一、何谓“等时圆”如图1所示,ad、bd、cd是竖直面内三根固定的光滑细杆,a、b、c、d位于同一圆周上,a点为圆周的最高点,d点为最低点。每根杆上都套有一个小滑环(图中未画出),三个滑环分别从a、b、c处释放(初速为0),用t1、t2、t3依次表示各滑环到达d所用的时间,则( ) First, what is the “isochronous circle” as shown in Figure 1, ad, bd, cd are three fixed smooth stems in the vertical plane, a, b, c, d on the same circumference, a point is the highest point of the circle , d is the lowest point. Each rod is covered with a small slip ring (not shown in the figure). The three slip rings are respectively released from a, b, and c (the initial velocity is 0). Each slip ring is represented by t1, t2, and t3 in sequence. d time used, then ()