使用Mathematica绘制圆滚线（Cycloid）

定义：

百度百科：“圆滚线指一个动圆沿同一平面内的一直线或另一圆周作无滑动地滚动时，动圆上任何点的轨迹。这种曲线首先由伽利略提出。圆上定点的初始位置为坐标原点，定直线为x轴。当圆滚动j 角以后，圆上定点从O点位置到达P点位置。当圆滚动一周，即j从O变动2π时，动圆上定点描画出摆线的第一拱。再向前滚动一周，动圆上定点描画出第二拱，继续滚动，可得第三拱，第四拱……，所有这些拱的形状都是完全相同的 ，每一拱的拱高为2a（即圆的直径），拱宽为2πa（即圆的周长）。”

Wikipedia: 

"A cycloid is the curve traced by a point on the rim of a circular wheel as the wheel rolls along a straight line without slipping. A cycloid is a specific form of trochoid and is an example of a roulette, a curve generated by a curve rolling on another curve.

The cycloid, with the cusps pointing upward, is the curve of fastest descent under constant gravity, and is also the form of a curve for which the period of an object in descent on the curve does not depend on the object's starting position."

Manthematica中如何绘制圆滚线？