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如图,两平行金属导轨位于同一水平面上,相距l,左端与一电阻R相连;整个系统置于匀强磁场中,磁感应强度大小为B,方向竖直向下,一质量为m的导体棒置于导轨上,在水平外力作用下沿导轨从静止 开始以加速度a向右匀加速运动,运动过程中始终保持与导轨垂直并接触良好,已知导体棒与导轨间的动摩擦因数为μ,重力加速度大小为g,导轨和导体棒的电阻均可忽略,求:

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    Last year I broke the record for the oldest person in the world to ride a rollercoaster. I'm 105, but the doctor agrees I'm in good 1

    The rollercoaster I rode 2 you round quite fast. It was, however, a bit too 3for me. I'd have preferred a really fast one that went upside down. But I was told I couldn't ride it, because my blood pressure could drop and I might have 4

    I wasn't nervous. I was securely 5 in, so I knew I wouldn't fall out. The rollercoaster ride went on for three or four minutes, and I 6 every second. Later, someone came to 7 me with the Guinness World Records Certificate.

    My 8 ways really began a couple of years ago, with the ice-bucket challenge. My great-granddaughter recommended me to have a bucket of 9 water poured on my head. I was 102 then. It 10 that I was the oldest person in the world to do it, and the video clip of me was very 11 After that, I started to think about what else I could do for different 12My family say I'm crazy, but they always go along with me. I do get people 13I take it easy, but I'm not taking any14.

    If someone now broke my world record, I'd say, “Good luck to you.” But the real15 is not what I've done; it's how hard I've always worked. I'm taking it easy now, so I have found a new challenge. I was suspended on a rope from a crane about 25 feet high and 16myself down. It was fun.

    I've got a few ideas for the next charity challenge on my 106th birthday, but nothing has been 17yet. All I can say is that it will be 18 good, if not better than the rollercoaster.

    I'm not sure if anyone is19 of all the fun I'm having. They just say I'm silly and that's about it. On 20I've had many good days. I've had a really good life.