搜题请搜索小程序“爱下问搜题”
问题 问答题

如图所示，两木板A、B并排放在地面上，A左端放一小滑块，滑块在F=6N的水平力作用下由静止开始向右运动．已知木板A、B长度均为l=1m，木板A的质量MA=3kg，小滑块及木板B的质量均为m=1kg，小滑块与木板A、B间的动摩擦因数均为μ1=0.4，木板A、B与地面间的动摩擦因数均为μ2=0.1，重力加速度g=10m/s2．求：

（1）小滑块在木板A上运动的时间；

（2）木板B获得的最大速度．

答案

（1）小滑块对木板A的摩擦力：

f1=μ1mg=0.4×1×10=4N

木板A与B整体受到地面的最大静摩擦力：

f2=μ2（2m+MA）g=0.1×（2×1+3）=5N

f1＜f2，小滑块滑上木板A后，木板A保持静止

设小滑块滑动的加速度为a1，则：

F-μ1mg=ma1

根据运动学公式，有：

l=

1
2
a1
t21

解得：

t1=1s

（2）设小滑块滑上B时，小滑块速度v1，B的加速度a2，经过时间t2滑块与B速度脱离，滑块的位移x块，B的位移xB，B的最大速度v2，则：

μ1mg-2μ2mg=ma2

vB=a2t2

xB=

1
2
a2
t22

v1=a1t1

x块=v1t2+

1
2
a1
t22

x块-xB=l

联立解得：vB=1m/s

答：（1）小滑块在木板A上运动的时间为1s；（2）木板B获得的最大速度为1m/s．

单项选择题

男性，70岁。头晕、乏力、纳差3个月余，不规则低热1个月就诊。体检：中度贫血貌，胸骨无压痛，肝、脾、淋巴结均不肿大。化验：Hb70g／L，WBC3.2×109/L，血小板43×109/L，骨髓涂片示增生低下，原粒细胞0．48（48％）。诊断为急性低增生性白血病。首选化学治疗方案是（）

A.小剂量阿糖胞苷

B.维甲酸

C.DA（DNR加Ara-C.

D.VP（VCR加PrednisonE.

E.HAD（三尖杉加Ara-C加DNR）
查看答案
单项选择题


In the following passage, there are 20 blanks representing words that are missing from the context. Below the passage, each blank has 4 choices marked by letter A, B, C and D respectively. There is only ONE right answer. Blacken the corresponding letter as required on your Machine-scoring ANSWER SHEET.

How many of us would temp for three years while we waited for the perfect job Not many of us, perhaps. But Wentworth Miller, the "Prison Break" star, said he chose to wait even longer time (91) he found the right job.
"Actually I think it might have been three or four years until I was lucky enough to get guest (92) on TV shows," said Miller, 34, who rose to international stardom (93) to "Prison Break" in 2006. That kind of patience and p will was portrayed in "Prison Break". Miller (94) engineering wizard Michael Scofield, who tries everything to break out of a Chicago (95) with his brother. Miller said he was fond of his character, (97) he said "exists in shades of grey".
Miller comes from a (97) of teachers. He said he didn’t seem fated for a career in the arts, (98) his passion for acting. After (99) from Princeton with an English degree, he moved (100) his parents’ request to Los Angeles to look for a stable life. He started (101) in a little company that made television movies. This (102) simply faxing, filing, walking the boss’ dog and going to the store for the boss’ lunch. Every weekend during the summer, Miller (103) go to the office (104) he didn’t have air-conditioning. "I would hang (105) in the conference room and set up camp and rob the company kitchen," recalled Miller.
(106) , Miller realized he still had questions about his choice. He decided to quit. But the boss said he was making a mistake and offered him a well-paid assistant position. "I eventually (107) that if I did the corporate job, it would be great if I was successful, but I would always wonder about the (108) . If I did the acting and was successful, I would never wonder (109) that job," he explained. He quit and temped (110) many people in the entertainment industry.

A．would

B．should

C．might

D．ought to
查看答案