than it is to ride at a constant speed?

起步比等速前進費力

Or wondered what causes your bicycle to move?

你想過自行車為什麼會移動嗎？

Or thought about why it goes forward instead of backwards or sideways?

為什麼是前進 而不是後退或向旁邊呢？

Perhaps not, and you wouldn't be alone.

也許你沒想過，但不止是你

It wasn't until the 17th century

一直到十七世紀

that Isaac Newton described the fundamental laws of motion

牛頓 (Isaac Newton) 描述了 三大運動定律

and we understood the answer to these three questions.

我們才明白了這三個問題的答案

What Newton recognized was that things tend to keep on doing

牛頓發現 事物傾向於保持原來狀態

what they are already doing. So when your bicycle is stopped,

所以，當你的自行車停止

it stays stopped, and when it is going,

它會保持停止狀態；如果它前進

it stays going.

則會保持前進狀態

Objects in motion tend to stay in motion

動者恆動

and objects at rest tend to stay at rest.

靜者恆靜

That's Newton's First Law.

這就是牛頓第一運動定律

Physicists call it the Law of Inertia, which is a fancy way of saying

物理學家稱為慣性定律 這只是比較酷的名稱

that moving objects don't spontaneously speed up, slow down, or change direction.

意思是運動中的物體不會自己 加速、減速、改變方向

It is this inertia that you must overcome to get your bicycle moving.

必須克服慣性，自行車才會移動

Now you know that you have to overcome inertia to get your bicycle moving,

雖然知道克服慣性，自行車才會動

but what is it that allows you to overcome it?

但克服慣性的到底是什麼？

Well, the answer is explained by Newton's Second Law.

牛頓第二運動定律說出了答案

In mathematical terms, Newton's Second Law says

牛頓第二定律，用數學觀點來說

that force is the product of mass and acceleration.

力 = 質量 × 加速度

To cause an object to accelerate, or speed up,

要讓某個物體加速

a force must be applied.

必須施力

The more force you apply,

施力越大

the quicker you accelerate. And the more mass your bicycle has,

加速越快。而且自行車質量越大

and the more mass you have too,

還有你的質量越大

the more force you have to use to accelerate at the same rate.

要施更大的力，才有相同的加速度

This is why it would be really difficult to pedal a 10,000 pound bicycle.

如果自行車有一萬磅重 會很難踩得動

And it is this force, which is applied by your legs pushing down on the pedals,

而就是雙腿踩踏板的力量

that allows you to overcome Newton's Law of Inertia.

讓你克服牛頓的慣性定律

The harder you push down on the pedals, the bigger the force

你越用力踩踏板

and the quicker you accelerate.

加速就越快

Now on to the final question:

最後一個問題是：

When you do get your bike moving,

當你移動自行車時

why does it go forward?

為什麼自行車會往前走？

According to Newton's Third Law, for every action,

根據牛頓第三定律，每個作用力

there is an equal and opposite reaction.

都有大小相等、方向相反的 反作用力

To understand this, think about what happens when you drop a bouncy ball.

要理解這點，想想彈力球掉到地上 會怎麼樣

As the bouncy ball hits the floor,

彈力球掉到地上時

it causes a downward force on the floor.

對地板施了個向下的力

This is the action.

這是作用力

The floor reacts by pushing on the ball with the same force,

地板的反應是，用相等的力量推球

but in the opposite direction - upward -

但方向相反，變成向上

causing it to bounce back up to you.

讓球彈回你手上

Together, the floor and the ball form what's called

地板和球，形成了一對

the action/ reaction pair. When it comes to your bicycle,

作用力與反作用力

it is a little more complicated. As your bicycle wheels spin

自行車則更複雜

clockwise, the parts of each tire touching the ground

當車輪順時針旋轉 輪子接觸地面部分

push backwards against the earth -

會向後推地球

the actions. The ground pushes forward with the same force

這是作用力。地面以相等的力量

against each of your tires - the reactions.

反方向推輪子，這是反作用力

Since you have two bicycle tires, each one forms an action/ reaction pair

因為自行車有兩個輪子 每個會與地面形成一對

with the ground. And since the Earth is really really really big,

作用力與反作用力 因為和自行車相比

compared to your bicycle, it barely moves

地球真的超級大， 所以你的自行車輪胎向後的力量

from the forced caused by your bicycle tires pushing backwards -

幾乎沒有讓地球移動

but you are propelled forward.

──然而你就被反作用力推向前了