问题 填空题

潜水艇的上浮和下潜是靠______来实现,当潜水艇静止在水面上时,受到的浮力______(填“大于”、“小于”或“等于”)重力,开始下潜至刚没入水中时受到的浮力______(填“变大”、“变小”或“不变”);当潜水艇从水中上浮至刚露出水面时,受到的浮力______(填“变大”、“变小”或“不变”).

答案

在水中,潜水艇受到水的浮力和重力的作用,

∵Fvg,潜水艇的体积不变,上浮或下潜时,排开水的体积不变,

∴潜水艇受到水的浮力不变;

把压缩空气压入潜水艇的压力舱,将海水排出,潜水艇自重G减小,当F>G时,潜水艇上浮;

打开压力舱的阀门,让海水进入压力舱内,潜水艇自重G增大,当F<G时,潜水艇下沉;

当F=G时,潜水艇悬浮,可以停留在任何深度;

由此可见,潜水艇能够上浮和下沉是通过改变自重来实现的;

当潜水艇静止在水面上时,根据漂浮条件知:受到的浮力等于重力,

开始下潜至刚没入水中时,排开水的体积逐渐变大,根据Fvg,则受到的浮力变大;

当潜水艇从水中上浮至刚露出水面时,潜水艇处于浸没状态,排开水的体积没有改变,所以受到的浮力不变.

故答案为:改变自重;等于;变大;不变.

阅读理解

阅读理解。

     "Depend on yourself" is what nature says to every man. Parents can help you. Teachers can help you.

Others still can help you. But all these people only help you to help yourself.

      There have been many great men in history. Many of them were very poor when they were young,

and had no uncles, aunts, or friends to help them. School were few and not very good. They could not

depend on them for education. They tried their best to learn something and never gave up till they became

well-known. One of the most famous teachers in England used to tell his pupils, "I cannot make worthy

men of you, but I can help you make men of yourselves."

      Some young men don't try their best to make themselves valuable to the human beings. If they see

their weak points and change their ways, they can be successful. They are nothing now, and will be

nothing as long as they live, unless they accept the advice of their parents and teachers, and depend on

themselves.

1. Many of great men succeeded because _____. [ ]

A. they had got good education

B. they had no uncles, aunts or friends to help them

C. they worked hard

2. A teacher can _____ according to the famous teacher in England. [ ]

A. make his pupils men of strength and courage

B. make his pupils worthy men

C. help his pupils become famous in the world 

3. If young people depend on their own efforts, _____.  [ ]

A. they are sure to be very famous in the world

B. they can be successful in their lives

C. they no longer need help

4. According to the passage, which is the following is right?  [ ]

A. All these people only help you to make you famous.

B. If young people see their weak points,they must be successful.

C. If young people accept the advice of their parents and teachers,

    and depend on themselves, they can be successful.

5. From the passage, we can learn that _____. [ ]

A. A successful person should depend on his teacher.

B. No matter what you do, depend on youself.

C. If you want to be famous,you cannot depend on education.

单项选择题