问题 问答题

如图所示,一根细线上端固定,下端拴着一个重量为G的小球,静止时细线保持竖直.现给小球一个水平向右的拉力F,把小球拉到某个位置静止.若当细线与竖直方向的夹角为θ时,水平拉力的大小为F,细线对小球的拉力为T,求F与T.

答案

取小球为研究对象,当细线与竖直方向成θ角时,小球受重力G、水平向右的拉力F和细线

对小球的拉力T的作用,静止时小球所受的三个力的合力为零.

如图所示,把重力分解为沿细线向下的分力G1=

G
cosθ
和水平向左的分力G2=Gtanθ.

根据共点力的平衡条件有

  F-G2=0,T-G1=0,

所以F=Gtanθ,T=

G
cosθ

答:水平拉力的大小为F=Gtanθ,细线对小球的拉力为T=

G
cosθ

阅读理解

     Their reunion was unlike any other.  Graduating from the Macon, Georgia,  class of 1959 gathered

together in 2009, even though many of them never had an opportunity to meet during their high school

days.

     Schools in the 1950's, like most other public locations, were segregated (隔离)in Georgia. Kids were

forced to attend different schools depending on the color of their skin, and those with white skin were

further separated by gender. Fifty years later these people who were not allowed to associate with each

other were finally able to connect.

     It all began with the personal journey of a man named Tom Johnson. He grew up in Macon and

enjoyed a very successful career which included serving as the publisher of the Los Angeles Times and

president of CNN. Tom's life changed drastically over the years, but he still felt the desire to reconnect

with his roots.

     In 2005 Johnson returned to Macon with his son, Wyatt. As the pair drove around town,  the father

recounted high school stories to his son. He talked about how students attended one of three schools:

BallardHudson, Lanier or Miller. Wyatt looked over at his father and said: "Dad  think about how

many friends you missed getting to know."

     Johnson thought about the people he never got the chance to meet. He decided to do something

about it.

     The first step was to find people who graduated from all three schools. Johnson wrote to each person

and proposed they all get together for a luncheon. He explained that even though they were kept apart

during their youth, they didn't have to be separated any more. His classmates received the message with

open hearts. A reunion date was set for October.

      More than 200 former Macon students traveled hundreds of miles to reunite with people who went

to high school with them. It was an unprecedented event, former students coming together to make up

for the time they had lost during the days of segregation.

1. White students in Georgia in the 1950's could ________ .     

A. attend school of a single sex

B. attend school of mixed sexes

C. attend school with students of other skin colors

D. attend any school they like

2. When the classmates received the message from Tom, they ________.

A. were all eager for the reunion

B. were doubtful about his real purpose

C. refused to listen to him

D. all began to miss him

3. We can learn from the passage that Tom Johnson ________.

A. began to miss his old school friends after he retired

B. wanted to reconnect with his roots after the dramatic changes in his life

C. wanted to reconnect with his roots in spite of his successful career

D. began to miss his old school friends after hearing his son's words

4. The passage mainly tells us ________.

A. the success story of Tom Johnson

B. the segregation in Georgia in the 1950's

C. an unusual reunion of old graduates

D. the old school system in Georgia

名词解释