1. Introduction: A Wheel Touching the Road
Picture a bullock cart rolling slowly down a village road. Or your own bicycle wheel turning. Watch the round wheel where it meets the ground.
How much of the wheel actually touches the road at one instant? Not a stretch of it. Not a long line of rubber pressed flat. Just one tiny spot. A single point. The next instant, the wheel turns a little, and a new point touches down.
Stop scrolling. Try it in your head before reading on. At one frozen moment, how many points of the wheel are pressing on the ground?
Now look at the spokes of that wheel. Each spoke runs from the centre out to the rim. The spoke that points straight down — the one heading toward the ground — what angle does it make with the road?
Hold that picture. The wheel meeting the road at one point, and that bottom spoke standing straight up from the road. We are going to build this whole topic out of that one everyday picture.
You can now picture the exact situation this topic is about: a round thing touching a straight thing at a single point.