depend on wot yu wanna rotate bout
if base is along x-axis, draw vertikal line thru point yuzed as
senter av the serkel.
yu kan spin the shape round vertikal line thru this senter
If yu want dif direkshun, need tu find the mass senter
or senter av area av semiserkel. yuno it will be on the vertikal line thru senter
av serkel. But how hi up from senter??? That require kalkulus.