Yeah a steam engine. Good question. I love those old steamies. My dad and I used to go the Grainger's Picnics when I was a kid. Growing up in Amish country, there were so many of those old steam tractors. *sniff* I can smell the coal burning now.
You're right... "inertia" was a bad word to use. I tried to use it as a metaphor for demonstration's sake, but it didn't come across.