问题 问答题

在绝缘水平面上放一质量m=2.0×10-3kg的带电滑块A,所带电荷量q=1.0×10-7C.在滑块A的左边l=0.3m处放置一个不带电的绝缘滑块B,质量M=4.0×10-3kg,B与一端连在竖直墙壁上的轻弹簧接触(不连接)且弹簧处于自然状态,弹簧原长S=0.05m.如图所示,在水平面上方空间加一水平向左的匀强电场,电场强度的大小为E=4.0×105N/C,滑块A由静止释放后向左滑动并与滑块B发生碰撞,设碰撞时间极短,碰撞后两滑块结合在一起共同运动并一起压缩弹簧至最短处(弹性限度内),此时弹性势能E0=3.2×10-3J,两滑块始终没有分开,两滑块的体积大小不计,与水平面间的动摩擦因数均为μ=0.5,g取10m/s2.求:

(1)两滑块碰撞后刚结合在一起的共同速度v

(2)两滑块被弹簧弹开后距竖直墙壁的最大距离s.

答案

(1)1.0m/s          (2)0.08m

(1)设两滑块碰前A的速度为v1,由动能定理有:

                                           

解得:v1="3m/s                                  "

AB两滑块碰撞,由于时间极短动量守恒,设共同速度为v

                              

解得:v="1.0m/s                                 "

(2)碰后AB一起压缩弹簧至最短,设弹簧压缩量为x1,由动能定理有:

      

解得:x1="0.02m                                     "

设反弹后AB滑行了x2距离后速度减为零,由动能定理得:

                    

解得:x2≈0.05m                                 

以后,因为qE>μ(M+m)g,滑块还会向左运动,但弹开的距离将逐渐变小,所以,最大距离为:S=x2+s-x1="0.05m+0.05m-0.02m=0.08m.                "

单项选择题 A1/A2型题
阅读理解

阅读理解。

     The hole in the Earth's ozone layer (臭氧层) has until now protected Antarctic from the worst effects of

global warming. But scientists have warned that as the hole closes up in the next few decades, temperatures

on the continent could rise by around 3℃ on average, with melting ice contributing to a global sea level

increase of up to 1.4 metres.

     In the past decades the western Antarctic has seen rapid ice loss as the world has warmed, but the other

parts of the continent have, paradoxically, been cooling, resulting in a 10% mcrease in ice in the seas around

the region. This is because the hole in the ozone layer has increased cold winds in Antarctic, making much

of the continent surface colder than usual.

     But now that the gasses that cause the ozone hole have been banned, scientists expect the hole to repair

itself within the next 50 to 60 years. By then the cooling effect will have faded out and the Antarctic will face

the full impact of global warming. This means an mcrease in average air temperatures of around 30℃ and a

reduction in sea ice by around a third.

     The biggest threat to the continent comes from warming seas. Robert Johnson, a scientist who monitors

Antarctic ice sheets, said, "The ice sheets in Antarctic are hundreds of metres thick. But once warm ocean

waters start flowing underneath, the ice will begin thinning and could break up very quickly." Thinning ice

sheets cause ice to break away from the continent and to melt even faster. Escaping ice from western

Antarctic has already resulted in a 10% rise in global sea level in recent decades.

     Johnson believes that international action to reduce global warming is required immediately or it may be

too late. "Everything is connected-Antarctic may be a long way away but it is an important part of the Earth's

system," said Johnson. "It contains 90%of the world's ice, 70% of the world's fresh water and that is enough,

if it melts completely, to raise sea levels by 63 metres."

     Even in aworse-case situation scientists don't expect the ice to entirely disappear, but predict that, because

of the melting ice sheets, average sea level rise will be around l.4 metres higher by the end of the century.

1. The underlined word "paradoxically" (in Paragraph 2) most probably means "_____". [ ]

A. rapidly

B. approximately

C. contradictorily

D. apparently

2. What is the effect of the hole in the ozone layer on Antarctic?[ ]

A. It is causing the ice to melt faster.

B. It is making much of the continent colder.

C. It is making the effects of global warming in the region worse.

D. It is reducing the amount of water in Antarctic.

3. What do scientists think is the biggest danger facing Antarctic? [ ]

A. Rising sea levels. 

B. Warming sea water temperature.

C. Water pollution.

D. Growing ice sheets.

4. Which of the following is true according to the passage? [ ]

A. Antarctic is currently experiencing the full effects of global warming.

B. The average temperature has increased by 3℃ in recent decades.

C. Antarctic contains most of the world's fresh water.

D. Ten percent of Antarctica's ice has already been lost.

5. The best title for the passage is _____. [ ]

A. Our planet in danger

B. Antarctic melting away

C. Action plan to save Antarctic

D. Let's save the ozone layer