Never before in history ________ a city been so completely destroyed. San Francisco is gone. Nothing is left of it ___
_____ memories and some houses far from the centre of the city. Its businesses are gone. The factories, hotels and palaces are all gone too. Within an hour __
____ the earthquake, the smoke of San Francisco’s fires could be seen 160 kilometres away. The sun was red in the dark sky. There was no stopping the fires. There was no way to organize ____
___ communicate. The steel railway tracks were now ___
____(use). All of the ways ___
_ man had made to keep the city safe were gone in the thirty seconds the earth moved.
Out at sea it was calm. No wind came up. ________ from all direction---east, west, north, and south, strong winds blew upon the ___
____ (luck) city. Man himself had to make ruins of some of the city’s best buildings ____
_____ that they would not be a danger to those in the streets. A list of buildings undestroyed was now only a few addresses. A list of the brave men and women would fill a library. A list of all ____
____ killed will never be made.
如图1所示,A.B.C三点为一直角三角形的三个顶点,∠B=30°,现在A.B两点放置两点电荷qA.qB,测得C点电场强度的方向与AB平行,则qA带________电,qA∶qB=________.
如图所示的电路中,1、2、3、4、5、6为连接点的标号。在开关闭合后,发现小灯泡不亮。现用多用电表检查电路故障,需要检测的有:电源、开关、小灯泡、3根导线以及电路中的各连接点。为了检测小灯泡、开关以及3根导线:
①在连接点1、2已接好的情况下,应当选用多用电表的 挡,进行检查;
②在连接点1、2同时断开的情况下,应当选用多用电表的 挡,进行检查;
③在开关闭合情况下,若测得3、4两点间的电压接近电源的电动势,则表明 。
如图所示为一小球做平抛运动闪光照片的一部分,图中背景格的边长为5cm。求出物体做平抛运动的初速度大小为___ ____ 。B点的速度大小为。(g取10 m/s2)
用甲、乙两种光做光电效应实验,发现光电流与电压的关系如图所示,由图可知,两种光的频率甲
乙(填“<”,“>”或“=”), (选填“甲”或“乙”)光的强度大。已知普朗克常量为
,被照射金属的逸出功为
,则甲光对应的遏止电压为 。(频率用
,元电荷用e表示)
放在水平地面上的一物块,受到方向不变的水平推力F的作用,F的大小与时间t的关系和物块速度v与时间t的关系如图所示。取重力加速度g="10" m/s2,则物块的质量m= kg,物块与地面之间的动摩擦因数μ= 。