题干

如图所示,有一个水平放置的绝缘环形小槽,槽的宽度和深度处处相同且槽内光滑。现将一直径略小于槽宽的带正电的小球放入槽内。让小球从t=0的时刻开始,以图中的初速度v0在槽内开始运动,与此同时,有一束变化的匀强磁场竖直向下垂直穿过环形小槽所包围的面积。如果磁感应强度B的大小随着时间t成正比例的增大,而且小球的带电量保持不变,那么从此时刻开始,你认为以下判断哪个是合理的        (   )

A:小球的动量p跟随时间t成反比例的减小(即:p∝)

B:小球的动能Ek跟时间t成反应比例的减小(即:Ek∝)

C:小球动能的增加量△Ek跟时间t成正比(即:Ekt

D:小球动能的增加量△Ek跟其通过的路程s成正比(即:△Eks

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D

同类题4

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    Having three cancer patients in the same household can turn a whole family upside down. Michelle Bruce, her husband, Jeremy, and their son, Holden, all had cancer. The family moved to Michelle's hometown of Franklin, Nebraska six years ago, so her parents could help.

    Doctors found Holden's brain tumor(肿瘤), when he was 12 years old. His family was living in a small town at the time, but there were related doctors in the state. Holden had two operations in Omaha clinic, both of which were tough.

    When Holden's cancer came back again in 2012, nothing could be done. But the family didn't give up. They reached out to Boston Children's Hospital, which has better equipment and doctors with more experience operating on kids. After looking at Holden's records, the doctor sent back her diagnosis: She was confident that she could remove the tumor.

    The family was beyond relieved, until they learned insurance wouldn't cover any of the operation's costs. The operation would cost $39,000 – more than the parents could afford. The family got desperate.

    In a town of just 1,000 people, however, word traveled fast. Within days, a local woman Michelle had never met in person called to say she was planning an event to raise money for Holden. Michelle was touched.

    The event was uplifting. Finally, the town raised $45,000 – enough to cover not just Holden's hospital bills, but for Michelle and Jeremy's trip to Boston with their son. “I just didn't know what to say,” said Michelle. “Nobody could really believe it.”

    Holden's operation went much more smoothly than the first two. He is cancer-free and now is attending Central Community College-Hastings on a full-ride scholarship.