题目

A Letter to A Student in Yushu Dear friend, When we learned over the radio and in the newspaper that Yushu was struck by 16.______ terrible earthquake on April 14th, 2010, we became very 17.________ (worry) and sad. It is said that thousands 18.______ houses 19._________(destroy) , so you can’t have your classes as usual at present. I wonder 20.___ ____ things have got better by now.    Don’t lose heart and be strong-minded 21.______ you are facing the great disaster, for people all over China are standing by you. I’d like to send you some books 22._______ were bought last week5．为验证向心力公式，某探究小组设计了如图所示的演示实验，在米尺的一端钻一个小孔，使小孔恰能穿过一根细线，线下端挂一质量为m，直径为d的小钢球．将米尺固定在水平桌面上，测量出悬点到钢球的细线长度l，使钢球在水平面内做匀速圆周运动，圆心为O，待钢球的运动稳定后，用眼睛从米尺上方垂直于米尺往下看，读出钢球外侧到O点的距离r，并用秒表测量出钢球转动n圈用的时间t．则：（1）小钢球做圆周运动的周期T=$\frac{t}{n}$．（2）小钢球做圆周运动的向心力F=m$\frac{4{π}^{2}{n}^{2}}{{t}^{2}}（r-\frac{d}{2}）$．