问题 问答题

如图所示,半径为R的圆盘匀速转动,在距半径高度h处以平行OB方向水平抛出一小球,抛出瞬间小球的初速度与OB方向平行,为使小球和圆盘只碰撞一次且落点为B,求:

(1)小球的初速度大小;

(2)圆盘转动的角速度.

答案

(1)设小球在空中的飞行时间为t1,初速度为v0,圆盘的角速度为ω,

h=

1
2
gt2得:

小球平抛时间t1=

2h
g

小球水平方向分运动R=v0t1

可得v0=

R
t1
=R
g
2h

(2)当OB再次与v0平行时,圆盘运动时间t2=nT(n=1,2,3,4,…),

T=

ω

依题意t1=t2

得:

2h
g
=
2nπ
ω

解得ω=nπ(n=1,2,3,4,…)

答:(1)小球的初速度大小为R

g
2h

(2)圆盘转动的角速度为ω=nπ (n=1,2,3,4,…)

阅读理解

Why should I teach my children history? That sounds like a stupid question to even ask.But,as I hear different home schooling teachers discuss history,I get the idea that there may be different reasons for teaching history.Let me briefly explain the three good reasons for studying history and two bad reasons for studying history.

The major reason I see for studying history is that we can learn from the past.I believe that the world would be a much better place if more people understood the successes and failures of the past and the things that made these successes and failures.However,as the unfortunately(不幸地)true statement goes ‘‘the one thing we seem to learn from history is that we don't seem to

learn from history”,perhaps at least in teaching history, to my children I can do a small part in changing this.

A second major reason for studying history is that it is hard to understand the current(目前的)political climate in the absence of an understanding of its historical context(背景).We can not

even understand why we are and where we are without history,much less try to figure out where we are going or how we should get where we want to be.

I teach my children history,for one more reason.I bought a set of historical audio tapes for our children.My seven-year-old son listened to them over and over.It was my hope that he would

become inspired by the accomplishments(成就)of people like the Wright brothers to accomplish things by himself.I think that it is good that we celebrate the accomplishments of people like Martin Luther King Jr. By doing so,young people are called on to stand for the principles that he stood for and accomplish what he didn’t accomplish.I also think that by studying people like Adolph Hitler,people can learn to stand against the things that he stood for.

小题1:What message can we get from the underlined sentence in Paragraph 2 ?

A.Many people aren’t clever enough to learn well from the past.

B.Many people fail to make good use of history and make the same mistakes.

C.Many people feel it hard to understand history.

D.Many people have no interest in studying history.小题2:In Paragraph 3,the author shows that history is useful because

A.it makes the current political situation go smoothly

B.it helps us realize the importance of historical events

C.it helps us understand why things are the way they are

D.it helps people accept the present situation where they live小题3:Some historical figures are mentioned in the last paragraph to show____.

A.people can be inspired to do good and also learn to fight against evil

B.people may also learn from bad historical figures

C.more celebrations should be held to honor their achievements

D.today’s people can also achieve what they achieved小题4:What would be talked about in the following paragraph?

A.How to teach history effectively.

B.Some negative reasons for studying history.

C.How to get more people to study history.

D.Some bad historical figures.

单项选择题