问题 填空题

如图所示,甲、乙、丙、丁四种情况,光滑斜面的倾角都是α,球的质量都是m,球都是用轻绳系住处于平衡状态,则球对斜面压力最大的是______图;球对斜面压力最小的是______图.

答案

甲图中:根据平衡条件得,斜面对小球的支持力N1=mgcosα<mg.

乙图中:受力如图,由平衡条件得,N2=

mg
cosα
>mg

丙图中:斜面对小球的支持力N3=0.

丁图中:设绳子与斜面的夹角为θ,绳子的拉力大小为F,则斜面对小球的支持力N4=mgcosα-Fsinθ<mg

根据牛顿第三定律得知,斜面对小球的支持力与小球对斜面的压力大小相等.

所以球对斜面压力最大的是乙图,压力最小的是丙图.

故答案为:乙,丙

单项选择题

St. Patrick’s Day is celebrated on March 17, his religious feast day and the anniversary of his death in the fifth century. Legend has it that this patron saint had given a sermon from a hilltop that drove all the snakes from Ireland. He also used the three-leafed shamrock to represent the Trinity how the Father, the Son, and the Holy Spirit could all exist as separate elements of the same entity—and converted the pagans to Christianity. The Irish have observed this day as a religious holiday for hundreds of years. People wear green in memory of the Emerald Isle and wear shamrocks.

The first St. Patrick’s Day parade, however, took place not in Ireland, but in the United States. Irish soldiers serving in the English military marched through New York City on March 17, 1762. Along with their music, the parade helped the soldiers, as well as fellow Irishmen serving in the English army, to reconnect with their Irish roots.

Over the next thirty-five years, Irish patriotism among American immigrants flourished, prompting the rise of so-called "Irish Aid" societies, like the Friendly Sons of Saint Patrick and the Hibernian Society. Each group would hold annual parades featuring bagpipes (which actually first became popular in the Scottish and English armies) and drums.

Up until the mid-nineteenth century, most Irish immigrants in America were members of the Protestant middle class. When the Great Potato Famine hit Ireland in 1845, close to a million poor, uneducated, Catholic Irish began to pour into America to escape starvation. Despised for their religious beliefs and funny accents by the American Protestant majority, the immigrants had trouble finding even menial jobs. When Irish Americans in the country’s cities took to the streets on St. Patrick’s Day to celebrate their heritage, newspapers portrayed them in cartoons as drunk, violent monkeys.

However, the Irish soon began to realize that their great numbers endowed them with a political power that had yet to be exploited. They started to organize, and their voting block, known as the "green machine," became an important swing vote for political hopefuls. Suddenly, annual St. Patrick’s Day parades became a show of strength for Irish Americans, as well as a must-attend event for a slew of political candidates. In 1948, President Truman attended New York City’s St. Patrick’s Day parade, a proud moment for the many Irish whose ancestors had to fight stereotypes and racial prejudice to find acceptance in America.

The St. Patrick’s Day parade first appeared in the U. S. probably because Irish soldiers ().

A. needed to remember their Irish roots and fellowship

B. needed a ceremony to go with their military music

C. wanted to find a new form of celebrating an old holiday

D. wanted to celebrate their military victory in the U. S.

填空题