Name: 當一個物理老師知道他的東西 ! (When a physics teacher knows his stuff !!)
Uploaded: 2025-08-26T11:03:14.000Z
Duration: 3 min 19 s

I have an object that weighs 15 kilograms and I can lift it up one meter, which I have done now.

我有一個重 15 千克的物體，我可以把它舉起一米，現在我已經做到了。

I have increased the potential energy of this object. 15 times 10 is about 150 joules.

我增加了這個物體的勢能。15 乘以 10 約為 150 焦耳。

If I let it fall, then that will be converted to kinetic energy.

如果我讓它掉下來，它就會轉化為動能。

If I would let it swing from one meter height and you would be there and it would hit you, you'd be dead. 150 joules is enough to kill you.

如果我讓它從一米高的地方掄起來，而你就在那裡，然後它砸在你身上，你就死定了。150 焦耳就足以要了你的命。

You lift up a very heavy object, even heavier than this, and then you let it go, you swing it, thereby converting gravitational potential energy into kinetic energy, and that way you can demolish a building.

你舉起一個非常重的物體，甚至比這個還重，然後你放開它，揮動它，從而將重力勢能轉化為動能，這樣你就可以摧毀一棟大樓。

You just let it hit... and it breaks a building.

你只要讓它撞擊......它就會撞壞一棟樓。

So you're using, then, the conversion of gravitational potential energy to kinetic energy.

那麼，你使用的是重力勢能到動能的轉換。

Now, I am such a strong believer of the conservation of mechanical energy that I am willing to put my life on the line.

現在，我是機械能守恆的堅定信奉者，我願意將自己的生命置之度外。

If I release that bob from a certain height, then that bob can never come back to a point where the height is any larger.

如果我從某一高度鬆開小球，那麼小球就永遠無法回到高度更大的地方。

If I release it from this height and it swings, then when it reaches here, it could not be higher.

如果我從這個高度釋放它，它就會擺動，那麼當它到達這裡時，就不會再高了。

There is a conversion from gravitational potential energy to kinetic energy back to gravitational potential energy, and it will come to a stop here.

從重力勢能到動能，再回到重力勢能，這中間有一個轉換過程，它會在這裡停止。

And when it swings back, it should not be able to reach any higher, provided that I do not give this object an initial speed when I stand here.

如果我站在這裡時沒有給這個物體一個初始速度，那麼當它擺動回來時，它就不可能到達更高的地方。

I trust the conservation of mechanical energy for 100%.

I'm going to release this object, and I hope I will be able to do it at zero speed, so that when it comes back, it may touch my chin, but it may not crush my chin.

我要釋放這個物體，我希望能以零速度釋放，這樣當它回來時，可能會碰到我的下巴，但可能不會壓碎我的下巴。

I want you to be extremely quiet, because this is no joke.

我要你們保持安靜，因為這可不是鬧著玩的。

If I don't succeed in giving it zero speed, then this will be my last lecture.

如果我不能成功地讓它的速度為零，那麼這將是我最後一次講課。