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标题: A man and his shadow [打印本页]

作者: ob 时间: 2004-11-3 03:49
标题: A man and his shadow

A man is walking along a level road lit only by a single streetlight. He is moving in a straight line, at a constant speed.

He passes the lamppost and leaves it behind, which makes his shadow lengthen.

Does the top of the man's shadow move faster, slower or at the same speed as the man?

作者: husonghu 时间: 2004-11-3 04:25
标题: 与人和灯杆的相对高度有关。

设：灯杆高 = I，人高 = h

如果 I=2h，一样快；
如果 I>2h，影子速度比人的速度慢；
如果 I<2h，影子速度比人的速度快。

不知对不对。

作者: sean9991 时间: 2004-11-3 21:49
标题: answer

Let the height of the streetlight and the man be H and h, respectively.

Assume H > h.

Let the speed of the man be v, then the speed of the top of his shadow is H / (H-h) * v, which is always greater than v.

作者: husonghu 时间: 2004-11-4 18:28
标题: 问的是人影顶部移动的速度，我答的是人影变长的速率，还应加上人本身速度. sean对.

问的是人影顶部移动的速度，我答的是人影变长的速率，还应加上人本身速度. sean对.

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