题目

在空中相同高度处以相同的速率分别抛出质量相同的三个小球．一个竖直上抛，一个竖直下抛，一个平抛，若不计空气阻力，从三个小球抛出到落地的过程中（ ） A．三个球动量的变化量相同 B．下抛球和平抛球动量的变化量相同 C．上抛球动量变化量最大 D．三球落地时的动量相同 答案：C 【详解】 ABC．三个小球以相同的速率抛出，可知竖直上抛运动的物体运动时间大于平抛运动的时间，平抛运动的时间大于竖直下抛运动的时间，所以上抛运动的时间最长，根据动量定理知，mgt=△p，可得上抛球动量变化量最大．下抛球动量变化量最小，故C正确，A、B错误． D．根据动能定理知，mgh=，After I mastered my first (协奏曲) at age 14, my parents decided to get me a nice violin. I tried out dozens of instruments before I found my match: a German violin. It was beautiful, but what I liked best about it was its voice. Confident and strong, it was everything I longed to be.I'm not sure how much that violin cost, but my parents made me promise never to let it out of my sight. They didn't understand that dragging a large violin caseran counter tomy daily middle school task of being invisible (看不见的). I was a strange, absent-minded kid. When I spoke up in class, my comments brought confused silence from teachers and wild laughter from students. Like a deer in a wolf pack, I tried to be quiet and still.In contrast, my new violin was almost shockingly loud. Together, we could drown out the rest of my middle school orchestra(管弦乐队) — which was encouraged, since the other kids made sounds like cats’ crying. For one glorious hour every day, I was showered with attention. Everyone wanted to hear what I had to say.Between classes, I bent under the combined weight of my violin case and a backpack filled with books. My posture suffered, but my confidence grew. With my violin by my side, I found my voice. More and more, I contributed to class discussions and even made a couple of friends.Today, I'mjust an amateur violinist with a regular day job. As I sit in my community orchestra, sometimes I feel jealous(嫉妒的) my fellow musicians’ instruments with their elegant voices. I may not be the best violinist around, but at least I'm still the loudest.【1】The underlined part “ran counter to” in Paragraph 2 probably means__________.A. was similar toB. went along withC. was the opposite ofD. had something in common with【2】We know from Paragraph 2 that the author___________.A. did not fit in at schoolB. was an excellent studentC. was strong and confidentD. did not attend classes regularly【3】What kind of feeling is expressed in Paragraph 3?A. Disappointment. B. Anger.C. Pride. D. Regret.【4】What can we infer from the text?A. The violin changed the author in some way.B. The author hated the loud voice of his violin.C. The violin always made the author embarrassed.D. The author was the best violinist in his community orchestra.