Friction on the Incline

I discovered a hardbound book leaning from the bottom shelf down to the rug. A thirty-degree slope. Flat cardboard cover. Perfect transit ramp. I collected a stray plastic bottle cap from beside the skirting board. Placed it inverted at the summit. Initial push applied. It slid, spun twice, and stopped abruptly on the carpet fibers. Insufficient momentum. I shifted the book two inches left to clear the pile. Second attempt. The cap accelerated smoothly down the spine, clearing the transition onto bare floor. Rolling dynamics verified.