阅读理解。
May: Happenings from the Past
May 5,1884
Isaac Murphy, son of a slave and perhaps the greatest horse rider in American history, rides Buchanan
to win his first Kentucky Derby. He becomes the first rider ever to win the race three times.
May 9,1754
Benjamin Franklin's Pennsylvania Gazette produces perhaps the first American political cartoon (漫画),
showing a snake cutsin pieces, with the words" Join or Dic" printed under the picture.
May 11,1934
The first great dust storm of the Great Plains Dust Bowl, the result of years of drought (干旱), blows
topsoil all the way to New York City and Washington, D.C.
May 19,1994
Jacqueline Lee Bouvier Kennedy Onassis, former first lady and one of the most famous people of the
1960s, died of cancer in New York City at the age of 64.
May 24, 1844
Samuel F.B Morse taps out the first message, "What Hath God wrought," over the experimental
long-distance telegraph line which runs from Washington, D,C, to Baltimore, Md.
1. We know from the text that Buchanan is _____.
A. Isaac's father
B. a winning horse
C. a slave taking care of horses
D. the first racing horse in Kentucky
2. What is the title of the first American political cartoon?
A. Join or Die
B. Pennsylvania Gazette
C. What Hath God Wrought
D. Kentucky Dorby
3. In which year did the former first lady Jacqueline die?
A. 1934
B. 1960
C. 1964
D. 1994
4. Which of the following places has to do with the first telegram in history?
A. Washington, D.C.
B. New York City.
C. Kentucky.
D. Pennsylvania.
的电阻，右端NQ与半径R为2m的光滑竖直半圆形绝缘导轨平滑连接；一根阻值不计的长为L=1.2m，质量m=0.5kg的金属杆ab放在导轨的EF处，EF与NQ平行。在平面NQDC的左侧空间中存在竖直向下的匀强磁场B，平面NQDC的右侧空间中无磁场。现杆ab以初速度v0=12m/s向右在水平轨道上做匀减速运动，进入半圆形导轨后恰能通过最高位置CD并恰又落到EF位置； (g取10m/s2)
，到达最高位置CD位置时，速度为
，由于恰能通过最高点，则：
落到水平导轨上
