题干

如图,两固定的绝缘斜面倾角均为θ,上沿相连.两细金属棒ab(仅标出a端)和cd(仅标出c端)长度均为L,质量分别为2m和m;用两根不可伸长的柔软导线将它们连成闭合回路abdca,并通过固定在斜面上沿的两光滑绝缘小定滑轮跨放在斜面上,使两金属棒水平.右斜面上存在匀强磁场,磁感应强度大小为B,方向垂直于斜面向上,已知两根导线刚好不在磁场中,回路电阻为R,两金属棒与斜面间的动摩擦因数均为μ,重力加速度大小为g,已知金属棒ab匀速下滑.求


①作用在金属棒ab上的安培力的大小;

②金属棒运动速度的大小.

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解:①设导线的张力的大小为T,右斜面对ab棒的支持力的大小为 N1 ,作用在ab棒上的安培力的大小为F,左斜面对cd的支持力大小为 N2

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    My mother asked me to call my grandmother. What could we possibly have a1about? Nothing! I just did what she told me and2called my grandmother.

    When she3the phone, I planned. to have a small,10-minute exchange.4, she invited me to lunch at her house so we could5together. Hesitantly, I agreed.

    After hanging up the phone I immediately6agreeing to the lunch date on Saturday.

    Surprisingly, my Saturday morning wasn't7.I found it was nice to see her. I forgot the time8her stories about her recent trip to Europe and her sewing class. I wasn't9that 60-year-old people had a social life. It turned out that he r10wasn't as mundane(平凡的)as I thought. I was happy to have spent time with her. We planned to meet11at her house. My mom didn't have to12me any more.

    This time I didn't feel the gloom(忧郁).13 I wanted to ask were building up in my mind: What was my grandfather like? What did you do at my age?

    As we looked through pictures, I was14.I had never seen, nor imagined, my grandmother as a young girl. She reminded me of myself. I learned that as humans, we15human experiences, no matter what age.

    She told me about the time she and her friends went swimming in the lake late at night and the first time she met my16."You would have adored him. He was the funniest man I'd ever met." We sorted through three shoeboxes of photographs, each with a17story.

    Ever since that18phone call to my grandmother, we have talked at least once a week. I regret not calling her earlier. She has19me what can't be learned in a textbook: being open to new things, enjoying the simple things, and appreciating what life has given me. I have20a new perspective(视角)on life.