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Sit on a swing, let someone push you, and away you go.
坐上秋千，身后一人使劲一推，你便飞了出去，翱翔半空。
But what happens if the person who pushed you walks away, and you want to swing faster and higher.
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？
If you've ever been in the second grade, you know the solution to that.
这个问题，只要你上了二年级，就知道该怎么做。
Just startpumping your legs-stretching them forward then drawing them up underneath you-and youcan make yourself swing higher.
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。
Have you ever wondered why this is?
但你是否想过这是什么原理？
Swings work by converting potential energy into kinetic energy, then kinetic energy back intopotential energy, over and over again.
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。
The kinetic energy is the fast part of swinging; it's the speed you have as you rush back and forth.
动能是秋千快的一部分能量，表现为来回运动时的速度。
Potential energy is the high part of swinging.
势能存在于秋千荡的的高度之中。
The higher you go on the swing, the more potential energy you have.
秋千荡得越高，所获得的势能也就越大。
Even though you're not moving at the very top of each swing, you can think of your height asyour potential for speed.
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。
A physicist would say that your kinetic energy-that is, your speed-is“stored” as potential energy at the top of each swing.
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。
More height is more speed, just waiting to happen.
这就意味着，荡得越高，速度也就越快。
You can increase the energy of your swing in two separate ways.
以下两种方式都可以增加摇摆的能量。
If someone pushes you, that increases your kinetic energy.
要是有人在身后推你，那将增加动能。
By pumping your legs, you can increase your potential energy.
摇晃双腿的方式，增加的是势能。
By raising your legs at the top ofeach swing, you can raise the overall center of mass of your body, effectively raising the height ofyour swing.
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。
Pump after pump, the energy you burn in your legs increases your swing's potential energy.
不断的摇晃，双腿产生的能量将增加势能。
That extra height will add up, giving you a wilder swing ride.
由此，秋千会荡得更高，更精彩。
