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当前位置:首页 > 教学参考> 学生园地> 趣味物理 > 内容详情

从开动着的车子里下来,要向前跳吗

作者:用户  人气: 1676 次  时间:2004年12月24日  星级:

这个问题,无论你把它向什么人提出,一定会得到相同的答案,根据惯性定律,是应该向前跳的。但是,你不妨请他把这个道理说得更详细些,问他:惯性对于这个问题究竟起着什么作用?我们可以预言,这位朋友会肯定地滔滔不绝地开始叙述;但是,只要你不去打断他的话头,他会很快就自己也迷惑起来了,他的结论竟是,由于惯性的存在,下车时候相反地竟是要向跟车行相反的方向跳的。

  真的,事实上,惯性定律在这个问题上只起着次要的作用,主要的原因却是在另外一点上。假如我们把这主要的原因忘了,那么我们就真会得到这样的结论:应该是向后跳而不是向前跳了。

  假设你一定得在半路上从车子里跳下来,这时候会发生些什么情况呢?

  当我们从一辆行驶着的车子上跳下的时候,我们的身体离开了车身,却仍旧保持着车辆的速度(就是要依惯性作用继续运动)要想继续前进。这样看来,当我们向前跳下的时候,我们当然不但没有消除了这个速度,而且还相反地把这个速度加大了。

  单从这一点看,我们从车子上跳下的时候,是完全应该向跟车行相反的方向跳下,而绝对不是向车行的方向跳下。因为,如果向后跳下,跳下的速度跟我们身体由于惯性作用继续前进的速度方向相反,把惯性速度抵消一部分,我们的身体才可以在比较小的力量作用之下跟地面接触。

  事实上呢,无论什么人,从车上跳下的时候,总是面向前方的,就是向行车的方向跳下的。这样做也确实是最好的方法,是由不知道多少次的经验所证明了的;这使我们坚决劝告读者在下车的时候不要做向后跳跃的尝试。

  那么,究竟是怎么一回事呢?

  我们方才那套理论跟事实所以有出入,毛病只是出在方才的解释只说了一半,没有说完。在跳下车子的时候,无论我们面向车前还是面向车后,一定会感到一种跌倒的威胁,这是因为两只脚落地之后已经停止了前进,而身体却仍旧继续前进的缘故。当你向前方跳下的时候,身体的这个继续前进的速度,固然要比向后跳下的更大,但是,向前跳下还是要比向后跳下安全得多。因为向前跳下的时候,我们会依习惯的动作把一只脚提放到前方(如果车子速度很快,还可以连续向前奔跑几步),这样就会防止向前的跌倒。这个动作我们是非常习惯的,因为我们平时在步行的时候都在不断地这样做着:在上一节中,我们就已经说过,从力学的观点上说,步行实际上就是一连串的向前倾跌,只是用一只脚踏出一步的方法阻止着真正跌倒下去。假如向后倾跌,那么就不能够用踏出一步的方法来阻止跌倒,因此真正跌倒的危险就大了许多。最后,还有一点也很重要:即使我们真的向前跌倒了,那么,因为我们可以把两只手撑住地面,跌伤的程度也要比向后仰跌轻得多。

  所以,在下车的时候向前跳跃比较安全,它的原因与其说是受到惯性的作用,不如说是受到我们自己本身的作用。自然,对于不是活的物体,这个规则是不适用的:一只瓶子,如果从车上向前抛出去,落地的时候一定要比向后抛出去更容易跌碎。因此,假如你有必要在半路上从车上跳下,而且还要先把你的行李也丢下去,应该先把你的行李向后面丢出去,然后自己向前方跳下,但自然最好是不要在半路上跳车。

  有经验的人──例如电车上的售票员和查票员──时常这样跳:面向着车行的方向向后跳下。这样做可以得到两重便利:一来减少了由于惯性给我们身体的速度,另外又避免了仰跌的危险,因为跳车的人的身体是向着车行的方向的。

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