问题 多选题

下列说法中正确的是(  )

A.重心一定在物体内部

B.相互接触的物体间一定有弹力

C.不受到弹力的物体不会受到摩擦力

D.摩擦力的方向不一定与物体运动方向相反

答案

A、重心的位置不一定在物体内部,也可以在物体之外,如空心圆环,故A错误;

B、相互接触的物体间要有弹力,还要发生弹性形变,故B错误;

C、弹力是产生摩擦力的一个必要条件,没有弹力就没有摩擦力,但有弹力不一定有摩擦力,故C正确;

D、摩擦力的方向与物体相对运动或相对运动趋势方向相反,不是与物体运动方向相反,故D错误.

故选CD

单项选择题
单项选择题

If you leave a loaded weapon lying around, it is bound to go off sooner or later. Snow-covered northern Europe heard the gunshot loud and clear when Russia cut supplies to Ukraine this week as part of a row about money and power, the two eternal battlegrounds of global energy. From central Europe right across to France on the Atlantic seaboard, gas supplies fell by more than one-third. For years Europeans had been telling themselves that a cold-war enemy which had supplied them without fail could still be depended on now it was an ally ( of sorts). Suddenly, nobody was quite so sure.

Fearing the threat to its reputation as a supplier, Russia rapidly restored the gas and settled its differences with Ukraine. But it was an uncomfortable glimpse of the dangers for a continent that imports roughly half its gas and that Gérard Mestrallet, boss of Suez, a French water and power company, expects to be importing 80% of its gas by 2030--much of it from Russia. It was scarcely more welcome for America, which condemned Russia’s tactics. And no wonder: it consumes one-quarter of the world’s oil, but produces only 3% of the stuff. Over the coming years, the world’s dependence on oil looks likely to concentrate on the Middle East, particularly Saudi Arabia. Russian oil had seemed a useful alternative.

Fear of the energy weapon has a long history. When producers had the upper hand in the oil embargo of 1973-74, Arab members of the Organisation of the Petroleum Exporting Countries (OPEC) cut supply, sowing turmoil and a global recession. When consumers had the upper hand in the early 1990s, the embargo cut the other way. After Saddam Hussein invaded Kuwait in 1990, the world shut in 5m barrels a day (b/d) of production from the two countries in an attempt to force him out. With oil costing $ 60 a barrel, five times more than the nominal price in 1999, and spot prices for natural gas in some European and American markets at or near record levels, power has swung back to the producers for the first time since the early 1980s. Nobody knows how long today’s tight markets will last. "It took us a long time to get there and it will take us a long time to get back," says Robin West, chairman of PFC Energy in Washington. A clutch of alarmist books with titles such as "The Death of Oil" predict that so little oil is left in the ground that producers will always have pricing power. The question is how worried consumers should be. What are the threats to energy security and what should the world do about them The answers suggest a need for planning and a certain amount of grim realism, but not for outright panic.

The word "now" of the first paragraph denotes()

A. because of the fact (that)

B.for the time being

C. currently

D. at present