问题 多选题
如图所示,相距为L的两条足够长的平行金属导轨,与水平面的夹角θ,导轨上固定有质量为m,电阻为R的两根相同的导体棒,导体棒MN上方轨道粗糙下方光滑,整个空间存在垂直于导轨平面的匀强磁场,磁感应强度为B.将两根导体棒同时释放后,观察到导体棒MN下滑而EF保持静止,当MN下滑速度最大时,EF与轨道间的摩擦力刚好到达最大静摩擦力,下列叙述正确的是(  )

A.导体棒MN的最大速度为

B.导体棒EF与轨道之间的最大静摩擦力为mgsinθ

C.导体棒MN受到的最大安培力为mgsinθ

D.导体棒MN所受重力的最大功率为

答案

AC

单项选择题

Allen: I’ve never been a big fan of the bears, especially the vacation show. On my last trip to Disneyland(1999) we passed it by, because we didn’t want to spend the time on something we didn’t really enjoy. If we’d known it would be closed by our next visit, I’m sure we would have watched it one more time. I was just making the point that it wasn’t until Disney announced the closing that this feeling suddenly sprang(弹跳) up for the Country Bear show at Disneyland.
But I think the idea of keeping some attractions open forever regardless of the number of visitors it gets is a mistake. We all have great memories of the Bears, but clearly very few kids today (or I should say families today) are making those same memories from this show. That may be a sad fact, but it’s a fact. For whatever reason, the show doesn’t pull in the numbers. Isn’t it right to use the Country Bear space to bring in more popular attraction so that more kids today can make those magic memories of an attraction they like Otherwise aren’t we just telling the kids today that they "have to" make great memories of the Bears, when it’s obvious that the kids themselves are not enjoying the Bears as we did
Roger: I am very disappointed that they have decided to retire the Country Bear Playhouse. I remember going to the show as a child, and still enjoying it as a teenager, and now as a young adult. It is true that there aren’t a lot of shows left at DL and with this one closing even less. Being at DL does require a lot of walking, and it is nice to have an attraction that is a place to sit and enjoy a good show. I understand that DL has to develop, but there has to be some history to the Disneyland that Walt Disney first designed. There should be some parts of DL that just never go away and this is one of them. By the time I have children it looks like there won’t be any attractions that I can say. I went as a child at the rate they are going.
Now the Bears are leaving. What I want to say is: stop trying to compete with everyone! DL is the best, because of attractions like the Country-Bears. If DL insists on destroying all of its magic to make way for the latest, well, then it might as well be just another theme park.

Where can this passage be found

A.In a Web discussion zone.

B.In a suggestion book.

C.In a report on the popularity of the Country Bears.

D.In high school students’ compositions.

问答题 简答题