Here are several excellent applets for visualizing cross-sectional solids. We use the Stewart Calculus TEC applet today in class, and the others in tomorrow's lesson.
http://www.stewartcalculus.com/tec/
http://web.monroecc.edu/manila/webfiles/pseeburger/secure/MyLarson/ch7/LC7_2xsection2.htm
http://web.monroecc.edu/manila/webfiles/pseeburger/secure/MyLarson/ch7/LC7_2xsection1.htm
http://web.monroecc.edu/manila/webfiles/calcNSF/JavaCode/other/myXSection.htm
AP As We Go - tonight's homework, submit answers online