题干

根据α粒子散射实验,卢瑟福提出了原子的核式结构模型.图中虚线表示原子核所形成的电场的等势线,实线表示一个α粒子的运动轨迹.在α粒子从a运动到b、再运动到c的过程中,下列说法中正确的是(   )

A:动能先增大,后减小

B:电势能先减小,后增大

C:电场力先做负功,后做正功,总功等于零

D:加速度先变小,后变大

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答案(点此获取答案解析)

C

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    A girl complained to her father about her hard life. She didn't know what she had to do and wanted to1up. She felt tired of fighting and fighting. One problem had been finished but2came over.

    Her father, a cook, took her into the3.He poured water into three pans (锅) and boiled it. When the water was boiling, in the first pan he put some carrots, in the second he put some eggs and in the last he put some coffee. He 4for them for a few minutes without any words.

    The girl waited impatiently, not knowing5her father was doing that. About 20 minutes later, her father turned6the stove, took out the carrots and put them in a bowl. He took out the7and put them in another bowl. After that the coffee was poured into a cup. Turning back to his daughter, he asked, “My sweet heart, what do you8?” “Carrots, eggs and coffee,” she answered.

    Her father told her to 9her eyes and let her touch the carrots. She did so and felt that the carrots were10. After that he asked her to take eggs and11them. Then, she got the eggs, warm and hard. At last, the father asked her to smell the coffee. She asked, “What did you do this for, Father?” He12that each one had had the same unlucky experience—the boiling water, but13had a different result. The strong and hard carrots had become soft and weak after being in the boiling water. The eggs became hard after being cooked. The coffee was very14and it changed the water. “Who are you?” asked her father, “When bad luck15your door, what's your decision? Are you carrots, eggs or coffee?”