题目

阅读下面短文，根据所给情节进行续写，使之构成一个完整的故事。When I was ten years old, I lived with my family in a small house in rural South Jersey. I often accompanied my mother to the A&P to buy groceries. Without a car, we walked, and I would help her carry the bags.My mother had to shop every carefully, as she was a waitress, and her salary barely sustained (勉强维持) us. One day, while she was weighing prices, a promotional display for the World Book Encyclopedia caught my eye. The volumes were beautifully cream-colored. Volume I was ninety-nine cents（2012?昭通）站在地上的人，利用如图所示的滑轮组把重为G的物体匀速提高h．（1）根据题目要求在图中用笔画线绕制滑轮组．（2）设想不用滑轮组，直接用手把物体匀速提高h，这时人手对物体做的功W1=GhGh（写出表达式）．（3）若用滑轮组提升此重物，且动滑轮重为G0（绳重及摩擦不计），这时绳端的拉力F=G+G02G+G02，人手拉绳做的功W2=（G+G0）h（G+G0）h（写出表达式）．（4）此实验说明使用滑轮组可以省力，但不能省功功．（5）该滑轮组的机械效率η=GG+G0GG+G0；（写出表达式）提高此滑轮组机械效率的一种方法为：减小动滑轮重，减小额外功减小动滑轮重，减小额外功．