Neil deGrasse Tyson Explains "What Goes Up, Must Come Down"

you ever hear the expression what comes up must come down um yes there's even a song it used to be a commercial what goes up must come down so i remembered hearing that as a kid and i said what what you telling me nothing will can ever leave earth like what right so then i remember like researching middle school is when i sort of came of age in all of this and then you realize okay um we went to the moon okay the and and there's now of course the astronauts came back right but they didn't just fall back just when that people were just tossed up and then oh we're falling back now one giant leap to get home so there's like golf carts or rovers on the moon they're not coming back right there's the base of the lam you know the lunar excursion module the upper part went up to dock with the but the water so there are things that have been sent into space that are never coming back right so where does this saying come from well it's a pre-space era saying all right so you toss something and gravity is pulling downward on it so here's what happens you it leaves your hand with a certain speed i don't care what the speed is okay but it's a certain speed and gravity is pulling down on it if you want to do it in in english units 32 feet per second for every second it's in motion okay so if i throw it up at let's say 96 miles an hour a good fastball okay 96 miles per hour after one second it's no longer going 96 miles per hour because the acceleration of gravity is acting opposite that speed subtracting away its velocity so it started out in 96 after one second how fast is it going um 32 feet per second minus 96. you taking that away from 96. right so now how fast is it going i don't know i don't know what it is after one second 96 minus 32 dude oh oh okay because well wait a minute yes no you said it's 96 miles per hour oh embarrassing oh yeah that's so that's a different chuck calling me out okay let's let's see i throw here's how i get out of that real quick i meant 96 feet per second oh okay there you go that's what i meant all along oh okay okay all right so i throw it 96 feet per second after one second how fast is it going so and now it's uh uh 96 minus 32. 64. okay all right that's after one second after two seconds how fast is it going upwards a half of that so it's another 32 minus off of that so now it's 32 right good that's after two seconds going 32 after one more second how fast is it going well it just stopped it stops stops it stops okay after another second how fast is it going now it's accelerated back to that 32 feet so now it's coming down 32 feet per second right okay after second second it's 64 and then back to 96 when you grab it okay so that entire path holding air resistance aside for a moment is perfectly symmetric right okay and so this is gravity always pointing downward of course yeah on the way up it's subtracting from your speed so we say deceleration but it's really a negative acceleration right it's subtracting from your speed it goes through zero and now it's adding to your speed coming back down sweet all right yeah so that's gravity doing its thing cool so i think to myself um suppose i threw it faster right it would go higher before it came back okay let me throw it at 200 feet per second a thousand feet per second a million feet per second whatever pick a number there's got to be some point where it is there a point where it never comes back right but you might say well it can always subtract from it so yeah it's always going to come back isn't it well that's what it seemed like because based on the example i just gave that's what it seemed like but wait but wait as the thing gets higher it gets farther away from earth and so earth's force of gravity on it begins to drop uh-oh so no longer is it pulling at 32 feet per second down oh so as it keeps getting higher earth is still pulling on it but by less and less when you explore the consequences of this there's a speed that you can launch from earth where your object gets to infinity before the earth stops its motion completely that is a specific speed from the surface of every object there is it is the velocity of escape yeah you know what we call it the old escape velocity it's called the escape velocity that's the official in my field we tell it like it is okay and so earth has an escape velocity so wait a minute so that and that's the same for every whatever the gravity is so every gravity every gravity corresponding corresponding escape velocity escape so for earth the escape velocity is seven miles per second seven miles per second so if you could toss something up at seven miles per second i am signing with the yankees immediately earth will slow it down to zero right when it gets to infinity right that's the escape philosophy that's the escape velocity so right when it slows down it's already it's inside of the it's at infinity right it's at infinity okay so now you might say well that's weird how can you be at infinity well so here's how that goes all right excellent if you threw it at eight miles per second aha eight miles per second when it gets to infinity it's still going okay all right okay it would have eaten seven of those miles per second by the time it's at infinity and by at infinity it's still going right still moving okay so you can have greater than the escape velocity and that means you've got a place to go even when you've gotten to infinity okay so that's a skateboard so escape philosophy for the moon is much less for mars it's a little less than earth on the sun is much higher yeah i was going to say um wait what's the