问题 选择题

甲、乙两物体相距s,它们同时同向运动,乙在前面做初速度为0、加速度为a1的匀加速直线运动,甲在后面做初速度为v0、加速度为a2的匀加速直线运动,则

A.若a1=a2,它们只能相遇一次

B.若a1>a2,它们可能相遇两次

C.若a1>a2,它们只能相遇一次

D.若a1<a2.它们不能相遇

答案

AB

A、甲从乙的旁边通过说明相遇时甲的速度大于乙的速度,若a1=a2,则以后甲的速度将都大于乙的速度,故不会再次相遇,故A正确;

BC、若a1>a2,则甲经过乙的旁边以后,乙的速度增加更快,故乙将会追上甲,从而再次相遇,故B正确,C错误;

D、若a1<a2,则甲的速度增加的比乙快,故甲一定会追上乙,此后乙再也不可能追上甲,故相遇一次。故D错误。

故选AB.

问答题 简答题
阅读理解

阅读理解。

    The more we do a thing, the more we tend to like doing it; and if we do not continue to do it, we feel

uncomfortable. This is called the force of habits, and the force of habits should be fought against.

     Habits, whether good or bad, are gradually formed. When a person does a certain thing, he is driven

by some unseen force to do the same thing repeatedly, then a habit is formed. Once a habit is formed, it

is difficult, and sometimes impossible, to get rid of. It is therefore very important that we should pay great

attention to the formation of habits. Children often form bad habits, some of which remain with them as

long as they live. Older persons also form bad habits as long as they live, and sometimes become ruined

by them.

    There are other habits which, when formed in early life, are of great help. Many successful men say

that much of their success has something to do with certain habits in early life, such as early rising, honesty

and so on. 

    Among the habits which children should not form are laziness, lying, stealing and so on. They are all

easily formed habits. Unfortunately, older persons often form bad habits, which should have been avoided.

    We should keep from all these bad habits, and try to form such habits as will be good for others and

ourselves. Remember: Habits cure habits.

1. Generally speaking, it is difficult for one to _____. [ ]

A. form good habits and easy for him to form bad habits

B. form bad habits and easy for him to form good habits

C. get rid of bad habits and easy for him to form good habits

D. get rid of good habits and easy for him to form bad habits

2. We should pay much attention to the formation of habits because _____. [ ]

A. habits are of great help to everyone of us

B. a man can never get rid of the bad habits

C. we are forced to do them repeatedly

D. it's hard and even not possible to get them away

3. _____ are formed little by little. [ ]

A. Good habits

B. Bad habits

C. Both good habits and bad habits

D. Neither good habits nor bad habits

4. According to the passage, early rising or honesty _____. [ ]

A. is such a habit as should not have been got rid of

B. is an early formed habit that is with one as long as he lives

C. is such a habit as will be kept from

D. is a habit that has something to do with success