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问题 多选题

一固定光滑斜面的两倾角分别为30°和45°，质量分别为2m和m的两个滑块用不可伸长的轻绳通过滑轮连接（不计滑轮的质量和摩擦），如图分别置于倾角分别为30°和45°两个斜面上并由静止释放，下滑加速度为a1，轻绳张力为T1；若交换两滑块位置，再由静止释放，下滑加速度为a2，轻绳张力为T2．则在上述两种情形中正确的有（　　）

A．质量为2m的滑块受到重力、绳的张力、沿斜面的下滑力和斜面的支持力的作用

B．质量为m的滑块均沿斜面向上运动

C．a1＞a2

D．T1=T2

答案

A、两个滑块都受到重力、支持力和拉力，下滑趋势是重力的作用效果，故A错误；

B、由于2m的物体的重力的下滑分量总是较大，故质量为m的滑块均沿斜面向上运动，故B正确；

C、根据题意有a1=

2mgsin30°-mgsin45°
3m
，a2=
2mgsin45°-mgsin30°
3m
所以可以知a1＜a2，故C错误；

D、由隔离法有：T1-mgsin45°=ma1，T2-mgsin30°=ma2，代入a1和a2可得T1=T2，故D正确．

故选BD．

单项选择题

商业银行扩张信用，创造派生存款的基础部分是（）。

A.缴存中央银行的存款准备金

B.现金

C.中央银行放出的信用

D.企事业单位及社会公众缴付的税款
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单项选择题

Ask someone what they have done to help the environment recently and they will almost certainly mention recycling. Recycling in the home is very important of course. However, being forced to recycle often means we already have more material than we need. We are dealing with the results of that overconsumption in the greenest way possible, but it would be far better if we did not need to bring so much material home in the first place.
The total amount of packaging increased by 12% between 1999 and 2005. It now makes up a third of a typical household’s waste in the UK. In many supermarkets nowadays food items are packaged twice with plastic and cardboard.
Too much packaging is doing serious damage to the environment. The UK, for example, is running out of it for carrying this unnecessary waste. If such packaging is burnt, it gives off greenhouse gases which go on to cause the greenhouse effect. Recycling helps, but the process itself uses energy. The solution is not to produce such items in the first place. Food waste is a serious problem, too. Too many supermarkets encourage customers to buy more than they need. However, a few of them are coming round to the idea that this cannot continue, encouraging customers to reuse their plastic bags, for example.
But this is not just about supermarkets. It is about all of us. We have learned to associate packaging with quality. We have learned to think that something unpackaged is of poor quality. This is especially true of food. But it is also applied to a wide range of consumer products, which often have far more packaging than necessary.
There are signs of hope. As more Of us recycle, we are beginning to realize just how much unnecessary materials are collecting. We need to face the wastefulness of our consumer culture, but we have a mountain to climb.

What can we learn from the last paragraph

A. Fighting wastefulness is difficult.
B. Needless material is mostly recycled.
C. People like collecting recyclable waste.
D. The author is proud of their consumer culture.
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