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【物理3—5模块】

 
（1）质量为1kg的小球以4m/s的速度与质量为2kg的静止的小球正碰。关于碰后的速度 与，下面哪些是可能的（   ）

A．

B．

C．

D．（2）静止在匀强磁场中的一个B核俘获了一个速度为v =7.3×104m/s的中子而发生核反应，生成α粒子与一个新核。测得α粒子的速度为2×104 m/s，方向与反应前中子运动的方向相同，且与磁感线方向垂直。求：⑴写出核反应方程。⑵求α粒子与新核轨道半径之比。

答案

(1) AB  (2)B+n→He+Li   120∶7

（1）AB

（2）⑴由质量数守恒和电荷数守恒得：B+n→He+Li

（2）由动量守恒定律：

可以求出反冲核的速度大小v2=103m/s方向和α粒子的速度方向相反

由带电粒子在匀强磁场中做匀速圆周运动的半径公式可求得它们的半径之比是:

120∶7

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Randy Kraus was paralyzed. His left side was useless. But his right hand was (1) enough to lift a bucket to his forehead. Once, he’d been a police officer and owned a private-eye agency. Once, he’d been p and able. Now, he felt he was nothing.
His (2) started with Parkinson’s disease, but it didn’t (3) there. In July 2002, the 60 year old Kraus went into the hospital for an operation to control the shaking. (4) , during the operation, he had a stroke. He was paralyzed. The (5) man, who loved golf, could think, but couldn’t (6) . Kraus wanted the doctors to (7) it to him straight. "You may never walk again, " they told him. "Maybe you won’t (8) be able to talk. "
Once at home, he (9) he couldn’t lift a fork or take a drink by himself. Physical treatment was so painful and slow. What did he have to (10) for So now Kraus held the gun against his head. (11) the cold metal on his skin, he began to consider not his pain, but the pain he (12) cause his wife, daughters and grandchildren. He didn’t pull the trigger.
"You are where you are", his exercise physiologist, Andrew Garud told him. "The pace would be slow; the pain would be (13) . But as long as you are (14) , you have the ability to get better. " After three months of (15) with Garud, Kraus wanted to see if he could (16) . He could. Then he took three steps, sat down and cried like a baby. One step (17) to another. Next he managed a short walk. It was the hardest (18) of Kraus’s life.
Garud kept saying he could (19) more. Now, Kraus can brush his teeth, shave himself and get around the house with a walker. Little success only the paralyzed can (20) understand.

A．puzzle
B．process
C．injury
D．trouble
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