escape velocity for a black hole aha there is no escape it's like that scene in uh chas commentaries movie a bronx tale where they shut the door on the motorcycle gang after they asked them to kindly leave the establishment gentlemen we would appreciate it if you would leave and the guys they look at the young they're all in suits all the what supposed mobsters and the motorcycle gang throws a glass against the bar and laughs at them like yeah you're gonna make us leave and then all of a sudden one of the guys goes over and locks the door and he looks at the motorcycle gang and he says now you just can't leave that's the black hole okay that's the gangster black hole that's against the black hole now that we had a total tangent off of the fact that if you're in a black hole you're not getting out you're not getting out because the escape velocity is basically what um what's the fastest thing there is that's the speed of light right therefore so the black's got to be faster than that right yeah so so black hole is has such a high gravity at that zone that not even light can escape so the escape velocity is the speed of light right so the escape fly shows up in all kinds of ways and places so this whole thing what goes up must come down it's yeah if you're just ordinary but if you're superman no and we live in the era of superheroes and space programs and everything so it's time we retired that statement right pretty sure that's what we should do or we could change it to what goes up at 6.9 miles per hour or less must come down okay no no so it's what goes up at what goes up at 6.9 miles per second per second sorry must come down must come down right exactly exactly and just to put some icing on this cake for the astronauts who went you can go into orbit but you're still attached to earth you want to go to the moon you have to achieve escape velocity otherwise you will not make it to the moon and you'll fall back right okay just like our the ball that you tossed or this this exercise that we conducted so that's why there's a secondary booster it's called a translunar injection which sounds like some medicine you don't want but there you are in orbit going about five miles per second but you gotta hit seven miles per second so you wait until it's just right and the geometry is just right then you fire right in line with the direction you are headed right that gets you up to near seven miles per second and that'll get you to the moon actually you don't need to get earth you don't need to get to the moon you just have to get to the spot where moon's gravity is stronger than earth there's gravity wow you just have to get just to that spot and then you fall towards the moon all right so wait a minute so could that is that how they did when they did the flyby did they just allow the moon's gravity to take flybuys they're not done that way but if you want to land that's how you do it okay if you want to land you just need enough fuel to get to that point at that point and then you then you just get there crosses over the hill and you just and now you're just going downhill that's going downhill oh my god that's that's pretty brilliant that these people figured this out yeah it's called science are you just discovering this now my god people do this stuff we've got to do more with this science stuff neil we definitely have to do more with the science fiction that's how we get to mars even we don't have we don't pack fuel to get to mars we pack fuel to get to the point where mars gravity takes over then we fall towards mars sweet because that's less fuel you don't want to take more fuel than you have to exactly so you're never really flying to mars you're flying at mars waiting for mars to come and grab you to come to mars cool you're not waiting you're you're this is very calculated right excuse me act like will it will they take us willing [Laughter] wouldn't that be funny if that's how you had to do it like you're getting oh my god i hope if you just barely miss it then you just sling yourself back to earth and yeah there you go that's so cool man that's and one last thing just an interesting thought experiment again neglecting air resistance here if you take a high-powered rifle and shoot it straight up high power rifles go thousand miles i mean they're they go faster than sound okay shoot it straight up it'll go all the way up okay right and every second it loses 32 feet per second speed it reaches zero and then it comes back and every second it gains 32 feet and it'll fall right back down the nozzle of your gun at the same speed that it left okay uh thereby killing you and you so there you go knighting the magazine if you're a yahoo the lesson here is don't fire your guns in the air okay not straight up no air resistance would slow down the bullet on the way down okay so uh anyhow check oh by the way escape velocity neglects air resistance so right so you'd have to compensate for the air resistance if you were doing doing it because the air eats up some of your energy exactly anyway that's it chuck that's what goes up will only sometimes must come down there you go there you have it tell me how we're going to reword it now it's what goes up must come down as long as it's going less than 6.9 miles per second okay there you have that catchy phrase just invented you heard it here on star talk all right we gotta run all right all right chuck always good to have you always a pleasure be able to grasp tyson here keep looking up [Music] you

Channel: StarTalk

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Id: 5wwEFfOzc4c

Length: 13min 26sec (806 seconds)

Published: Tue Oct 27 2020