- #1
Physgeek64
- 247
- 11
Homework Statement
Find the centre of mass of a semi-circle
Homework Equations
##y_{cm}=\frac{1}{M} \int y dm ##
The Attempt at a Solution
So ## y= R cos \theta ## where theta is measured from the vertical, and the base of the semi-circle is along the horizontal
Now apparently from here you can change coordinated to polar coordinates and replace ##dm## with ## \rho r dr d \theta## to obtain the correct answer of ##\frac {4}{3 \pi}## But I'm confused as to why you can't also replace ##dm## with ##2 \rho R sin \theta R d \theta## to split to 'dm' segment into rows perpendicular to the horizontal base?
Many thanks :)