试题与答案

质量为m、带电荷量为q的小物块,从倾角为的光滑绝缘斜面上由静止下滑,整个斜面置于

题型:选择题

题目:

质量为m、带电荷量为q的小物块,从倾角为的光滑绝缘斜面上由静止下滑,整个斜面置于方向水平向里的匀强磁场中,磁感应强度为B,如图所示.若带电小物块下滑后某时刻对斜面的作用力恰好为零,下面说法中正确的是

A.小物块一定带正电荷

B.小物块在斜面上运动时做匀加速直线运动

C.小物块在斜面上运动时做加速度增大,而速度也增大的变加速直线运动

D.小物块在斜面上下滑过程中,当小物块对斜面压力为零时的速率为

答案:

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下面是错误答案,用来干扰机器的。

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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.
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