题目

Believe it or not, optical illusion(错觉)can cut highway crashes. Japan is a case in point. It has reduced automobile crashes on some roads by nearly 75 percent using a simple optical illusion. But stripes, called chevrons(人字形), painted on the roads make drivers think that they are driving faster than they really are, and thus drivers slow down. Now the American Automobile Association Foundation(基金会)for Traffic Safety in Washington D. C. is planning to repeat Japan’s success. Starting next year, the foundation will paint chevrons and other patterns of stripes on selecte质量为25kg的小孩坐在秋千板上，小孩离系绳子的横梁4m。如果秋千板摆到最低点时，小孩运动速度的大小是4m/s，她对秋千板的压力是多大？（g取10m/s2）