continuing the discussion of newton's third law of motion here's another example a jet engine a jet engine is a tube open at one end closed at the other we pipe in fuel and we set fire to it and the fuel burns producing hot gas okay so we're generating gas inside the tube that has the largest volume and we're getting it very hot which makes it expand to an even larger volume there isn't room for it inside the tube so what happens the gas shoots out the end of the tube that gas has a mass we don't often think of gases that have mass but they do and it has a velocity a very large velocity that gives it momentum that's the action the reaction is that the body of the engine and whatever it's bolted to like an airplane wing is pushed in the other direction and the momentum the mass times velocity of the all this material being pushed in that direction is equal to the mass times velocity of the escaping gases over here a jet engine is a newton's third law engine we create an action and the reaction is what pushes the plane along some people say no no it's there's all this still air here the gas is pushing on the air well let's address that by switching from a jet engine to a rocket engine rocket is very much the same thing except it's usually positioned this way it's a tube open at one end and closed at the other we pipe in fuel and we pipe in oxygen why do we have to pipe in oxygen here we didn't over there well the jet engine is flying through the air we can grab oxygen from the air as we go to burn the fuel our rocket is planning to go into space there is no air therefore we have to take our own oxygen with us so we pipe in fuel we pipe in oxygen we mix them we set light to them they burn and a huge mass of gas comes shooting out the end you've seen rocket launches on tv i'm sure think of those huge billowing clouds of gas an enormous m mass of fuel turning into hot gas coming out of the rocket and it's coming out again with an enormous velocity there's your action the reaction is whatever this is attached to is going in the opposite direction with the same mass times velocity action equal and opposite reaction now practical details the jet engine doesn't just look like a simple tube nor does a rocket engine well a bottle rocket that simple cardboard tube firework that you set light to and it goes that is a simple tube with chemicals that burn to produce hot gas on the inside but no a jet engine doesn't look much like a simple tube it's a lot fancier with all kinds of parts as seen here why well let's say we want to get more momentum more thrust out of our engine the first thing that we do is try to design it in ways that will increase the v increase the velocity of the escaping gas how can you do that well one approach would be to narrow the end of the tube wouldn't it the same amount of gas has to get out if it's got a smaller hole to get out through it'll have to go much faster that's like when you have a garden hose water coming out the end if you want to really make it come out fast you put your thumb over the end right make the hole smaller comes out faster okay so the the jet engine designers do everything they can to increase v the velocity of the escaping gases and that leads to the design of the engine once we've done all we can to increase v the velocity of the gases how can we still get more momentum now we have to start increasing m and we do that by burning more fuel so we need to carry bigger fuel tanks but we can do that how about our rocket same applies the designers have done all they can to the shape of the the tube the rocket engine to make sure that the gases come out with the maximum velocity and having done all they can in that area then we have to increase the amount of fuel now here we have a problem increasing the m the mass of the fuel burned means we have to carry more fuel which increases this mass so since we have more mass up here that means we have to burn more fuel which means we have to carry more fuel which means we have to burn more fuel which means we have to do you see that there's kind of a law of diminishing returns here rapidly our rocket is getting bigger and bigger and bigger and we're not getting a whole lot more for it our rocket has to defy the earth's gravity and go straight up through the atmosphere and into space have you ever seen the size of the rockets that we use they're enormous the saturn v rocket that sent a little capsule with just room for three people to cram in all the way to the moon is seen here laying on its side in a giant building at the johnson space center in houston in all it's 360 feet long 33 feet wide giant cylinder here another one another one the capsule the very far end there sorry the picture looks a little funky the thing is so huge you can't get it in one picture it's cobbled together with a picture here a picture here and a third picture over there these huge fuel tanks tank full of fuel plus a tank full of oxygen the space shuttle when we use that space shuttle itself big engines but where do you carry the fuel well that was that big brown tank strapped onto the belly of the space shuttle many times the size of the shuttle itself that was the fuel tank again look at the sheer size of the thing with people standing there for scale by the way you can go visit this it's now located at the california science center in exposition park right here in los angeles even that didn't get the job done and that was why they assembled those two booster rockets one on each side and fire all the engines the three on the shuttle and the two boosters at the same time for the initial push and they after a while would detach and fall off so we've kind of reached the limit we can going about as fast as we can using conventional rockets and yes we can beat the earth's gravity and go into space what if we want to go even faster why do we need to well we've sent people to the moon the moon astronomically speaking is incredibly close to us it's only a quarter million miles away that may sound like a lot but i had an old car that i drove quarter million miles before it eventually died on me i mean it's not an unimaginable distance so what's the one-way travel time about three days yeah that's manageable now we're talking about sending people possibly to mars how long does it take to get to mars at those speeds about six months now we have a bit of a problem what's going to happen when we lock up three four half a dozen astronauts in a very small tin can for six months and don't let them out well the type of aggressive go-getter leader personality that you need for an astronaut isn't really compatible with that kind of socializing they're going to kill each other by the end of the flight so we really need a way to get there faster newton's third law is still our guide how can we do it there is a way have you heard of something called an ion engine very popular in science fiction i believe all the big blue pulsing engines on the back of spacecraft in george lucas's galaxy far far away are according to george ion engines those however are pure fiction nothing to do with reality but an ion engine is real and we have them and we're using them okay what's an ion ion is an atom that has gained or lost one or more electrons remember how an atom works it has the nucleus of protons and neutrons in the middle it has electrons orbiting around the number of electrons all with negative charge matches the number of protons or with positive charge making the atom electrically neutral if we add or remove one or more electrons the atom becomes an electrically charged ion okay so an ion electrically charged atom atom with the wrong number of electrons mismatch between the electrons and the protons and the neat thing about an ion since it's electrically charged we can get it to move by placing it in an electric field so an ion engine a real one is a long narrow tube inside the spacecraft with a coil of water wrapped around it we place an ion an electrically charged atom into the end of the tube and we put electric current through the coil of wire generating this electric field under the influence of the electric field the ion is repelled and it accelerates down the tube and shoots out the end with a huge velocity velocity of hundreds of thousands of miles an hour of course the ion doesn't have much mass atoms are tiny but the tiny mass multiplied by the huge velocity gives us momentum and that's the action it pushes the spacecraft along in the other direction an iron engine does not deliver the kind of enormous thrust to [Music] counter gravity and push it out into space unlike the george lucas version so we still need our chemical rockets to get us off the planet and into space but once we are out far from the earth far from the pull of the earth's gravity we turn on the iron engine and we run it for several weeks and this steady gentle push just keeps accelerating the spacecraft and accelerating the spacecraft and so after a couple of weeks we turn off the iron engine this spacecraft is now traveling several times faster than it was when we began slow but steady acceleration if we are hoping to get momentum mass times velocity from our ions we want to use more mass rather than less so we want to choose a relatively heavy ion would we want to use a substance that is normally a gas a liquid or a solid well in solids the atoms are all attached together we've got to waste effort prying individual atoms off of the mass what we want is a gas where the atoms are independent from one another and we want the heaviest available gas consult a periodic table look at the elements and try to figure out where the heaviest gas is it's way down the right hand side in what are called the noble gases which go helium neon argon krypton xenon xenon chemical symbol xe is the heaviest stable gas that we have and so yes on a real spacecraft we have a bottle of xenon so we can take atoms of the gas one at a time ionize them place them in the iron edge and there are spacecraft traveling to destinations around our solar system at this time running on ion engines still an application of newton's third law we're trying here to de-emphasize the mass so we can use a small amount of material but get a lot of velocity from it newton's third law of motion