问题 选择题

以下列举了几种物体的运动,若都忽略空气对运动物体的阻力,则其中不遵守机械能守恒定律的是(  )

A.物体沿着斜面匀速下滑

B.物体由光滑斜面顶端滑到斜面底端

C.在竖直方向弹簧振子所做的简谐运动

D.铅球运动员抛出的铅球从抛出到落地前的运动

答案

A、物体沿着斜面匀速下滑,动能不变,势能减小,所以物体的机械能不守恒,故A错误.

B、物体由光滑斜面顶端滑到斜面底端,,除了重力之外没有其他的外力对物体做功,所以机械能守恒,故B正确.

C、在竖直方向弹簧振子所做的简谐运动,只有动能和势能相互转化,所以机械能守恒,故C正确.

D、铅球运动员抛出的铅球从抛出到落地前的运动,只受到重力的作用,所以机械能守恒,故D正确.

题目选不遵守机械能守恒定律的,故选A.

问答题 简答题
单项选择题

The Frugal Gourmet Cooks American

by Jeff Smith

Our real American foods have come from our soil and have been used by many groups— those who already lived here and those who have come here to live. The Native Americans already had developed an interesting cuisine using the abundant foods that were so prevalent.

The influence that the English had upon our national eating habits is easy to see. They were a tough lot, those English, and they ate in a tough manner. They wiped theft mouths on the tablecloth, if there happened to be one, and they ate until you would expect them to burst. European travelers to this country in those days were most often shocked by American eating habits, which included too much fat and too much salt and too much liquor. Not much has changed! And, the Revolutionists refused to use the fork since it marked them as Europeans. The fork was not absolutely common on the American dinner table until about the time of the Civil War, the 1860s. Those English were a tough lot.

Other immigrant groups added their own touches to the preparation of our New World food products. The groups that came still have a special sense of self-identity through their ancestral heritage, but they see themselves as Americans. This special self-identity through your ancestors who came from other lands was supposed to disappear in this country. The term melting pot was first used in reference to America in the late 1700s, so this belief that we would all become the same has been with us for a long time. Thank goodness it has never worked. The various immigrant groups continue to add flavor to the pot, all right, but you can pick out the individual flavors easily.

The largest ancestry group in America is the English. There are more people in America who claim to have come from English blood than there are in England. But is their food English Thanks be to God, it is not! It is American. The second largest group is the Germans, then the Irish, the Afro-Americans, the French, the Italians, the Scottish, and the Polish. The Mexican and American Indian groups are all smaller than any of the above, though they were the original cooks in this country.

Which of the following statements is true().

A. People from other cultures or nations start to lose their self-identity once they get settled down in America.

B. The "melting pot" is supposed to melt all the foods but in reality it doesn’t.

C. The special sense of self-identity of people from other countries can’t be main- mined once they become Americans.

D. The "melting pot" finds it capable of melting all the food traditions into the American tradition.