问题 问答题

如图所示,长为l的轻绳一端系于固定点O,另一端拴一个质量为m的小球。将小球从O点正下方l/4处以一定的初速度v0水平向右抛出。经过一段时间,绳被拉直,在极短的时间内小球的速度变为0,此后小球以O为悬点在竖直平面内摆动。已知绳刚被拉直时与竖直方向成60°角。求

①小球水平抛出时的初速度v0

②在绳被拉直的短暂过程中,悬点O受到绳拉力的冲量。

答案

(1)     (2),方向沿绳向下

①从开始运动到绳被拉直,小球竖直方向的位移为h=l/4, 水平方向的位移s=, 经历时间t=

抛出时的初速度 v0=

②绳刚被拉直时,小球的速度大小 v=,方向与竖直方向的夹角θ=arctan=60°。说明此时球的速度方向沿绳的方向。

根据动量定理,绳对小球的冲量Imv=

根据牛顿第三定律,绳对O点的作用力的冲量大小为,方向沿绳向下。

选择题
阅读理解

第三部分:阅读理解(共20小题;每小题2分满分40分)

US first lady Michelle Obama is among the world’s 100 most powerful women on a list topped by German premier Angela Merkel for four years in a row, according to a Forbes ranking released recently.

Michelle Obama debuted(初次露面)at No.40,coming in ahead of talk show host Oprah Winfrey at 41 and Britain’s Queen Elizabeth at 42.

Sheila Bair, chair of the US Federal Deposit Insurance Corp, which insures bank deposits,remained NO.2 after debuting on the Forbes list last year. She has gained increased fame as the US recession(经济衰退) lasts.

The list is based on factors such as economic impact, media reach and career accomplishments. Former US Secretary of State Condoleezza Rice fell off the list after leaving office in January. Rice’s successor (继任者),Hillary Clinton, came in at NO.36, dropping from No.28 last year when her presidential bid made her the woman with the highest public profile on the list.

“women im power are rising to leadership positions in business, government and philanthropy (慈善)by making daring and unconventional moves,”Forbes said.“Gone are the days of women feeling they must stick with one employer and patiently wait for promotions.”

“Highly ambitious women… are moving across companies and industries, making big leaps with each change, and repositioning themselves for opportunities that allow them to gain a breadth of experience,” the business magazine said.

Merkel,55, became the first female premier of Germany in 2005 and succeeded in retaining(保留) power in a federal election not long ago.

Michelle Obama,45,has won fans for her down-to-earth personality, her support of causes including healthy eating and the arts.

1.Condoleezza Rice is not included on the list probably because_______________ .

A. she exercises less influence

B. she keeps her new job a secret

C. she has become an ordinary woman

D. she is silent after leaving office

2.Hillary Clinton was ranked No.28 last year because_____________

A. she was the former first lady

B. she was running for president then

C. she frequently appeared in public

D. she got support from her husband

3. We learn from the passage that powerful women today_____________

A. change their jobs more frequently

B. are better political leaders than men

C. are not content with being housewives

D. have changed their traditional concept

4. What would be the best title for the passage?

A. The most powerful women in the world

B. Women in power are rising to leadership postitions

C. Forbes: Germany’s Merkel again most powerful woman

D. Forbes: The rise and fall of the women in power

D. The latest source of biofuel