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how do you do ladies and gentlemen and
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boys and girls I am juliia Suna Miller
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and physics is my business and our
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business today is the very special third
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law in Newton's three laws of
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motion and to view it at least once in
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our lifetime in the Latin I have it here
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Lex three oxion contrarium
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simpal
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reion now if you didn't know any Latin
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at all You' see something in there about
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action and reaction and indeed that's
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what it sums up to be third law to every
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action there is always an equal and
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opposite a contrary reaction or
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the mutual actions of any two bodies are
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always equal and oppositely
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directed now a strange introduction to
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this law would be as follows consider me
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standing still and quiet on the earth
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and I jump up in this
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manner I ask did you feel the Earth
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recoil and the answer is it must be it
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did indeed recoil even though though you
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may not have felt it why here is the
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earth enormous Mass here I am a teeny
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wey creature of little Mass I push down
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on the floor and with a force F which
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acts on me and on the earth and as
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before in the second law my little Mass
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you detect has an enormous acceleration
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up I go but the Earth having an enormous
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Mass has a teeny wiy acceleration
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which you cannot
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detect
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so
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consider Newton's third law in a toy
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here is a little vehicle with some
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Wheels to which is fixed a rubber
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balloon and I blow some air in the
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balloon now I'm going to let the air
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come out of a certain hole orifice here
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the air comes out this way and what must
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the vehicle do the vehicle must go the
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other way watch it
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there it goes and so I am reminded of a
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certain exercise that goes like this
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here is a rocket vehicle we have some
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stuff in here in a state of
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combustion the products of the
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combustion come out here millions of
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little particles of stuff now many
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people have the distorted notion that
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this stuff coming out here must push
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against the atmosphere in order for this
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to go
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and I say that is utterly erroneous this
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system this vehicle works better where
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there is nothing because we want the
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little mass of stuff coming out there to
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come out with the greatest possible
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velocity and hence have the greatest
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rate of change of momentum the product
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of M and V being
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momentum now I'm going to go back to the
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two cars because the case of the two
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cars I said earlier very important
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important massive car less massive car I
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stretched the spring between them one in
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the same Force acts on both and we
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witness an astonishing thing here is the
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massive car here is the little car less
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inertia there is the spring now what did
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we see the same Force acted on both but
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you notice
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notice that the smaller one had the
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bigger acceler
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ation and the bigger one had the Lesser
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acceleration and therefore in the same
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time of travel and they would both
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travel the same time from Time Zero to
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the time of collision clearly the
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smaller one has the higher velocity so
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writing the momentum of both cars we
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would equate it as follows the large car
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has a little velocity the little car an
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enormous velocity and these momenta are
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equal very important that you see that
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point that the same Force acts on both
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the accelerations are inversely as the
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masses and the momenta are equal in
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another program I will talk about their
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energies and we will discover a very
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strange thing now more on this
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conservation of momentum which is
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essentially Newton's third law here I
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have a CO2 cartridge gas in here under
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enormous pressure here is a carriage in
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which I put
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it I'll look around that end and you
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will notice that there is a barrier on
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this end of the cartridge of the
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carriage now what am I going to do I am
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going to make a hole in this end of the
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cartridge the gases come out this way oh
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I erased that before the gases come out
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out this way and they have a certain
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momentum their mass and their velocity
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the vehicle must go the other way watch
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it now
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watch there it goes there it
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goes Miller's Earthbound rocket now a
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very oh very important thing it fled
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away fled away momentum I'm sure I would
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have asked a question had I had it in
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hand and question I would feel it to be
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very cold very cold and I want to know
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why is it cold a second question there
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could indeed be frost on it and the
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question is where cometh the frost in
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the first question we have a very
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powerful piece of
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physics the gas is coming out must do
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work this costs energy and I leave it
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there for further discussion in another
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program now let's go to something still
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more dramatic again conservation of
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linear
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momentum here is an array of Steel
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spheres highly elastic steel is highly
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elastic by that I mean when they are
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deformed they
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recover they lie in the lowest potential
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configuration I take one up the plane
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let it go having given it some potential
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energy and it collides with the system
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at rest and watch what happens on the
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remote
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end
