阅读理解

    When the man's wife died，their youngest baby was 2 years old．They had six other children一three boys and three girls．aged from 4 t0 16．

    The man's parents and his wife's parents came to visit them．

    ‘‘We've been talking about how to make this work，”they said．“There's no way you can take care of all these children and work to make a living．So，we've decided to place each child with a different uncle and aunt．”

     The man refused．Over the next few weeks，he moved the family to a tiny town and his business developed quickly．People came from far away to do business with him．And the children helped him both at home and at work．

The children grew up and got married．Five of the seven went off to college．The children's success was a source of pride to the father．

    Finally，the youngest daughter，who was 2 years old when the mother died，got married．With his life's work done，the father died．

This man's work had been the lonely but joyful task of raising his family．This man was my father．I was the 1 6-year-old，the oldest of seven．

如图所示，位于竖直平面内光滑的 $\frac{1}{4}$ 圆弧轨道半径为R，轨道的最低点B的切线沿水平方向．质量为m的小球（可视为质点）从轨道最上端A点由静止释放．若空气阻力忽略不计，重力加速度为g．求：

一幅画中适当留下不着色彩的空白，更能达到一种“恰是未曾着墨处,烟波浩渺满目前”的境界。“空白效应”说明了（   ）。