And with no net force, an object at rest stays at rest, and an object in motion stays that way.

沒有淨力，靜止的物體就會保持靜止，動作中的物體就會保持它的動作

Even if it’s in midair 10 km above the earth’s surface.

就算它處在距離地表10公里的上空

Now of course it’s not like there aren’t forces acting on the airplane.

當然不是沒有力量施加在飛機上的

Gravity pulls down on the plane itself plus all of the people and baggage inside, and every single air molecule

重力加上飛機內的乘客跟行李對飛機產生一股向下的力，還有每個空氣分子

that is shot through the engine or collides with the fuselage or wings pushes on the plane as well.

會射擊引擎或是碰撞機身、機翼，也會推動飛機

But if all of these forces are balanced – in particular, if the air molecules push

不過，當這些力產生平衡，尤其當空氣分子推動

the plane up enough to counteract gravity – then the plane stays up.

飛機向上的力足夠去抵銷重力時，飛機就會維持飛行狀態

Getting air molecules to push the plane up is the crucial part of flying

讓空氣分子去推動飛機向上是讓飛機飛行關鍵的一部分

And planes do this by making sure the undersides of the wings crash into more air molecules more violently than the upper sides.

飛機做到這點的方式是透過確保機翼下方能夠碰撞到比機翼上方更多的空氣分子

When a plane is parked on the ground, air molecules

當飛機停在地面上，空氣分子

bounce off of the top and bottom of the wings in roughly equal amounts, or with “equal pressure.” And no lift.

在機翼頂端跟底端反彈的力度大約相等，或是"壓力相等"，沒有向上舉起的力

But in motion, the curved shape of the wings and their slightly inclined angle means that

但處在運動狀態時，機翼的曲線形狀和稍微傾斜的角度代表

the bottoms smash into more air molecules than before, and smash harder into those molecules.

機翼底部會碰撞到比之前更多的空氣分子，且更大力的碰撞空氣分子

So the pressure on the bottom of the wing goes up.

所以機翼底部的壓力會上升

In addition, fewer air molecules now strike the top of the wing and those that do strike

此外，碰撞機翼頂部的空氣分子將會變少而且撞擊力度

it less forcefully, partly because it’s being “shielded” by its own forward motion

會變小，一部分原因是因為它被自己向前的運動 "防護"了

(the way running into the rain keeps your back drier) and partly because a curving stream of air

(就像是在雨中跑步會讓你的背相較之下比較乾燥)，另一部分是因為空氣呈曲線流動

has lower pressure on the inside of the curve since the molecules get thrown centripetally to the outside.

流動曲線內側的壓力比較低，因為空氣分子會向心地由內側向外側流動

But whatever the reasons, the pressure on the top of the wing goes down.

不過無論是哪個原因，機翼頂部的壓力變小了

So, low pressure on the top plus high pressure on the bottom, and the plane has lift.

所以，頂部的低壓力加上底部的高壓力，飛機就能夠升起

And if the force imbalance is big enough, it can lift the plane up into the air against gravity.

如果力量的失衡夠大的話，就能夠使飛機成功抵抗重力，升到空中！

Now, all this crashing into air molecules to lift the plane also pushes to slow the plane down,

現在，這些讓飛機上升的空氣分子的碰撞也會讓飛機慢下來

which it would, if not for engines. Engines also push air (in this case,

如果沒有引擎的話。引擎也會推動空氣 (在這個例子裡，是向後推)

backwards), either via a propellor, or a jet, or a jet driving a propellor.

透過螺旋槳、噴射器，或是噴射器搭配螺旋槳

For various reasons, it turns out that you want to have a really big propellor driven by a really small jet

由於種種原因，得出你需要用很小的噴射器來推動很大的螺旋槳

for the most efficient engine.

這是最有效率的引擎

But even in inefficient engines, the spinning fan blades generate their “thrust”, which is essentially horizontally lift

不過就算是沒有效率的引擎，旋轉的扇葉還是會產生他們的 "推力"，是個水平方向的力

by moving quickly through air with a curved shape and a slightly inclined angle – they’re essentially mini-wings.

透過在空氣中用曲線形、微傾斜角度的扇葉快速運動 ---本質上就像迷你機翼一樣

And so an airplane is essentially a meta-wing: it flies by moving mini-wings fast enough

所以一架飛機本質上就是一個綜合式機翼: 它的飛行是透過夠快速的迷你機翼運動

to push air molecules backwards, which moves the plane forward fast enough that its big

來推動後方的空氣分子，這使飛機向前飛行的速度夠快，讓大機翼能夠將

wings push air molecules down.

空氣分子向下推擠