题干

如图所示,物体A在水平力F的作用下,静止在竖直墙壁上.当水平力减小为
F
2
时,物体A恰好沿竖直墙壁匀速下滑.此时物体A所受摩擦力的大小(   )

A:减小为原来的 12

B:和原来一样

C:增大为原来的2倍

D:无法判断

上一题 下一题 0.0难度 选择题 更新时间:2018-01-08 09:38:31

答案(点此获取答案解析)

B

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       Some thing in chocolate could be used to stop coughs and lead to more effective medicines,say UK researchers.

Theirstudy found that theobromine, found in cocoa, was nearly a third more effectivein stopping coughs than codeine, which was considered the best cough medicineat present.

    The   Imperial College London researchers who published their results online said thediscovery could lead to more effective cough treatment. “While coughing is notnecessarily harmful, it can have a major effect on the quality of life, andthis discovery could be a huge step forward in treating this problem,” said Professor Peter Barnes.

Tenhealthy volunteers were given theobromine, codeine or placebo, a pill thatcontains no medicine, during the experiment. Neither the volunteers nor theresearchers knew who received which pill. The researchers then measured levelsof capsaicin(辣椒素),which is used in research to cause coughing and as a sign of how well themedicines are stopping coughs.

Theteam found that, when the volunteers were given theobromine, the capsaicin needto produce a cough was around a third higher than in the placebo group. Whenthey were given codeine they need only slightly higher levers of capsaicin tocause a cough compared with the placebo.

   There searchers said that theobromine worked by keeping down a verve activity(神经活动), which caused coughing. They alsofound that unlike some standard cough treatments, theobromine caused no sideeffects such as sleepiness.