I have here a pendulum I have an object
that weighs 15 kilograms and I can lift
it up one meter which I have done now
that means I've done work MGH is the
work I have done believe me
I've increased the potential energy of
this object 15 times 10 is 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 to
call 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're 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 400% I may not trust myself 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 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 I will close my eyes I don't
want to see this - please be very quiet
I almost didn't sleep all night three
two one zero
physics works and I'm still alive