When a physics teacher knows his stuff !!

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

Channel: Lectures by Walter Lewin. They will make you ♥ Physics.

Views: 8,601,812

Rating: 4.9451451 out of 5

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Id: 77ZF50ve6rs

Length: 3min 19sec (199 seconds)

Published: Fri Mar 04 2016