问题 问答题

我国北方冬季天气寒冷,雪后路面结冰会给交通运输带来极大的影响.若汽车橡胶轮胎与普通路面的动摩擦因数为0.9,与冰面的动摩擦因数为0.1.当汽车以某一速度沿水平普通路面行驶时,急刹车后(设车轮立即停止转动),汽车要滑行8m才能停下.重力加速度g取10m/s2.求:

(1)汽车行驶时的速度大小;

(2)汽车在冰面上以同样速度行驶,急刹车后滑行的距离比普通路面增大多少.

答案

(1)汽车普通路面上刹车时,只有摩擦阻力对汽车做负功,根据动能定理有:

-μ1mgx1=0-

1
2
m
v20

得汽车的初速度为:v0=

2μ1gx1
=
2×0.9×10×8
m/s=12m/s

(2)汽车以初速度v0=12m/s在冰面上刹车时,只有阻力对汽车做负功,根据动能定理有:

-μ2mgx2=0-

1
2
m
v20

得汽车刹车位移为:x2=

1
2
v20
μ2g
=
1
2
×122
0.1×10
m=72m

比普通路面多滑行:△x=x2-x1=72-8m=64m

答:(1)汽车行驶时的速度大小为12m/s;

(2)汽车在冰面上以同样速度行驶,急刹车后滑行的距离比普通路面增大64m.

阅读理解

Are two holidays not enough? Do you dream of three, or even four? (61)这不再是梦想! Last month, the State Council(国务院) encouraged primary and high schools to give students a spring or autumn break without changing the total number of student vacation days.

Students in China currently only have long holidays in two seasons —summer and winter. These holidays are usually 10 to 12 weeks in total.“If we had spring and autumn breaks, (62)I would have time to taste all four seasons instead of burying (埋) myself in my studies,”Gao Yiran, a 15-year-old boy told China Daily. Seasonal holidays during autumn and spring may be something new in China, but not in other countries.

In the US, students usually have three holidays. Summer holiday is the longest one, and runs from mid-June until early September. Summer camp is one of kids’ favorite places to go to, where they can make new friends and try some new activities. Winter holiday starts from mid-December and ends in early January. Most kids and their parents go to spend time together over the Christmas period. Spring break comes in mid- March and (63) it is usually a 10-day holiday.

Schools in Japan start in April, as most people think that spring is the perfect time for new things. For school children ,summer holiday lasts from July 20 to August 31. Most kids are busy with club activities including sports and dancing. Spring break starts at the end of March and continues till early April. It is a good chance for a short family trip around the city.

阅读短文,按要求完成下面的任务。

任务一 将61处的中文翻译成英文。

任务二 将62处斜体部分的英文翻译成中文。

任务三 写出63处“it”指代的内容。

任务四 64.What do Japanese kids usually do in their spring break?

任务五 65.为该文段拟写一个标题。

单项选择题