题干

如图所示,阻值为R,质量为m,边长为l的正方形金属框位于光滑的水平面上。金属框的ab边与磁场边缘平行,并以一定的初速度进入矩形磁场区域,运动方向与磁场边缘垂直。磁场方向垂直水平面向下,在金属框运动方向上的长度为L(L> l )。已知金属框的ab边进入磁场后,金属框在进入磁场过程中的运动速度与此过程ab边在磁场中的位移之间的关系和金属框在穿出磁场过程中运动速度与此过程cd边在磁场中位移之间的关系分别为v=v0-cx;v=v0-c( l +x)(v0未知),式中c为某正值常量。若金属框完全通过磁场后恰好静止,则有 (     )

A:线框bd边进入一半时线框加速度大小为a = 3cl3B24mR

B:线框进入和穿出过程中做加速度逐渐减小的减速运动;

C:线框在穿出磁场这个过程中克服安培力做功为 mc2l2

D:磁感应强度大小为B= mcRl

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答案(点此获取答案解析)

B,D

同类题3

短文续写

    Dad pushed his finger into my basket. “What's that?”he asked. “Mew, ”said the basket.

    Dad suddenly pulled his hand away. “Oh no,” he said. “Not a cat. No way. ”

    I pulled Checkers from the basket. “It's not a cat. It's a kitten. Can I keep him? Please?”

    Dad frowned for a minute, thinking. Then he rubbed Checkers under the chin(下巴). “You'll have to take good care of him. ”

    I hugged Checkers. “I will, “I promised, he won't be any trouble at all.” And Checkers wasn't—until he grew into a cat. He wanted to be the first one in the house- and the first one out of the house. Sometimes he blocked dad in the way. “That cat!”Dad complained. Checkers climbed onto our roof. He had no trouble getting down. Usually he aimed for Dad. “That cat!” Dad yelled. Checkers flushed the toilet and watched the water circle away. “That cat!”Dad was almost mad. Checkers hid in strange places. He liked to surprise people.

    One day he hid under the couch(睡椅). Dad walked by with a glass of orange juice. Checkers jumped out. He curled around Dad's ankle and attacked Dad's toes. Orange juice spilt all over the floor. Dad yelled and put Checkers outside. “Get out of my way, cat!” he said angrily. And he slammed the door.

    Dad washed the injured toes and peeled open a bandage. He wrapped it around his big toe, and then he opened four more for his ankle.

    I watched from the window. Checkers looked frustrated as he walked down the road.

    “I'm sorry Checkers scratched you, ”I said. “He was just playing. He didn't mean to hurt you.”

    “Don't worry, ”said Dad. “That cat will be back. ”

    After supper I called for Checkers. But Checkers didn't come. Nighttime came. I called and called. I knocked his food dish with a spoon loudly. But still no Checkers. I left the porch light on. But the next morning Checkers still wasn't there.

    I carried a picture of Checkers to all the neighbors. I drew posters and hung them all over. But nobody found Checkers.

    Days went by. Life wasn't the same without that cat. I was angry with Dad. I didn't think he missed Checkers at all.

    Then one night the phone rang.



    Dad nodded, his face turning red.

注意:1.所续写的短文词数应为150左右;2.应使用5个以上短文中标有下划线的关键词语;3.续写部分分为二段,每段的开头语已经为你写好;4.续写完成后,请用下划线标出你所使用的关键词语。