问题 多选题

一条河宽400m,船在静水中的速度是4m/s,水流速度是5m/s,则(  )

A.该船一定不能垂直河岸渡到对岸

B.当船头垂直河岸横渡时过河所用时间最短

C.船横渡到对岸时,船对岸的最小位移为400m

D.该船渡河的速度最小是4m/s

答案

设船在静水中的航速为v1,水流的速度v2

A、由题,船在静水中的航速小于水流的速度,根据平行四边形定则可知,船的合速度方向不可能垂直于河岸,则

船不能到达正对岸.故A正确;

B、将小船的速度分解为垂直河岸和沿河岸方向,在垂直于河岸的方向上,河宽一定,当在该方向上的速度最大时,渡河时间最短,所以当船头方向垂直河岸,在该方向上的速度等于静水航速,时间最短,

为tmin=

d
v1
=100s.故B正确;

C、船实际是按合速度方向运动,由于v1、v2的大小一定,根据作图法,由三角形定则分析可知,当船相对于水的速度v1与合速度垂直时,合速度与河岸的夹角最大,船登陆的地点离船出发点的最小距离.设船登陆的地点离船出发点的最小距离为S.根据几何知识得

d
s
=
v1
v2
代入解得  S=500m.故C错误,

D、船在静水中的速度是4m/s,水流速度是5m/s,则最小速度为1m/s,故D错误;

故选:AB

单项选择题
阅读理解

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

1________

Examinations are almost always stressful Students rarely know exactly what to expect on the test, and those who suffer from exam fear or exam anxiety can see their grades suffer as a result of this stress. For some students,exam fear causes them to “ seize up ’’ and forget what they have studied.

2______Good study habits and learning how to relax can help students overcome exam fear.

3     ______According to Penn State University and Kids Health,proper study habits and preparation are the keys to cutting out exam fear. Avoid last-minute “cramming” for exams and actively listen while in class.

Focus on positive thoughts, relax the night before your test and reward yourself after the test is over. Often, people who suffer from exam anxiety cannot shut out negative thoughts or emotions about the test, and this can contribute to feeling more anxiety than normal 4______

Keep your mind and body healthy by getting  enough sleep, eating well and exercising. Healthier people who are well-rested often do better on exams.

5_____ Exam fear is normal When that anxiety severely interferes (干扰)with your ability to take the test, however, outside help might be needed. Ask your teachers and family for support and help.

A.How To Succeed hi An Exam

B.How To Overcome Exam Fear

C.Develop good study habits.

D.Understand that you are not alone and ask for help if necessary.E.   Remind yourself that it’s just one test and that you can get through it

F.   Battling this type of performance anxiety can be hard, but it is not impossible.

G.   Remind yourself to read the instructions carefully.