题面摘自 Henry E. Dudeney 公版文本;英文为古腾堡原文整理,中文为本站自译,提示、解答骨架和闲谈保留本站原创结构。
We have here a stop-watch with three hands. The second hand, which travels once round the face in a minute, is the one with the little ring at its end near the centre. Our dial indicates the exact time when its owner stopped the watch. You will notice that the three hands are nearly equidistant. The hour and minute hands point to spots that are exactly a third of the circumference apart, but the second hand is a little too advanced. An exact equidistance for the three hands is not possible. Now, we want to know what the time will be when the three hands are next at exactly the same distances as shown from one another. Can you state the time?
我们这里有一个三针秒表。秒针每分钟绕表面一周,末端靠近中心的地方有一个小环。我们的表盘显示了主人停止手表的确切时间。您会注意到三只手几乎等距。时针和分针指向的点正好相隔圆周的三分之一,但秒针有点太超前了。三只手的距离精确相等是不可能的。现在,我们想知道当三只指针彼此之间的距离完全相同时,时间是什么时候。你能说出时间吗?
提示 1
先说出现象:哪些量会变,哪些约束不会变。
提示 2
找守恒量、相似关系、平衡条件或不变量,不急着代公式。
提示 3
把物理图景或谜题结构翻成一个最小方程组,再处理边界情况。
完整解答
解题主线是先把 Dudeney 谜题 33 的条件整理成一个稳定模型,再选择最少的变量。第一步确认约束,第二步写出关键关系,第三步检查特殊情形。这里给的是原创解法骨架;若要核对原始题面,请回到公版来源。
这类题最怕一上来套公式。先把图景或语言条件说清楚,答案通常会少绕很多路。