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简介

想在朋友们面前秀个酷酷的、能让他们惊讶的小把戏吗?那试试这个活动吧!找个土豆、一根吸管——铅笔也能凑合——和你的朋友们一起挑战,在土豆上挖洞,看谁花的力气最少。很有可能,他们找不到轻松完成任务的方法。 最好的方法是——你使用的气力最少——会让所有人大吃一惊!怎么做到的!来调查一番,玩得开心吧!你能找到一个可靠的方法来完成这项任务吗?你能解释它背后的物理吗?

背景

人体自有一套奇妙的工具:手指、牙齿、胳膊以及更多。然而,有时,这些还不足以完成这项任务。我们使用工具,来增强人类推或者拉的能力,来让任务变得更容易。想想手推车、剪刀、刀、弹弓和螺丝钉,所有这些工具能帮助我们完成各种任务。科学家把这些工具称为简单的机械。楔子就是一个例子。它有两个平面:一端是厚的(例如斧子的背面,或者针鼻子);它逐渐缩小为另一端的尖锐边缘(斧刃或者针尖)。楔子可以被用来分开两个物体、分割某个物体、使物体们保持在一起或者阻止它们移动。楔子的应用实例有钉子、刀、订书钉和门楔。楔子,能集中楔形上厚端往薄端两侧或者尖端的所有推力。结果很明确——完成这项任务需要的气力就少多了。试试黄油餐刀。将刀刃顶端推入一大块奶酪里,然后转动刀柄,小心地将手柄推入奶酪里。切奶酪时,哪边需要的力气更小?锋利的一边,对吧?

第 1 段(可获 2 积分)

楔子是六种简单的器具之一。其他的五种分别是滑轮、杠杆、车轮、斜面及螺丝。这些器具看起来很直观简洁,但有时也会有让人意想不到的作用出现。在这个活动中看看你在哪里可以找到一些楔。你不要忘记要有创造力,并且要考虑到其他的因素在起作用!

材料

  • 土豆,至少有一英寸厚(最好是又新鲜又硬的)。
  • 切菜板
  • 切土豆用的刀(这一步应有成年人帮助)
  • 吸管,每一步试验时都需要一根(任何弯曲的部分都应该仔细地去除)
  • 一头尖一头平的铅笔(一头尖的筷子也可以)
第 2 段(可获 2 积分)

准备

  • 请一位大人来切土豆片,切片至少要大约一英寸厚。
  • 将土豆片(们)放在切菜板上。
  • 你在这个活动中的目标,是要在土豆片上钻出铅笔或者吸管大小的洞。从上面列出的材料中,你能想到什么方法吗? 除了你自己的想法,你可以尝试下面列出的8种方法。每个都试试;看看哪个或哪些是比较容易的,并试着解释为什么。
  • 这里有几个技巧,可以帮助你像科学家一样进行本次活动:
    • 在你尝试一种方法之前,先进行假设(或者预判)。 完成下面的句子,会帮助你开始:“假如我…(描述你将要尝试的方法),我…(期望/不期望)在土豆片上打个洞,因为…”
    • 每次尝试,使用一个新的吸管。吸管在测试期间,可能会受损,所以要在类似的条件下尝试不同的方法,你得为每次测试准备一个新的吸管。
    • 每次测试后,花点时间来调查结果。你的假设是正确的吗?为什么你认为是这样的? 你从这次测试中学到了什么吗?
  • 出于安全原因,在测试时,要保证土豆一直是在切菜板上。
第 3 段(可获 2 积分)