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one oh that's terrific watch it look
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it nearly
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instantaneously indeed I'm led to ask
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how fast does the impulse get down there
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answer with the same velocity as the
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velocity of sound in steel which is
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about 15,000 ft per second so if this
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were a foot long or that's more likely a
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foot long that's 9 12 in watch
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it 115,000 of a second you notice too
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that the whole system experiences some
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motion that's because they all wish to
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seek a lower potential
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plane now one at Collision MV one goes
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away MV watch two two MV plus MV two MV
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now let us disregard the intermediate or
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the other motions that we witness
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because a discussion of this would
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divert me from my purpose here all I
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wish to say is that whatever the
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momentum before the Collision so that is
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the momentum after one
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one
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two
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two three oh notice they are seeking a
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lower potential plane the energy of a
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system runs
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downhill free oh that's pretty notice
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they are seeking a lower potential plane
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four watch it now notice they are
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seeking a lower potential plane just as
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my energy is running downhill now as I
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do the
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show
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four five
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up five and so we say without question
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the momentum before the Collision is
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equal to the momentum
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after consider another classical
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demonstration of mine which is easy to
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do and quite dramatic here is a steel
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pipe this demonstration I will call
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virtual v i r
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t there are some demonstrations that I
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do for real and I call them real
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demonstrations this one we will imagine
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an imagination is a very necessary
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ingredient of this business we will
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imagine a steel pipe closed up tightly
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on one end with a cap mounted on Wheels
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and in this chamber I put some dry ice
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solid
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CO2 then with a wooden plug I plug it up
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very firmly very firmly and I let it
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stand here on the table in the hot air
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of the classroom or the TV studio and
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what happens the CO2 sublimes goes into
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gas or vapor the pressure builds up more
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and more the pressure gets enormous
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inside something has to go what goes the
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cap with an enormous velocity enough
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indeed to kill a man so if you should do
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this I warn you have some caution and
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what happens to the vehicle it goes the
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other way big Mass little velocity
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little Mass large velocity conservation
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of linear
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momentum powerful business powerful
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consider toys have a large role in my
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business because the physics of toys is
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an enchanting thing to engage in toys I
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would have you understand ladies and
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gentlemen and boys and girls toys are
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intended for child's play but the
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physics of them is hardly that here is a
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little propeller with some uh
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inclined blades and I have mounted
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it in a chamber wherein resides a
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spring so I am storing some wound up
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elastic energy in the spring and I
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release it I did that one prematurely
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what happens how is Newton's third
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law how does it play a role here
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answer the pitch blade engages the air
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dry waves the air down with a momentum
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MV the vehicle must go up watch it there
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it is oh that's a pretty thing and then
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it's spinning as a top about which I
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will speak in another program so the
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question to be asked is how does a bird
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fly here is a bird flying here is a bird
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I'm probably viewed as a vulture bird
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bird I can only fly if I push the air
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down down and that's why I go up now
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when a bird Soares as they do ever so
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gracefully that is quite another thing
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again to be considered
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subsequently or a balloon here's a
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balloon incidentally I'm going to do a
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program on bubbles and the like and I
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will raise the question of why is it
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hardest to blow up a balloon at the very
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beginning and easiest to blow it up the
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bigger you get it but anyway I've put
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some air in there now you know if I let
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the air out here momentum MV momentum
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the other way there is like that and one
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last
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exploration one last
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exploration Hero's
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engine here is a little vehicle you can
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make out of a tin can you put some tubes
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through it and bend the tubes at right
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angles to their uh lengths put put some
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water in
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here boil the water the steam more
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exactly Vapor which becomes Steam on
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condensation The Vapor comes out the
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orifices and this turns around ever so
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fast this is a substantial demonstration
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of Newton's third law and here is a
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bigger scale one on which we could run a
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very quantitative experiment so much
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water so much heat energy put in so so
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much energy gotten in the angular
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velocity of the
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vehicle and
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so what must we now say we have
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explored in the passing shows the first
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law in two parts the second law which is
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wrapped up in fals Ma and the third law
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referred to as action and reaction which
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I will close in this way here is a very
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freewheeling
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vehicle and you know that if I stand on
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it at rest and go forward the cart must
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go
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backward which it does and I say that
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when I Walk This Way the earth goes the
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other way I thank you for your attention
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