英语听力文摘 English Digest 323、荡秋千的精彩

　　Swinging Physics: Potential And Kinetic¹ Energy Working Together
动能与势能相互作用
动能是所有能看到的运动的能量，包括物体的，微粒的或质点集的运动等。
势能(Potential energy )是位置的函数，例如把一个重物举到一定的高度，当它下落时能够作功，所以它具有某种潜在的能量.
Sit on a swing, let someone push you, and away you go.
But what happens if the person who pushed you walks away, and you want to swing faster and higher. If you’ve ever been in the second grade, you know the solution to that. Just start pumping your legs–stretching them forward then drawing them up underneath² you–and you can make yourself swing higher. Have you ever wondered why this is?
Potential Into Kinetic
Swings work by converting³ potential energy into kinetic energy, then kinetic energy back into potential energy, over and over again.
The kinetic energy is the fast part of swinging; it’s the speed you have as you rush back and forth⁴.
Potential energy is the high part of swinging.
The higher you go on the swing, the more potential energy you have.

　　Speed and Swing
Even though you’re not moving at the very top of each swing, you can think of your height as your potential for speed. A physicist⁵ would say that your kinetic energy–that is, your speed–is “stored” as potential energy at the top of each swing.
More height is more speed, just waiting to happen.
How Do You Increase The Energy?
You can increase the energy of your swing in two separate ways.
If someone pushes you, that increases your kinetic energy.
By pumping your legs, you can increase your potential energy. By raising your legs at the top of each swing, you can raise the overall center of mass of your body, effectively raising the height of your swing.
Pump after pump, the energy you burn in your legs increases your swing’s potential energy. That extra height will add up, giving you a wilder swing ride.
译文：
坐上秋千，身后一人使劲一推，你便飞了出去，翱翔半空。
但要是再身后推你的人走开了，而你仍想秋千荡得更高更快，那该如何？这个问题，只要你上了二年级，就知道该怎么做。首先用以摇晃双腿，再向前伸展开来，最后将腿滑向身体之下，秋千就能荡得更高。但你是否想过这是什么原理？
势能转化为动能
秋千的工作原理是将势能转化为动能，然后又将动能再转化为势能，一遍遍的重复这个过程。
动能是秋千快的一部分能量，表现为来回运动时的速度。
势能存在于秋千荡的的高度之中。
秋千荡得越高，所获得的势能也就越大。
速度与秋千
即使在秋千的顶点你并没有运动，但你仍然可以将你的高度认为是潜在的速度。物理学家会告诉你，动能，也就是速度，储存在每次摆动顶端的势能里。
这就意味着，荡得越高，速度也就越快。
如何增加能量？
以下两种方式都可以增加摇摆的能量。
要是有人在身后推你，那将增加动能。
摇晃双腿的方式，增加的是势能。当双腿达到每次摇摆的顶部，身体重心也提高，这就有效的提升了秋千高度。
不断的摇晃，双腿产生的能量将增加势能。由此，秋千会荡得更高，更精彩。

1 kinetic   

adj.运动的；动力学的

参考例句：

• There exist many sources of energy both potential and kinetic.存在着许多势能和动能的能源。
• The kinetic theory of gases is the best known example.气体动力学理论就是最有名的例子。

2 underneath   

adj.在...下面，在...底下；adv.在下面

参考例句：

• Working underneath the car is always a messy job.在汽车底下工作是件脏活。
• She wore a coat with a dress underneath.她穿着一件大衣，里面套着一条连衣裙。

3 converting   

n.吹（炉冶）炼，转炉炼钢v.（使）转变( convert的现在分词 )；（使）转化；皈依；改变（信仰）

参考例句：

• Jim's got some half-baked idea about converting his garage into a sauna. 吉姆初步计划把他的车库改造成一个蒸汽浴室。 来自《简明英汉词典》
• Quite recently man has succeeded in converting sunlight directly into electrical energy. 最近人们已经成功地把太阳光直接转化为电能。 来自《现代汉英综合大词典》

4 forth   

adv.向前；向外，往外

参考例句：

• The wind moved the trees gently back and forth.风吹得树轻轻地来回摇晃。
• He gave forth a series of works in rapid succession.他很快连续发表了一系列的作品。

5 physicist   

n.物理学家，研究物理学的人

参考例句：

• He is a physicist of the first rank.他是一流的物理学家。
• The successful physicist never puts on airs.这位卓有成就的物理学家从不摆架子。