题目

My phone number is 872-4567.读作_____. A . eight six two five four six seven B . eight seven two four five seven six C . eight two seven four five six seven D . eight seven two four five six seven 答案：D 三个相同的支座上分别放置着三个质量和直径都相同的光滑圆球a、b、c，支点P、Q在同一水平面上，a球的重心位于球心，b、c的重心分别位于球心的正上方和正下方，如图所示，三球皆静止．试分析三种情况下支点P、Q对球的弹力方向．