Algodoo

I never knew exactly how these things worked until recently, and decided to build one on algodoo!
"seismograph" comes from the words "seismos", meaning shake, and "graph" meaning write. It is a device usually used to detect earthquakes. Here's how!
The entire device shakes with the earthquake, except for a weight. This is the purple ball. It doesn't move much, so it serves as a near fixed point. We measure the position of it on the graph below it to determine roughly how the earth shakes. it is usually displayed on a time-shake graph.
Suggest more ideas for me to build!
Thanks