在匀强磁场中有一带电粒子做匀速圆周运动，当它运动到M点，突然与一不带电的静止粒子碰撞合为一体，碰撞后的运动轨迹应是图中的（实线为原轨迹，虚线为碰后轨迹，不计粒子的重力）（ ）
A．
B．
C．
D．
设碰撞前带电粒子的动量为P．带电粒子在磁场中做匀速圆周运动的半径为r=
=mv qB
．P qB
带电粒子在M点与一不带电的静止粒子碰撞过程动量守恒，总动量仍为P，总电量仍q，则由r=
得知，碰撞后的共同体做匀速圆周运动的半径与碰撞前带电粒子的半径相同，则共同体仍沿原轨道做匀速圆周运动．故A正确．P qB
故选A
| 完形填空。 Mrs. Brown lived in a small town near a big farm. One Friday afternoon after she finished 1 housework, she went to her small shop. She opened the window of the shop and 2 out side. She liked to see the green farm. There were many cows, horses and small animals 3 the farm. Suddenly she saw a kangaroo under her window. It was interesting to see that it was 4 an old jacket. It stood there and looked hungry. Mrs. Brown gave it some bread 5 water. The kangaroo ate and drank them 6 . Then it became happy and jumped away. Suddenly something dropped on the 7 from its jacket pocket. It was a wallet!She picked it up, opened it and found there was $ 300 and a photo in it. That was a man's photo with his name "John" on it. John? She looked more _ 8 and remembered she had a brother many years ago. His name was also John. "Is this man my lost brother?" she thought. She took the wallet with her and began to 9 the man. She asked many people in the town, and then she came to the farm and asked the farmer. To her great surprise, it was John, her lost brother. They were very excited and 10_ to see each other. | ||||
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