问题 问答题

风洞实验室能产生大小和方向均可改变的风力.如图所示,在风洞实验室中有足够大的光滑水平面,在水平面上建立xOy直角坐标系.质量m=0.5kg的小球以初速度v0=0.40m/s从O点沿x轴正方向运动,在0~2.0s内受到一个沿y轴正方向、大小F1=0.20N的风力作用;小球运动2.0s后风力方向变为y轴负方向、大小变为F2=0.10N(图中未画出).试求:

(1)2.0s末小球在y方向的速度大小和2.0s内运动的位移大小;

(2)风力F2作用多长时间,小球的速度变为与初速度相同;

(3)小球回到x轴上时的动能.

答案

(1)球受重力、支持力和风的推力,在x方向不受外力,做匀速直线运动,在y方向受到恒定的力,故y方向做匀变速直线运动;

设在0~2.0s内小球运动的加速度为a1,则根据牛顿第二定律,有F1=ma1

解得:a1=

F1
m
=0.40m/s2根据运动学公式,有

Vy=a1t=0.80m/s

Sy=

1
2
a1t2=0.80m

Sx=V0t=0.80m

故合位移为

S=

S2x
+
S2y
=0.8
2
m≈1.13m即2.0s末小球在y方向的速度大小为0.80m/s,2.0s内运动的位移大小为1.13m.

(2)根据牛顿第二定律,风力F2作用时物体的加速度为:a2=

F2
m
=0.20m/s2

风力F2作用为:t=

vy
a2
=4.0s

故风力F2作用4s的时间,小球的速度变为与初速度相同.

(3)小球回到x轴上,则y方向的分位移为零,根据运动学公式,有

0=Sy+

v
2y
-
v2y
2(-a2)
解得

vy=-

2a2Sy+
v2y
=-0.4
6
m/s

故合速度为

v=

v2x
+v
2y
=
0.42+(0.4
6
)
2
=0.4
7
m/s

故小球回到x轴上时的动能为Ek=

1
2
mv2=0.28J.

阅读理解

阅读理解。

     I felt very sad because I was not able to get the ticket to the film Hero last Friday. I learned from the

newspaper that tickets could be bought at the cinema box office in Richland Hills any day between 10:00 a.m.

and 4:00 p. m. Since I work from 9:00 a. m. to 5:30 p. m., the only time I could go to the cinema was during

my 45 minutes' lunchtime. It is a pity that the cinema is on the other side of the town and the bus service

between my company and Richland Hills is not very good. But if you are lucky, you can make the round trip

in 45 minutes.

     Last Monday, I stood at the bus stop for 15 minutes waiting for a bus. By the time I saw one come round

the corner, there was not enough time left to make the trip, so I had to go back. The same thing happened on

Tuesday, and again on Wednesday. On Thursday my luck changed. I got on a bus right away and arrived at

the cinema in twenty minutes. But when I got there, I found a long line of people at the box office. I heard

one man said he had been waiting in line for over an hour. I found I would not have enough time to wait in line.

I caught the next bus and went back across the town.

     By Friday I understood my only hope was to make the trip by car. It was expensive, but I felt it would be

worth it to see the film. The trip by car only took 10 minutes, but it felt like an hour to me. When I reached

the cinema, I was delighted to see that nobody was waiting in line. But I quickly found out that it was because

they had already sold out all the tickets.

1. It seemed that the writer of the story works as a ____. [ ]

A. worker

B. doctor

C. farmer

D. clerk

2. The writer tried to go to the cinema every day, but really got there ____. [ ]

A. five times

B. three times

C. four times

D. twice

3. We can infer from the underlined sentence that the writer was ____. [ ]

A. worried

B. sorry

C. surprised

D. pleased

4. In the end the writer got ____ ticket(s). [ ]

A. one

B. two

C. three

D. no

5. When the writer reached the cinema, he was delighted because ____. [ ]

A. he understood

B. it was expensive, but he felt it would be worth it to see the film

C. the trip by car only took 10 minutes

D. he saw that nobody was waiting in line

多项选择题