English subtitles for clip: File:Physicsworks.ogv

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Now I want to return
to the conservation of mechanical energy.

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I have here a pendulum,

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I have an object that weights 15 kilograms
and I can lift it up 1 meter,

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which I have done now.

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It means I have done work?

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mgh is the work I have done,
believe me.

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I've increased the potential
energy of this object

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15 times 10, so 150 joules.

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if I let it fall, then,
that'll be converted to kinetic energy.

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if I would let it swing

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from 1 meter height

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and you would be there
and it would hit you,

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you'd be dead.

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150 joules is enough to kill you.

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they use this device,
it's called a "wrecker ball",

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they use it to demolish buildings

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you lift up a very heavy object,
even heavier than this

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and then you let it go, you swing it

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thereby converting gravitational
potential energy into kinetic energy

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and that way you can
demolish a building

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you just let it hit

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[GLASS SHUTTERS]

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and it breaks the building

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and that's the whole idea of "wrecking"

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so you're using then,

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the conversion of gravitational
potential energy

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to kinetic energy

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now, I am such a strong believer
of the conservation of mechanical energy

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that I am willing to put my life on the line.

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If I release that ball from a certain height

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then that ball can never come back to a point

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where the height is any larger.

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If I release it from this height,
and it swings

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then what reaches here
could not be higher.

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There is a conversion
from gravitational potential energy

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to kinetic energy,
back to gravitational potential energy

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and then it would come to a stop here.

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And when it swings back,

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it should not be able
to reach any higher

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provided that I do not give this object
an initial speed when I stand here.

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I trust the conservation
of mechanical energy 400 percent.

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I may not trust myself.

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I'm going to release this object,

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and I hope I will be able
to do it with zero speed,

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so that when it comes back,
it may touch my chin,

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but it may not crash my chin.

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

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If I don't succeed in giving it
zero speed, then

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this would be my last lecture.

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[LAUGHTER]

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I will close my eyes,
I don't want to see this.

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So please be very quiet.

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I almost didn't sleep all night.

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3... 2... 1... 0

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Physics works, and I'm still alive!