PBS
The Math of Bicycles: Wheel Figure This Out (Grades 4 7)
Several good activities here. Look particularly at Wheel Figure It Out (which is the opening page) and Gearing Up. There are answers to the very good questions at the bottom of each.
Math Science Nucleus
I. Science Ma Te: Integrating Science, Math and Technology
This site offers a wealth of online textbook-related materials that encourage the discovery of science in the world around us. Enter the site to access material on specific topics. Each section contains reading material (complete with...