解答这道题---**四、圆周运动** 质点在以某点为圆心半径为r的圆周上运动，即质点运动时其轨迹是圆周的运动叫“圆周运动”。它是一种最常见的曲线运动。例如电动机转子、车轮、皮带轮等都作圆周运动。 如图所示，在竖直平面内有一光滑圆弧轨道，a为轨道最低点，c为轨道最高点，b点、d点为轨道上与圆心等高的两点，e为ab段的中点。一个质量为m的小物块在轨道内侧做圆周运动。 **Chart/Diagram Description:** * Type: Diagram illustrating circular motion on a vertical track. * Main Elements: * A circle representing a smooth circular track in a vertical plane. * Labeled points on the track: 'a' at the bottom (lowest point), 'c' at the top (highest point), 'b' on the left side, 'd' on the right side. Points 'b' and 'd' are at the same height as the center of the circle. Point 'e' is located on the arc between 'a' and 'b', described as the midpoint of the 'ab' section (referring to the arc segment or some definition not explicitly clear geometrically from the image alone, but position is shown). * A small orange rectangular block is shown near point 'a', representing an object of mass 'm'. * Several arrows (A, B, C, D, E) are shown pointing towards the left side of the circular track from the left, indicating possible force directions or other vectors. Arrow A points towards the track near 'a'. Arrow B points towards 'e'. Arrow C points towards the arc between 'e' and 'b'. Arrow D points towards 'b'. Arrow E points towards the arc between 'b' and 'c'.