问题 选择题

从高处以足够大的初速度v0水平抛出一小球,若只在重力作用下,小球经过时间t1落地,落地时的速度大小为v1;若小球抛出后除受重力外,还受到水平风力作用,如图所示,则经过时间t2落地,落地时的速度大小为v2.下列关系正确的是(  )

A.t1<t2,v1<v2

B.t1=t2,v1>v2

C.t1=t2,v1<v2

D.t1>t2,v1=v2

答案

小球只受重力时做平抛运动,竖直方向做自由落体运动,则有:H=

1
2
g
t21
,t1=
2H
g
,落地时速度 v1=
v20
+
v2y
=
v20
+(gt)2

小球抛出后受重力和水平风力的作用时,在竖直方向上做自由落体运动,在水平方向上做匀加速运动.根据分运动与合运动具有等时性,知两次运动的时间相等,即t1=t2

两次运动在落地时竖直方向上的分速度相等,在水平方向上受风力作用时,落地水平速度大于抛出的初速度,根据平行四边形定则,知v1<v2.故C正确,A、B、D错误.

故选:C.

阅读理解

Asian athletes have had a spare time in the first two days of the World Indoor Championships in Birmingham, England. But Chinese hurdler(跨栏运动员) Liu Xiang surprised everyone by taking the bronze medal in the men's 60-meter hurdles.

Liu became the first Chinese male athlete to get a world indoor medal in the 18-year history of the championship and was the only Asian medallist this time. 

"Karaoke (卡拉OK)! That's what I'm going to do. I'm going to get a big group of my friends together and sing. I love both Western and Chinese music but especially Michael Jackson," said Liu after clocking 7.52 seconds in the final to take his place on the podium (领奖台).

"Frankly speaking, I didn't think I was in the shape to win a medal," Liu said. "On my arrival here, my aim was just to fight my way into the final. After I did that, I thought I might just get into the top six. I knew British hurdler Colin Jackson was going to be here and he's the world record holder indoors and out."

"There was also Cuba's reigning (统治的) Olympic champion Anier Garcia and Allen Johnson, from the United States." It was Johnson, the reigning world outdoor champion, who was successful in 7.47 seconds with Garcia second and Jackson finished back in fifth. 

Now Liu will never underestimate his chances again and is aiming for glory in Paris later this year.

"There is the World Championship outdoors in the summer and I have now got to think that I can get a medal. I can never again think that I can't get a medal," he said. Liu's success here marks his rapid transition into the senior ranks.

"The university gives me a lot of flexibility. My courses are very easy and the teachers are very helpful," said Liu.

They might have to be even more accommodating(乐于助人的) now if Liu starts taking on celebrity status, although an invite to his homecoming karaoke party may help.

小题1:The first sentence "Asian athletes have had a spare time in the first two days…" means "________".

A.Asian athletes can do things at their will in the first two days

B.Asian athletes haven't any achievements in the first two days

C.Asian athletes could match athletes from other continents

D.Asian athletes are tired of competing in the first two days小题2:The underlined sentence means "________".

A.I was not satisfied with my performances

B.winning a medal can't show what I have done

C.I didn't think I was able to win a medal

D.I haven't told anyone about my achievements in the hurdles小题3:Liu ________ the World Championship outdoors in Paris later this year.

A.is busy preparing for

B.has great pressure on

C.is more confident of his ability in

D.pays little attention to小题4:Which of the following is TRUE according to the passage?

A.Liu Xiang was the youngest athlete to take part in the competition in England.

B.Liu Xiang was the first Asian medallist to get a world indoor medal.

C.Liu Xiang is not only a good athlete but a music-lover.

D.Liu Xiang finds it hard to get along with teachers in university.

单项选择题