某人在静止的湖面上某高度处竖直上抛一个铁球，铁球上升到最高点后自由下落，穿过湖水并陷入湖底的淤泥中一段深度（假设泥中阻力大小恒定）．不计空气阻力，以v、F、Ek分别表示小球的速度、所受合外力和动能三个物理量的大小．下列图象中能正确反映小球运动过程的是（ ）
A．
B．
C．
D．
A、小球在空中先做竖直上抛运动，再做自由落体运动，速度随时间先均匀减小，再反向均匀增加；进入湖水后加速度减小，但仍会做加速运动；进入淤泥后开始减速运动，故速度图象应为A，故A正确，B错误；
C、小球在空中运动过程中只受重力，方向竖直向下；而进入水中后，受力减小，合力方向向下，而进入淤泥后合力向上，故C正确；
D、小球的动能Ek=
mv2，在空中运动时为匀变速直线运动，v=gt，故动能和时间为二次函数关系，故D错误； 1 2
故选AC．
| 完形填空 My mother used to ask me what was the most important part of the body. Through the years I would __1__what I thought was the correct answer. When I was younger, I thought __2__was very important to us as humans, so I said, "My ears, Mommy". She said, "No. Many people are __3__. But you keep thinking about it and I will ask you again soon." Several years passed __4__she asked me again. Since making my first __5__, I had often thought of the correct answer. So this time I told her, "Mommy, sight is very important to everybody, so it must be our eyes." She looked at me and told me, "You are __6__fast, but the answer is not correct because there are many people who are blind." __7__the years, Mother asked me a couple more __8__and always her answer was, "No, but you are getting __9__every year, my child." Then last year, my Grandpa died. Everybody was crying when it was our __10__to say our final good-bye to Grandpa. Mom asked me, "Do you know the most important body part yet, my dear?" I was __11__when she asked me this now. I always thought this was a __12__between her and me. She saw the__13__on my face and told me, "This question is very important. It shows that you have really __14__your life." I saw her eyes well up with tears. She said, "My dear, the most important body part is your __15__." I asked, "Is it because it holds up your head?" She replied, "No, it is __16__on them a crying friend or loved one can rest their head. I only hope that you have enough __17__and friends that you will have a shoulder to cry __18__when you need it." Then and there I knew the most important body part is not a __19__one. It is sympathetic to the pain of __20__. | ||||
|