题干

频闪照片是研究物体运动的重要手段。通常一只掉在水平地面上的弹性小球会跳起,而且弹跳的高度会越来越低。如图所示,是小球弹跳过程的频闪照片,小球在1、2 位置的高度一样。下面说法正确的是()

A:小球在1、2位置的重力势能相同,机械能1比2位置时大

B:小球在1、2 位置的重力势能相同,且机械能大小也相同

C:小球在2位置的重力势能比l位置时小,且机械能也比1位置时小

D:小球弹跳上升到最高点3位置时,速度为0,不受任何力作用

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A

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Benjamin West, the father of American painting, showed his talent for art when he was only six years of age. But he did not know about brushes before a visitor told him he needed one. In those days , a brush was made from camel's hair. There were no camels nearby. Benjamin decided that cat hair would work instead. He cut some fur from the family cat to make a brush.

    The brush did not last long. Soon Benjamin needed more fur. Before long, the cat began to look ragged (蓬乱). His father said that the cat must be sick. Benjamin was forced to admit what he had been doing.

    The cat's lot was about to improve.  That year, one of Benjamin's cousins, Mr.Pennington, came to visit. He was impressed with Benjamin's drawings. When he went home, he sent Benjamin a box of paint and some brushes. He also sent six engravings (版画)by an artist. These were the first pictures and first real paint and brushes Benjamin had ever seen. In 1747,when Benjamin was nine years old,Mr.Pennington retured for another visit .He was amazed at what Benjamin had done with his gift.He asked  Benjamin's parents if he might take the boy to Philadelphia for a visit.

In the city, Mr.Pennington gave Benjamin materials for creating oil paintings.The boy began a landscape (风景) painting.Wiliams ,a well-known painter,came to see him work . Wiliams was impressed with Benjamin and gave him two classic books on painting to take home .The books were long and dull. Benjamin could read only a little,having been a poor student.But he later said,”Those two books were my companions by day,and under my pillow at night.”While it is likely that he understood very little of the books,they were his introduction to classical paintings.The nine-year-old boy decided then that he would be an artist.