This activity uses simple materials to allow students to explore how earthquakes affect structures, causing some buildings to vibrate while others appear to stand still. The consistent rhythm in which the "buildings" (i.e. the plasticine balls) swing at is known as their natural frequency.
When you look at a building, it appears to be a rigid structure. All buildings, however, have some flexibility. This is particularly noticeable at the top of a tall building on a windy day. The shorter the building the stiffer it is, and the higher its natural frequency.
This activity has been adapted from the San Diego Supercomputer Center's TeacherTECH Science Series.