Riding home a couple nights ago, I came to a stop at the exit of a church parking lot, waiting for traffic. It was dark and raining. Not to worry; my bike is lit up like a two-wheeled UFO but those were the conditions.
I balanced in place until a break in the traffic occurred, then started forward – only for my front wheel to drop into a water-filled (and thus invisible) pothole deep enough to bring it to a complete and sudden stop.
Notice I said; “it” as in “the wheel” and not “me” because that meant the mass of my bike and its rider were able to rotate around the front wheel axle. Inasmuch as almost all that mass is above the axle, forward motion is translated into upward-then-forward motion.
I wasn’t going very fast, and was just getting ready to hop over the handlebars to land on my feet when the rotation paused, halted in a nearly vertical position, and then went back down. No harm done, though it could not have appeared graceful to an onlooker. I actually did look around to see if anyone was watching, then felt silly.
Relating this story to someone, they said; “If you’d been moving very fast, that could have been much worse!” But no, it wouldn’t. Suppose I’d been at cruising speed, I’d have cleared the diameter of the pothole before the wheel could have dropped far enough into it to stop forward motion. It’s happened before, it’s not pleasant, but my bike is equipped with front shocks for just such an eventuality.
Thinking about the incident, I can’t help being pleased by the visualization of momentum and leverage. As the bike and rider rose upward, gravity pulled back down until the energy expended was equal to the pre-existing forward energy. It could be mapped by a curve taking into account the compression of the front shocks, the changing geometry of weight in relation to direction and axle – actually lots of complexities. Then falling back down, momentum was stopped by the back wheel hitting the pavement, flexing my legs and deforming the inflated tire. And presumably, expending some of that energy as heat in the air of the tire, spokes of the back wheel, frame of the bicycle, and so forth.
It makes me want to dig into a calculus book but since it would take me a month of Sundays to figure it all out, I’ll just have to be content with the visualization.