臂丛神经损伤常用的矫形器（） A.肩矫形器 B.肘关节矫形器 C.腕手矫形器 D.对

验证机械能守恒定律, 可以利用平抛运动的闪光照片来验证，图甲是一小球做平抛运动的闪光照片的一部分，图中背景小方格的实物边长可以用游标卡尺测量，图乙是卡尺读数部分的示意图，由图乙可知小方格的边长是L=____________mm。如果相机的闪光频率为f,小球的质量为m，当地重力加速度g已知，那么小球在a→b→c→d的整个过程中减少的重力势能为       ，增加的动能为         （这两空用字母L、f、m、g表示），这样代入数据就可以验证平抛运动过程中的机械能守恒。

Everybody dreams. Some people think dreams can tell us about the future. Other people think dreams can tell us about ourselves. It’s like our (11) is talking to us.

Why are dreams (12) strange and hard to understand Some people think our brain uses (13) to talk to us. When we fly, swim, or fall down in our dreams, it has a (14) meaning. Lots of doctors help people (15) their dreams. They find that dreams tell us about our (16) and fears.

These days, many scientists (17) that dreams are very important. During the day, we have many (18) , and our brain receives a lot of information. When we dream, our brain (19) information that is not important, and puts the most important information into our (20) . As we learn more about the brain, we may find answers to our questions about dreams.

13().

A.symbols 

B.examples 

C.numbers 

D.pictures

风洞实验室能产生大小和方向均可改变的风力。如图所示，在风洞实验室中有足够大的光滑水平面，在水平面上建立xOy直角坐标系。质量m=0.5kg的小球以初速度v₀=0.40m/s从O点沿x轴正方向运动，在0~2.0s内受到一个沿y轴正方向、大小F₁=0.20N的风力作用；小球运动2.0s后风力方向变为y轴负方向、大小变为F₂=0.10N（图中未画出）。试求：

（1）2.0s末小球在y方向的速度大小和2.0s内运动的位移大小；

（2）风力F₂作用多长时间，小球的速度变为与初速度相同；

（3）小球回到x轴上时的动能。

凤翔彩绘泥塑用色不多，以（）为主，以黑墨勾线和简练笔法涂染，色彩对比强烈，使人爱不释手。

A.以大红大绿和黄

B.以大红大绿和蓝

C.以大红和蓝、黑