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问题 填空题

大小分别为7N、5N、3N的三个共点力分别作用在同一个物体上，则三个力合力的最大值为______N，合力的最小值为______N．

答案

当三个力作用在同一直线、同方向时，三个力的合力最大，即F=7N+5N+3N=15N．

当三个力作用在一个物体上，不在一条直线，并且夹角可以改变，7N与5N的最大值为12N，最小值为2N；

3N能在7N与5N最大与最小之间，所以能使物体处于静止状态，故此时三个力的合力为零，即它们合力的最小值为0．

故答案为：15，0．

单项选择题 A型题

下列关于抛射剂的叙述错误的是（）

A.抛射剂可分为压缩气体与液化气体

B.压缩气体常用作喷雾剂动力

C.抛射剂在气雾剂中起到动力作用

D.抛射剂可兼作药物的溶剂或稀释剂

E.气雾剂喷雾粒子大小、干湿与抛射剂用量无关
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单项选择题

Believe it or not，optical illusion can cut highway crashes.
Japan is a case in point．It has reduced automobile crashes on some roads by Rearly 75 percent using a simple optical illusion．Butst．ripes,called chevrons(人字形)，painted on ther rads make drivers think that they are driving faster than they really are，and thus drivers slow down.
Now the American Automobile Association Foundation for Traffic Safety in Washington D．C. is planning t。repeat Japan’s success．Starting next year，the foundati。n will paint cheyrons and。ther pattems of stripes on selected roads around the country to test how well the patterns reduce highway erashes．
Excessive(too great)speed plays a major role in as much as one fifth of all fatal traffic accidents， according to the foundation．To help reduce those accidents，the foundation will conduct its tests in areas where speed-related hazards(danger)are the greatest curves，exit slopes，traffic circles，and bridges．
Some studies suggest that straight，horizontal bars painted across roads can initially cut the average speed of drivers in half．However,traffic。ften returns to full speed within months as driVers become used to seeing the painted bars．
Chevrons，scientists say，not only give drivers the impression that they are driving faster than they reallv are but als0 make a lane appear to be narrower．The result is a longer lasting reduction in highway speed and the number of traffic accidents．

The passage mainly discusses______.

A．a new way of highway speed control
B．a new pattern for painting highways
C．a new way of training drivers
D．a new type of optical illusion
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