【英语时差8，16】荡秋千

 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 underneath¹ you--and you can make yourself swing higher. Have you ever wondered why this is?

Swings work by converting² potential energy into kinetic³ 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. 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.

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. 

1 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。

2 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》

3 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。

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。