问题 问答题

如图所示,在高为h=5m的平台右边缘上,放着一个质量M=3kg的铁块,现有一质量为m=1kg的钢球以v0=10m/s的水平速度与铁块在极短的时间内发生正碰被反弹,落地点距离平台右边缘的水平距离为l=2m.已知铁块与平台之间的动摩擦因数为0.5,求钢球落地时铁块距平台右边缘的距离s(不计空气阻力,铁块和钢球都看成质点).

答案

=1.6m 

设钢球反弹后的速度大小为v1,铁块的速度大小为v,碰撞过程时间极短系统动量守恒    ①

钢球做平抛运动水平方向   ②

竖直方向  ③

由②③①解得t=1s,v1=2m/s,v=4m/s

铁块做匀减速直线运动,加速度大小=5m/s2 ⑤,最终速度为0,则其运行时间=0.8s  ⑥

因此钢球落地时铁块已经在0.8s时停止,所以它距离平台右边缘的距离

=1.6m ⑦

阅读理解

阅读理解。

     A new weapon is on the way in the fight against smoking in Europe. Soon when smokers buy cigarettes,

they might see a shocking photo of a blackened lung or a cancer patient staring back at them from the packet.

     Some boys may think of smoking as cool and sexy. Their friends won't agree when they see their packets

of cigarettes lying on the table.

     The European Union announced on October 22, that it had chosen 42 photos that showed the damage

cigarettes could do to the body. It called on member nations to put these pictures on packets to discourage

young smokers.

     To catch the attention of teenagers, the special packets warn of long-term medical dangers, like cancer.

Short-term effects, like bad skin, are also on the list.

     "The true fact of smoking is disease, death and horror. That is the message we should send to the young,"

said David Byrne, an EU health official. "Hopefully these pictures will shock students out of their love for

cigarettes."

     The EU head office hoped the pictures would work better than current written warnings on packs of

cigarettes. The warning included "smoking kills" and "smoking can lead to a slow and painful death".

     So far, Ireland and Belgium have shown interest in the photos. Canada has used similar pictures and

warnings on cigarette packs since 2000. The country has recently seen a fall in the number of smokers.

     According to studies, smoking is the single biggest cause of avoidable death in EU. Every year more

than 650,000 smokers die, more than one person a minute.

1. What would be the best title for the text?

A. New Ways to Stop Smoking.

B. Pictures to Shock Smokers.

C. New Packets of Cigarettes.

D. Dangers of Smoking.

2. Which of the following is NOT the true fact of smoking?

A. Disease.

B. Death.

C. Horror.

D. Happiness.

3. We can learn from the text that _____.

A. the EU countries have put the new warning method into practice

B. only a small number of the EU countries have used the new warning method

C. the new warning method has worked in some EU countries

D. countries in the EU still use the old warning method

4. Which country is most successful in stopping smoking?

A. Ireland.

B. Belgium.

C. Canada.

D. EU.

5. The underlined sentence in the last paragraph suggests that _____.

A. it's hard to stop smoking in EU

B. deaths caused by smoking could have been avoided

C. smoking is the biggest cause of deaths in EU

D. EU has the largest number of deaths caused by smoking

填空题