I have here a pendulum.
I have an object that weighs 15 kilograms 
and I can lift it up one metre, 
which I have done now
That makes I have done work. 
mgh is the work I have done, believe me.
I have increased the potential energy of this object, 
15 times 10, to 150 joules.
If I let it fall, then that will be 
converted to kinetic energy
If I would let it swing, 
from one metre height, ...
... you would be there, and it will hit you.
You would be dead.
150 joules is enough to kill you.
They use these devices, 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 ...
[SHATTERING GLASS]
... 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 am 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.
I want you to be extremely quiet, 
because this is no joke.
If I don't succeed in giving it zero speed ...
... then, this will be my last lecture.
[AUDIENCE LAUGHTER]
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 ...
[LITTLE SCREAM]
[LAUGHTER]
Physics works and I am still alive!
[APPLAUSE]
SUBTITLES © 2016 Andrew Lucas
www.EnergyUnearthed.com
