问题 选择题

如图a所示,石料在钢绳拉力的作用下从水面上方以恒定的速度下降,直至全部没入水中。其钢绳拉力F随时间t变化的规律如图b所示。若不计水的摩擦力,则该   

A.石料的重为1400N

B.石料的重为500N

C.石料的密度为2.8×103kg/m3

D.石料的密度为3.2×103kg/m3

答案

AC

题目分析:图可知,

当石块没有浸入水中时,拉力等于重力,即:F=G=1400N,故A正确;

石料的质量:

当石块全浸入后,拉力等于重力减去浮力,

根据阿基米德原理,

得:

因为石料完全浸没,

石料的密度:

,故C正确.

故选AC。

点评:解决本题的关键是通过图乙确定石料的重力及钢丝绳受到的拉力,并会利用称重法计算出石料受到的浮力.

阅读理解

In 1859,during the Gold Rush,a twenty- year-old immigrant(移民)from Bavaria named Levi Strauss stepped off the boat in San Francisco. He had with him a special cloth called Serge de Nimes that was later called denim in America. Levi Strauss hoped to sell the denim as material to make tents and covers for wagons to the men who were going to the goldfields to look for gold.

“You should have brought trousers to sell. In the goldfields we need strong trousers that don‘t wear out,”one young miner advised Strauss. So Levi Strauss took some of his denim to the nearest tailor and had him make the miner a pair of trousers. The miner was so pleased that he told other miners about the wonderful newcomer,Levi,and soon Levi Strauss had to open a shop,producing enough trousers for the miners. The miners wanted trousers that were comfortable to ride in and were low cut so that they could bend over easily to pick up the gold from under their feet. Besides they wanted the trousers to have big useful pockets. One miner complained that the gold in his pockets kept tearing them. So Levi put metal corners in the pockets to make them stronger. Very soon,miners and cowboys from all over came to get fitted up with Levi‘s trousers. Today more than a hundred years later, Levi’s trousers walk the world a Levi‘s blue jeans(牛仔裤)。

小题1:The denim cloth was originally(起初)meant for____.

A.making tent covers and wagons

B.making tents for wagons

C.making trousers for the miners

D.making wagon covers and tents小题2:What kind of material do you think is denim?

A.A strong type of cloth that is not torn by strong wears.

B.A special type of cloth that cannot be washed away by rain.

C.A smooth type of cloth that is for underwear.

D.A wonderful type of cloth that is as good as metal.小题3:A miner advised Strauss to sell trousers because ____.

A.the miners didn‘t have enough trousers

B.the miner‘s trousers had not been strong enough

C.the miner‘s trousers did not wear out easily

D.the tailor on the goldfield could not make trousers小题4:Why did the miners need strong pockets in the trousers?

A.Their pockets kept tearing the gold.

B.Their pockets were not made of metal.

C.The gold they found was too sharp and heavy to tear their pockets.

D.The gold they collected easily tore their pockets.

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