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I said that everything, in some sense,that was done forcenturies after Newton's filling in thedetails to F equals MA.I don't want to give you the wrongimpression, there are a lot of fascinatingdetails.Newton's universe is rich and wonderful,and Centuries of work by many, many, manybrilliant people went into producinginsights and understandings and I wish Ihad the time to tell you about at leastthatpart of it that I know, but we're inastronomy class.What I'm going to do is give you sort of ahighlight reel of those of the aspects ofwhat has been learned.That will impact, what we do and that wewill need.And we will go through and try to develop,not asdeep an understanding as we did for

gravity, which will be central.Some intuition and some understanding, forsome other important concepts in physics.And as I said we'll organize it by the n,the structure of matter, and the f, theforces that act.[COUGH]And we will start with the si, with the nside.We'll start with what we know bout the endof the nineteenth century about thestructure of matter.By the end of the nineteenth century.

There we have a pretty comprehensiveunderstanding of the structure of matter.And its center is what is known as the theatomic theory of matter.All known matter ismade up of a hundred or so types of atoms,immutable units.They're indexed by an number called z thatruns from one to, whatever 100.And roughly high elements with higher zhave heavier atoms, more massive atoms.And these characterize the chemicalelements.

We understand some rules by which theybind togetherto form compounds and molecules andvarious kinds of,of, of, objects.And bulk matter can appear in one of threecommon states.We have solids, we have liquids, andgases.And in each, the bulk properties are

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understood as a consequence ofthe microscopic dynamics of the atoms andthe molecules that form it.And as a good and very helpful to usexample, one of the forms of energy wediscussedwhen we talked about a non-conservation ofmechanical energy was heat.Friction, for example, converts kineticenergy to heatand heat causes a object's temperature torise.So we know what's we mean when somethingis hot, we mean it has a high temperature.In the context of an atomic theory, wehave a good understanding of temperature.Temperature is simply a measure of theaverage random motions of atoms andmolecules inside an object, which explainswhy heatis sort of lowest common denominator ofenergy.Heat is the energy that something has whenit's doing its own thing.

It's not moving in bulk, but itsown internal degrees of freedom arerandomly fluctuating.And for example, in an ideal gas, that's agas that's madeup of atoms or molecules that have nointernal degrees of freedom andare just non-interacting and bumping offeach other and the walls.Then you can show that temperature givesyou the average kinetic energyof a molecule or an atom or whatever itis, is proportional to the temperature.

And temperature in this expression ismeasured in Kelvin.Now Kelvin degrees are the same ascentigrade degrees.That just determinesthe units in which we measure thisconstant here K, which is calledBoltsmans constant.What is not arbitrary of course is whereyou putthe zero of T, because doubling thetemperature relative to thatzero involves doubling the energy, and

there is a temperature atwhich the average kinetic energy ofmolecules descends to complete zero.This is absolute zero or negative 273 orso degrees centigrade.And this is the origin of that.And using these equations you can see thatat higher temperatures the molecules bumparound more.A gas exerts a pressure on any container

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in which you put it.The pressure is a force per unit areaapplied in, equally in all directions.And the pressure times the volume for anideal gas,is given by that same Boltsman constanttimes the temperature,times the number of atoms in this gas.Most gases are not ideal.They're all kinds of subtleties here, butthe essence of what we seehere, is that temperature's a measure ofthe random motion of the atoms.And that the pressure and the volumeincrease with rising temperature.Now, if you take a gas and you cool itat sufficiently low temperatures and ifthe pressure is sufficientlyhigh, we'll see that later.You form a liquid phase which is similartoa gas in that it doesn't have a fixedshape.The atoms are moving around but they're

weakly bound.And as a result, you form a phase calledthe liquid which is almost incompressible.Which means, under reasonable pressures,itmaintains a constant density, the volumeof a chunk of given amount of liquid ispretty much fixed.In both cases, in both phases, the densitydecreases with temperature.What that is especially useful to us for,is it tells us that if you have acollection of fluid under gravity, then

the warmfluid, which is expanded and is thereforeless dense.Will rise because it floats above thecooler liquid.And this brings us to the point thatan equilibrium with gravity, the pressurein acollection of fluid will not be constantandthe same at all points on the container.This is the case in the absence ofexternal forces.