Procedure

  • “Twisting a straw”: Hold the straw at about three fourths of the way up, and try to twist the straw in. If you are wondering if the edges of a straw are sharp, rub your finger along an edge. Does it feel sharp? Do you think it will cut into the potato? Make a hypothesis, perform the test and investigate the results. Did it work? Was it easy? What have you learned from this test?
  • “Twisting the flat side of a pencil”: Hold the pencil about three fourths of the way up from the flat end and try to twist the flat end into the potato. Do you think this will work? Make a hypothesis, perform the test and see what you learn. What happened on this test?
  • “Twisting the pointy side of a pencil”: Turn your pencil around and, holding it three fourths of the way up, try to twist the pointy side of your pencil into the potato. Will this be easier or more difficult than previous methods? Make a hypothesis, perform the test and investigate the results.
  • Did any of the twisting methods work? Let us try something different: jamming the item on the potato.
  • “Jamming a straw”: Hold your straw about three fourths of the way from one end (without pinching the straw) and jam the far end onto the slice. Can you force the straw through the potato? Make a hypothesis, perform the test and investigate the results. What have you learned from this test? Was your straw sturdy enough or did it bend under the force?
  • “Jamming the flat side of a pencil”: Repeat the previous method with the flat side of the pencil.
  • “Jamming the pointy side of a pencil”: Repeat with the pointy side of the pencil.
  • “Jamming the plugged straw”: Back to the straw with the sharp edges. (Use a new straw for this test.) Hold it so your thumb blocks off one end of the straw. Carefully jam the other end onto the slice. Keep making a hypothesis before you test, and investigate after the test. Was your prediction correct? Why do you think this works or does not work?
  • “Jamming a folded straw”: Now, try to hold the straw in yet another way. Fold one end of the straw over and hold it firmly closed so little to no air can escape through this end of the straw. Jam the other end onto the slice. Do you think this will work? Did it work? Why did it work or not work?
  • Look back at all the methods you tried. Which worked well; which worked, but with difficulty; and which did not work at all? Can you see a trend? Can you explain why some methods work extremely well, others are hard and still others do not work at all?
  • Extra: Try these methods on other fruits or vegetables or on other materials of varying hardness (with permission, of course, and have an adult supervise you). Will it work? Why or why not?
  • Extra: Can you come up with other ways to cut a hole this size through a potato? See if you can use other materials you find around the house (but be sure to ask permission, and have an adult supervise you).
  • Extra: As explained in the background section, the methods that worked for this activity used—among other things—a simple machine called a "wedge" to cut the hole in the potato. Can you find other examples of wedges around your house?
第 4 段(可获 2 积分)

现象与结论

用空吸管插入土豆(来挖)异常地省力,对不对?如果用尖尖的铅笔来插也可能会成功,但会费力些;因为扭断一支铅笔会很难而且很易失败,所以也可能成功。(@雨天:当然,用铅笔挖的话,洗就更困难了)

无论哪种方式,用的都是尖的那端,楔子原理。楔形能更容易将土豆推出。一开始,在插入土豆前,你所需花费的是插入的力气。然后你再慢慢地用力把它推进,或是扭进去,而在插入时,它就已经提供了那瞬间的力。而恰恰是楔形把这种瞬发力都集中在一个小表面积的吸管或铅笔尖的边缘上,使得它们可以穿透土豆。

第 5 段(可获 2 积分)

如果你向你的朋友们发出这个实验挑战,通过插入空吸管来挖土豆洞的这个方法一定会让他们印象深刻的;吸管竟可以强到穿透土豆而不被扭曲弯折。你知道这是为什么吗?当插入吸管并将之推入土豆,无形中你把空气困在了吸管中。正是这空气使吸管保持直挺。空气分子不断挤压管壁,这样吸管总不会瘪。空气是一种气体,气体很容易被压缩。同时,也是因为它,马铃薯的肉得以进入吸管,而又不会使吸管变形。

用空吸管比用铅笔要好操作是因为可以利用吸管的形状和空间来推动土豆;而用铅笔,虽然能推动更多的土豆量,但换个角度来想,你失去了更多的土豆材料,而且花费的力气也要大些,所以更难。

清理

土豆可以放在堆肥箱里。吸管可以扔进垃圾桶,其他厨房用具可以和脏盘子放在一起,到时候统一清洗。

第 6 段(可获 2 积分)

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