如图所示,在倾角为α的斜面上,放一质量为m的小球,小球和斜坡及挡板间均无摩擦,当

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问题:

如图所示,在倾角为α的斜面上,放一质量为m的小球,小球和斜坡及挡板间均无摩擦,当挡板绕O点逆时针缓慢地转向水平位置的过程中,则有(  )

A.斜面对球的支持力逐渐增大

B.斜面对球的支持力逐渐减小

C.挡板对小球的弹力先减小后增大

D.挡板对小球的弹力先增大后减小

考点:滑动摩擦力、动摩擦因数力的合成共点力的平衡
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已知2a=3b=k(k≠1),且2a+b=ab,则实数k的值为

A.6

B.9

C.12

D.18

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管理跨度与管理层次的正确关系是( )。

A.层次减少跨度不变

B.层次越多跨度越小

C.层次越少跨度越小

D.层次越大跨度越大

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(三)

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 nearly 75 percent using a simple optical illusion. But stripes, called chevrons, painted on the roads 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 to repeat Japan’s success. Starting next year, the foundation will paint chevrons and other patterns of stripes on selected roads around the country to test how well the patterns reduce highway crashes.

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 often 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 really are but also make a lane appear to be narrower. The result is a longer lasting reduction in highway speed and the number of traffic accidents.

The advantage of chevrons over straight, horizontal bars is that the former().

A.can keep drivers awake

B.can cut road accidents in half

C.will look more attractive

D.will have a longer effect on drivers

题型:多选题

固定矫治器治疗中的牙釉质脱矿发病率达()

A.10%

B.20%

C.30%

D.50%

E.90%

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齿轮泵不能输送的介质是()。

A、油类

B、粘稠液

C、有固体颗粒的介质

D、水

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