Pappus's centroid theorem, resulting in volume = (area of R) * (2πycentroid), which should equate to the volume of a sphere, 4/3πr3.

PREREQUISITES
  • Understanding of centroid calculations in geometry
  • Familiarity with integration techniques for area and volume
  • Knowledge of Pappus's centroid theorem
  • Basic principles of uniform mass distribution
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Homework Statement


Find the center of mass of a semi-circular plate of radius r

Find the volume when the plate (above) is rotated around a line along its straight side

Homework Equations



2(pi) integral of r dr

The Attempt at a Solution



I honestly don't know how to do centroids. :-( I'd like to know how to actually solve this problem.
 
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The x coordinate of the center of mass should be zero (intuitively).
For the y coordinate, assuming that the mass is uniformly distributed, use this equation (which can generalize for x):
ycentroid = [tex]\frac{\int\int_R y dA}{A}[/tex]

The volume is thus computed from the theorem of Pappus which states that
volume = (area of R) * (distance traveled by the centroid)
where the distance traveled by the centroid is 2*pi*ycentroid

Hope that helps, let me know if you need further clarification.
(Also, the volume when rotated about the straight side should come out to be that of a sphere, which is 4/3*pi*r^3, so you can use that to make sure you did it right.)
 
Last edited:

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