从同一地点同时开始沿同一直线运动的两个物体Ⅰ、Ⅱ的速度图象如图所示．在0-t时间内，下列说法中正确的是（ ）
A．Ⅰ、Ⅱ两个物体所受的合外力都在不断减小
B．Ⅰ物体所受的合外力不断增大，Ⅱ物体所受的合外力不断减小
C．Ⅰ物体的位移不断增大，Ⅱ物体的位移不断减小
D．Ⅰ、Ⅱ两个物体的平均速度大小都是v1+v2 2 
A、B、图象Ⅰ表明物体做加速度不断减小的减速运动，图线Ⅱ表明物体做加速度不断减小的加速运动，根据牛顿第二定律，合外力与加速度成正比，故两个物体的合外力都不断减小，故A正确，B错误；
C、两个物体的速度都是正值，表明两个物体都在向前运动，故C错误；
D、对于匀变速直线运动，平均速度等于初末位置速度的平均值，而图中两个物体都做非匀变速直线运动，故D错误；
故选A．
Do you know how to study better and make your study more effective(有效的). We all know that Chinese students usually study very hard for long . This is very good, but it doesn’t a lot, for an effective students must have enough sleep, enough food and enough rest and exercise. Every day you need to go out for a walk or visit some friends or some nice places. It’s good for your study. When you return your studies, your mind will be refreshed(清醒) and you’ll learn more study better. Psychologists(心理学家) that learning takes place in this way. Here take English learning an example. First you make a lot of progress and you feel happy. Then your language study seems the same. So you will think you’re learning and you may give up. This can last for days or every week, yet you needn’t give up. At some point your language study will again take another big . You’ll see that you really have been learning all along. If you get enough sleep, food, rest and exercise, studying English can be very effective and . Don’t give up along the way. Learn slowly and you’re sure to get a good result.
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