按要求作图：
(1) 如图甲所示，请你画出图中的反射光线和折射光线．
(2) 氢气球下吊一小重物G在空气中做匀速直线运动，若不计空气阻力和风力影响，小重物G恰沿MN方向匀速上升，请在乙、丙两图中选出正确的图，并在正确的图上画出小重物G的受力示意图．
（1）
（2）
题目分析：（1）过O点做出垂直于界面的法线，分别做出反射光线和折射光线，注意反射角等于入射角．从空气斜射入水中，折射角小于入射角，折射光线要靠近法线．如图：
（2）重物在空中沿MN斜向上做匀速直线运动时，如果不计空气阻力和风力影响，向上的升力和重力应该是一对平衡力，氢气球和和重物应该在同一条竖直直线上，故丙图正确．
重力的方向总是竖直向下的，因此过重心沿竖直向下的方向画一条带箭头的线段，用符合G表示，如图所示：
点评：（1）此题主要考查了有关光的反射、光的折射光路图的画法，对两个定律应熟练掌握；
（2）本题考查了平衡力的辨别和重力和重力的方向的理解和掌握，重力是一种最常见的力，重力的方向始终是竖直向下的．
I lived on a farm with my parents and my elder brothers and sisters when I was a little boy. We had the 36 to play and run wherever we wanted to go. We often 37 games just by pretending we were cowboys. Sometimes we played hide-and-seek. One of us would 38 and the others would come and find him. I remember one 39 I hid behind some old wood in the cottage. All was 40 . I couldn’t hear a sound, just the wind 41 . The situation became tense (紧张的) 42 the possibility of someone finding me, and the longer I 43 , the more tense I became. 44 , I heard footsteps coming down the yard. My 45 beat fast and I stood 46 , frozen like a block of ice. I saw my brother getting closer and closer. When he saw me, he laughed 47 , shouting at the top of his voice so that 48 could hear him say, “Eh, I’ve 49 our Tommy!” I can still see the 50 on his face today. Afterwards he said he didn’t think I would hide there. I shrugged my shoulders and said, “Why, it’s a good place, isn’t it?” I don’t have any 51 of these events, just memories, as we had no camera at that time. 52 it was a long time ago, I can see them now as if they 53 yesterday. Now I live in town on a busy street. My girls don’t have the freedom that we 54 have to run off down the road and play whatever they want. Anyway, is it 55 to let children out of sight these days?
|