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Michael Jordan once said, 'I don't know whether I'll fly or not.
迈克尔·乔丹曾说过，“我不知有一天我是否会飞起来。
I know that when I'm in the air sometimes I feel like I don't ever have to come down.'
但我知道当我在空中时，我有时会想永远也不落地。”
But thanks to Isaac Newton, we know that what goes up must eventually come down.
但是伊萨克·牛顿让我们知道上升的东西总会下降。
In fact, the human limit on a flat surface for hang time,
事实上，人类对于平面的滞空时间的极限，
or the time from when your feet leave the ground to when they touch down again, is only about one second,
或者说脚离地又触地的时间，差不多是一秒钟，
and, yes, that even includes his airness, whose infamous dunk from the free throw line has been calculated at .92 seconds.
是的，甚至包括乔丹的空中传奇，他著名的罚球线起跳扣篮是0.92秒。
And, of course, gravity is what's making it so hard to stay in the air longer.
当然，地心引力是使得物体在空中停留不长的原因。
Earth's gravity pulls all nearby objects towards the planet's surface, accelerating them at 9.8 meters per second squared.
地心引力会将物体以9.8m/s^2的加速拉向地球表面。
As soon as you jump, gravity is already pulling you back down.
在你跳起得那一刹那，引力已经开始作用在你身上了。
Using what we know about gravity, we can derive a fairly simple equation that models hang time.
对于我们熟以为知的引力，可以推算出一个可以计算停留时间的简单等式。
This equation states that the height of a falling object above a surface
这个等式显示出坠落物体离地面的高度
is equal to the object's initial height from the surface plus its initial velocity multiplied by how many seconds it's been in the air,
等于物体本身离地的高度加上它最初的速度，乘以在空中的时间（秒）
plus half of the gravitational acceleration multiplied by the square of the number of seconds spent in the air.
加上（1/2）×9.8m/s^2，再乘以逗留时间的平方。
Now we can use this equation to model MJ's free throw dunk.
现在我们可以用这个等式来模拟乔丹的罚球线起跳扣篮。
Say MJ starts, as one does, at zero meters off the ground, and jumps with an initial vertical velocity of 4.51 meters per second.
比如说，乔丹从离地零米跳起时的垂直速度为每秒4.51米。
Let's see what happens if we model this equation on a coordinate grid.
来看一下我们在坐标轴上模拟的情况。
Since the formula is quadratic, the relationship between height and time spent in the air has the shape of a parabola.
因为这个等式是一个二次方程，高度与时间的关系有一个抛物线的形状。
So what does it tell us about MJ's dunk?
所以说，关于乔丹的扣篮，这可以告诉我们什么呢？
Well, the parabola's vertex shows us his maximum height off the ground at 1.038 meters,
这个抛物线的顶点可以告诉我们乔丹达到最高的高度是1.038米，
and the X-intercepts tell us when he took off and when he landed, with the difference being the hang time.
而x轴交点可以告诉我们他起跳和落地的时间，之间的不同便是滞空时间。
It looks like Earth's gravity makes it pretty hard for even MJ to get some solid hang time.
看起来地心引力使得乔丹很难滞空很长的时间。
But what if he were playing an away game somewhere else, somewhere far?
但如果他是在其他地方、更遥远的地方打比赛呢？
Well, the gravitational acceleration on our nearest planetary neighbor, Venus, is 8.87 meters per second squared, pretty similar to Earth's.
在我们的近邻金星上，加速度为8.87m/s^2，与地球很相近。
If Michael jumped here with the same force as he did back on Earth,
如果乔丹是在金星上以与地球上相同的力气跳起，
he would be able to get more than a meter off the ground, giving him a hang time of a little over one second.
他可以跳的比一米更高，在空中还可以停留比一秒更长的时间。
The competition on Jupiter with its gravitational pull of 24.92 meters per second squared would be much less entertaining.
而如果这场比赛是在木星上举行的话，因为其24.92m/s^2的加速度，比赛会变得无聊。
Here, Michael wouldn't even get a half meter off the ground, and would remain airborne a mere .41 seconds.
乔丹不但跳不到半米高，而且在空中只能停留0.41秒。
But a game on the moon would be quite spectacular.
但如果在月球上的话，这场比赛会变得十分精彩。
MJ could take off from behind half court, jumping over six meters high,
乔丹可以从中场跳起，高度会超过六米，
and his hang time of over five and half seconds, would be long enough for anyone to believe he could fly.
他的滞空时间可以超过5.5秒，这个时间长到足够让我们相信他可以飞。
