時差N小時:動能與勢能相互作用
坐上秋千，身后一人使勁一推，你便飛了出去，翱翔半空。
But what happens if the person who pushed youwalks away, and you want to swing faster andhigher.
但要是再身后推你的人走開了，而你仍想秋千蕩得更高更快，那該如何?
If you've ever been in the second grade, you knowthe solution to that.
這個問題，只要你上了二年級，就知道該怎么做。
Just startpumping your legs-stretching them forward then drawing them up underneath you-and youcan make yourself swing higher.
首先用以搖晃雙腿，再向前伸展開來，最后將腿滑向身體之下，秋千就能蕩得更高。
Have you ever wondered why this is?
但你是否想過這是什么原理?
Swings work by converting potential energy into kinetic energy, then kinetic energy backintopotential energy, over and over again.
秋千的工作原理是將勢能轉(zhuǎn)化為動能，然后又將動能再轉(zhuǎn)化為勢能，一遍遍的重復(fù)這個過程。
The kinetic energy is the fast part of swinging; it's the speed you have as you rush back andforth.
動能是秋千快的一部分能量，表現(xiàn)為來回運(yùn)動時的速度。
Potential energy is the high part of swinging.
勢能存在于秋千蕩的的高度之中。
The higher you go on the swing, the more potential energy you have.
秋千蕩得越高，所獲得的勢能也就越大。
Even though you're not moving at the very top of each swing, you can think of your heightasyour potential for speed.
即使在秋千的頂點(diǎn)你并沒有運(yùn)動，但你仍然可以將你的高度認(rèn)為是潛在的速度。
A physicist would say that your kinetic energy-that is, your speed-is“stored” as potentialenergy at the top of each swing.
物理學(xué)家會告訴你，動能，也就是速度，儲存在每次擺動頂端的勢能里。
More height is more speed, just waiting to happen.
這就意味著，蕩得越高，速度也就越快。
You can increase the energy of your swing in two separate ways.
以下兩種方式都可以增加搖擺的能量。
If someone pushes you, that increases your kinetic energy.
要是有人在身后推你，那將增加動能。
By pumping your legs, you can increase your potential energy.
搖晃雙腿的方式，增加的是勢能。
By raising your legs at the top ofeach swing, you can raise the overall center of mass of yourbody, effectively raising the height ofyour swing.
當(dāng)雙腿達(dá)到每次搖擺的頂部，身體重心也提高，這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.
不斷的搖晃，雙腿產(chǎn)生的能量將增加勢能。
That extra height will add up, giving you a wilder swing ride.
由此，秋千會蕩得更高，更精彩。