In the presence of an external force, likegravity, pressure will, in fact,increase with depth at a rate proportionalto the density of the fluid.And let's see a demonstration of this andmaybeit'll help us understand what's going on.In this image, what we see is very simplyme holding a slinky, a spring.And what you see when you look at it

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is the following not very surprising factwhich isthat near the top of the spring the springis more stretched than it is near thebottom.And the way you understand that is quitesimple.A slinky stretches more the more you pullit and the top half ofthe slinky is holding up its own weight aswell asthe entire half weight of the bottom halfof the slinkywhereas the coils of the slinky very nearthe bottom arenot holding up anything and so they'rerelatively relaxed and unstretched.The higher up the slinky you go the moreof the weight the slinky is supporting.And, therefore, if you look at itcarefully, you'll see that the, degree towhich the slinky is stretched decreasesuniformly from top to bottom.What we see here is me holding a cup of

water.Now this is to demonstrate the decrease inpressure with height or increase inpressure with depth.Now, this is true for the air.We know that, that high altitude airpressure decreases, butremember, the decrease in pressure is dueto the extra weightof the air.And since air is not very dense, we'regoing to assume.That air pressure is the same everywhere

in this room.Not so for the cup because the density ofwater is much larger.The water at the bottom of the cup, justlikethe spring at the top of the slinky, isholdingup all of the water above it, whereas thewaterat the top of the cup is not holding upanything.And so I expect the pressure to increasefrom the top of the cup towards

the bottom, respectful of the density ofthe water in the cup.And, there's a way to see this.The way you see this is we drilled holesin theedge of the cup and, when I open thoseholes, ifthe pressure in the fluid is identical tothe pressure atthe top, which is the pressure of the air

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around it.Then the fluid will happily stay in thecup,held in by air pressure, but of course itwon't.When I remove my fingers,the fluid will come splashing out,impelled bythe excess pressure at the bottom of thecup,relative to the air pressure which issimilarto the pressure at the top of the cup.And in case you're not certain that thisis due to theweight of the water and the gravity, Iwill drop the cupat which point fluid becomes weightless,the pressure at the top ofthe up and the bottom of the cup are nowthe same.And indeed the fountain ceases.So I hopeI've convinced you that in a fluid in

equilibrium under the influenceof gravity, pressure increases with depthdepending on the density of the fluid.This is going to be important inastronomical context.In understanding the structure of thingslike planets and starswhich contain fluid and are certainly heldtogether by gravity.We talked about gases and liquids.Matter comes in a solid state.In the solid, the positions of atomsare roughly fixed.

They can oscillate about those positions.This allows a solid to maintain its shapeunder external force.You can apply pressures and stresses and asolid will react, but only a little bit.It will not, in fact, change its shape tosuit a container.The slinky we had was a very good exampleof a solid object.It deformed in response to gravity but itdidn't completely stretch out.It gave some small proportionateresponse to the stresses that were placed

upon it.And the larger those stresses, the largerthe response but, the response waslimited.The slinky retained fundamentally, itsshape even under duress.In all of these phases, fundamentally, theinteraction is between each atom ormolecule and those near to it.There is no long distance interaction.

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Or there's a weak interactionbetween atoms in, on side of the slinkyand the other.Each side of, bit of this linking knowsabout the bits around it.And the result is that if you, for examplecompress this slinky at one point or moveitat one point, then the perturbationtravels through thematerial as each piece communicates theinformation if you wish.To the next, bit over.An, these, mechanical deformationsof a solid, of liquid, or a gas, arecalled soundwaves, and they travel with a speed,characteristic of the material.Now, the most familiar to us of course,are the longitudinal sound waves in theair.That is what we hear with our ears.But let's use that slinky.For us a very nice demonstration of wave

propagation.To do that, let's see what happens when Ilet go of the top of the Slinky.Remember, the Slinky was extended in sucha way that it was in equilibrium.The net force on every bit of it vanished.And this had to do with the fact that itwas more extended at the top and less atthe bottom.So that this compensated for the pull ofgravity on every individual piece.Now, when I release the top, the top ofthe Slinky

starts falling down, but near the bottomeverything is still in equilibrium.The bottom of the Slinky is not falling.What we see is a density wavepropagating at the appropriate speeddetermined by the Slinky.Through the slinky.And it is only when this wave reaches thebottomof the slinky that the bottom beginsactually to fall.Until that time, it is in equilibrium, andthe information

that something has happened at the top hasnot reached it.What we're seeing is a sound wave.You bang on something at one point andit takes time for that deformation topropagate throughthe material and reach the other end.