Tip:
Highlight text to annotate it
X
WALTER LEWIN: I have here a pendulum.
I have an object that weighs 15 kilograms, and I can lift it up one
meter, which I've done now.
That means I have done work.
m*g*h is the work I have done, believe me.
I have increased the potential energy of this object.
15 times 10, it's about 150 joules.
If I let it fall, then that will be converted to kinetic energy.
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.
They use these devices--
they're called a wrecker ball--
they use them to demolish buildings.
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.
You just let it hit, and it breaks a building.
And that's the whole idea of wrecking.
So you are using, then, the conversion of gravitational potential energy to
kinetic energy.
Now, I am such a strong believer of the conservation of mechanical energy
that I am willing to put my life on the line.
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.
If I release it from this height and it swings, then when it reaches here,
it could not be higher.
There is a conversion from gravitational potential energy to
kinetic energy back to gravitational potential energy.
And it will come to a stop here.
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.
I trust the conservation of mechanical energy for 100%.
I may not trust myself.
I'm going to release this object, and I hope I will be able to do it at 0
speed, so that when it comes back, it may touch my chin but it may
not crush my chin.
I want you to be extremely quiet, because this is no joke.
If I don't succeed in giving it 0 speed, then this
will be my last lecture.
I will close my eyes.
I don't want to see this.
So please be very quiet.
I almost didn't sleep all night.
Three, two, one, zero.
Physics works.
And I'm still alive.
See you Wednesday.