阅读理解。
Can you imagine that you can save your own life during a heart attack (心脏病发作) by coughing (咳嗽)?
Let's see how…
A heart attack can happen to anyone. Let's say it's 4:17 p.m. and you're driving home (alone of course)
after an unusually hard day on the job. Not only was the workload (工作负担) extremely heavy, you also had
a disagreement with your boss, and no matter how hard you tried, he just wouldn't see your side of the situation.
You're really upset and the more you think about it, the more nervous you become.
All of a sudden you start experiencing severe pain in your chest that starts to spread out into your arm and
up into your jaw (下巴). You are only about five miles from the hospital nearest your home, unfortunately you
don't know if you'll be able to make it that far.
What can you do? You've been trained in CPR but the guy who taught the course didn't tell you how to
perform it on yourself.
Since many people are alone when they suffer a heart attack, this article seems in order. Without help the
person whose heart stops beating properly and who begins to feel faint (晕眩的) has only about 10 seconds
left before losing consciousness. However, these victims can help themselves by coughing repeatedly and very
powerfully. A deep breath should be taken before each cough, and the cough must be deep and last long, as
when producing sputum (痰) from deep inside the chest. A breath and a cough must be repeated about every
two seconds without stopping until help arrives, or until the heart is felt to be beating normally again. Deep
breaths get oxygen into the lungs (肺) and coughing movements squeeze (挤压) the heart and keep the blood
circulating.
The squeezing pressure on the heart also helps it regain normal rhythm (节奏). In this way, heart attack
victims can get to a phone and, between breaths, call for help.
Now, do you understand the whole matter? Tell as many other people as possible about this. It could save
their lives!
1. According to the passage, CPR is most probably something done to _____. [ ]
A. regain someone's breath and heart beat
B. help someone treat a heart attack by himself
C. teach people how to stay in good state
D. train people how to stay calm when facing dangers
2. According to Paragraph 5, coughing during a heart attack helps _____. [ ]
A. to relax the heart
B. to get oxygen into the lungs
C. to keep the blood circulating
D. to reduce the pain in the chest
3. We can learn from the passage that _____. [ ]
A. a long day's work usually results in heart attacks
B. not only the chest aches when one suffers a heart attack
C. people should attend a CPR course to survive heart attacks
D. one should cough loudly immediately one's chest aches
4. In which section of a newspaper can you read this passage? [ ]
A. Health Care.
B. Advertisement.
C. Family.
D. Teaching.
．现将导轨沿与水平方向成
角倾斜放置．在底部接有一个
的电阻．现将一个长为
、电阻
的金属棒自轨道顶部沿轨道自由滑下，经一段距离后进入一垂直轨道平面的匀强磁场中（如图12－17所示）．磁场上部有边界，下部无边界，磁感应强度
．金属棒进入磁场后又运动了
后开始做匀速直线运动，在做匀速直线运动之前这段时间内电阻R上产生了
的内能（
）．求：
时的加速度a的大小及方向；
方向为沿轨道向上；（2）32.5m
，又有
代入得：
负号表示其方向为沿轨道向上．
，金属棒做匀速运动时根据平衡条件得：
即
