问题 计算题

如图所示,放在长木板上的木块质量为m,当木板与水平方向夹角为α时,木块静止在长木板上。

(1)求此时木块所受的弹力和摩擦力各为多大?

(2)当把长木板的倾角增大到θ(θ>α)时,木块刚好沿长木板匀速下滑,求木块和木板之间的动摩擦因数。

答案

解:(1)木块受力情况如图所示,根据重力的作用效果把重力分解为F1、F2

由二力平衡可得FN=F2=mgcosα,Ff=F1=mgsinα

(2)当倾角增大到θ时,木块刚好匀速下滑,木块受力情况仍如上图。由二力平衡可得

在沿斜面方向上:Ff=F1=mgsinθ

在垂直于斜面方向上:FN=F2=mgcosθ

由Ff=μFN,得μ==tanθ

不定项选择 案例分析题
阅读理解

Once a year we show our love for our fathers by celebrating(庆祝) Father’s Day. This is the time when you have the chance to show your dad how much you love him. On Father’s Day, children usually give gifts to their fathers and take them out for a nice meal. But when and how did Father’s Day get started?

Father’s Day was introduced in the early part of the 20th century. It was to show love for our fathers in the same way that people did to mothers on Mother’s Day. This tradition began in Spokane, Washington, from an idea of a woman named Sonora Smart Dodd.

Mrs Dodd was raised by her father after her mother died. Looking back on all the things that he did for the family made Mrs Dodd realize just how special her father was. She realized that her father was a very loving man who should be honored and praised for all the things that he did for his family. Since her father was born in the month of June, Mrs Dodd decided to celebrate Father’s Day on June 19th, so the first Father’s Day was held in June, 1910.

By the year 1926, a committee (委员会) was formed in New York City to honor Father’s Day, and in 1956, it was recognized as a day of celebration. President Nixon made Father’s Day a national holiday in the year 1972.This is when it was decided that it would always be celebrated on the third Sunday of every June.

1. What is the best title for this passage?

A. The history of Father’s Day.           B. The story of Sonora Smart Dodd.

C. Something about Mother’s Day.      D. How to show your love for your father.

2. According to Paragraph 1, what do children usually do on Father’s Day?

A. Spend the whole day with their fathers.

B. Say aloud that they love their fathers.

C. Cook something nice for their fathers.

D. Prepare gifts for their fathers.

3. We learn that Father’s Day _______.

A. started from a story made up by a woman

B. is celebrated on the first Sunday of every June

C. is surely only celebrated by children in America

D. started from a woman’s wish to honor and praise her father

4. Americans have been celebrating Father’s Day as a national holiday since_____.

A. 1910    B. 1926    C. 1956    D. 1972