A uniform sheet of metal is cut in the shape of a semicircle of radius R and lies in the xy plane with its center at the origin and diameter lying along the x axis. Find the position of the CM using polar coordinates. (Center of mass). [In this case the sum that defines the CM position becomes a two-D integral of the form [tex]\int[/tex]r[tex]\sigma[/tex]dA where [tex]\sigma[/tex] denotes the surface mass density (mass/area) of the sheet and dA is the element of area dA= rdrd[tex]\phi[/tex].] Ok I thought I knew how to start this before I read the bracketed section... Could I get a hint on starting this with polar coordinates (never done this actually...) and why is the bracketed section even necessary? Thanks alot!