all right, gang.

today, we're gonna talk about begin with the m. momentum. momentum, hey. momentum. hey, which has the greater mass, a roller skate or a mack truck? mack truck. well, i would check your neighbor on that. how many say, "well, gee, i might be wrong. it must be a trick, maybe a roller skate." yeah, which has more mass, a roller skate or a mack truck? come on. a mack truck, by far, right? hands down. which has more mass, a mack truck moving or a roller skate moving? same speed. mack truck. how many say, "now, they get the same mass."? show of hands. good. okay. how about this, which has more oomph, a mack truck moving or a roller skate moving? mack truck. check your neighbor, more oomph. mack truck got something more than the roller skate.

and what is it, gang? what we're gonna talk about today, momentum. it's got more momentum, more oomph, okay? oomph. and let's define momentum. momentum is not just mass, not just inertia, but inertia in motion. so we define momentum to be the inertia of something, it's mass, multiplied by how fast it's moving, it's velocity. so momentum is mv. so when i asked you the question, which has more momentum, a roller skate or a mack truck moving at the same speed, you would say-- well, if the speeds are the same, the momentum will be the same. any fool knows that, right? but fools aside, what would you guys say? mack truck. the mack truck. the mack truck because it's got more mass. can you think of a case where the moving roller skate

might have a greater momentum than a moving mack truck? you take the mass of the mack truck, humongous, multiply it by its speed. you get a number. now, take the mass of a roller skate, tiny, tiny, multiply by its speed, you get some number. and you could make that number bigger, couldn't you if v was enormous for the roller skate, isn't that true? so momentum really involves not only inertia but how fast the inertia is moving, inertia and motion. you have that kind of idea. that's what we're gonna talk about today. and today's stuff is all common sense, and it's an outgrowth of newton's laws. if that roller skate is running down the hill and that mack truck is running down the hill and you gotta stop them. you gotta step up in front, put your hand there and force them to a stop. there's gonna be a difference, isn't there? it's gonna be a lot harder to stop the mack truck. why? because you're gonna have to decelerate it.

you're gonna have to decelerate it. and it takes not only a lot of force to-- i mean, how much force are gonna take to decelerate the mack truck compared to the roller skate? a lot or a little. a lot. why? 'cause the mack truck has a lot of mass. so i need to decelerate that, gonna take a lot of force. remember we talked about acceleration. acceleration, what? the amount of force applied upon a particular mass will yield a particular acceleration. are we getting now so we can read equations? okay? that's newton's second law. and this is how you get acceleration, and what is acceleration by definition? if someone asked you, "okay, that's how you get it by pushing on a mass, but what is acceleration?" you would say-- check the neighbor. what is acceleration anyway? change. it's a change. a change in what?

a change in velocity-- over time, over a period of time. --over time, okay? here's how you get acceleration and here's what it is. so you know what i can say. this must be equal to this. and now i can say force multiplied by time-- watch this. can you guys do a little algebra? force multiplied by time must be equal to mass multiplied by change in velocity. i've just cross multiplied. this is gonna allow me to look at things with a little more perception than otherwise. see where i have the m and the v, i can put them together. what do i call this quantity here?

check the neighbors. see if the neighbor knows what the quantity mass times speed equals. hey. begin with a m. how many sitting next to someone who has no idea? okay. change seats, change seats. yeah? okay. see what i had this change in, gang? change in. i'm getting tired of writing change in. ain't gonna write change in anymore. i'm gonna use a greek symbol for change in. what's the symbol, gang? - delta. - delta, that's right. so i'm gonna write that from now on as f multiplied by t equals change in. get it? delta mv. this is what we're gonna talk about today. so this delta mv, that's delta momentum. we have a name for force times time. what's the name of the force of an object

multiplied by the duration of time in which that force acts? we have a name for that. it's not so common a name as this one, but see if you sit next to someone who knows what is the name of force multiplied by the time during which that force acts. go. talk it up. what is it, gang? impulse. what's the name of this? what? - impulse. - impulse. all right. impulse. okay. so we write. impulse equals change in momentum, that's what we're gonna talk about today. you wanna change the momentum of something then you have to apply an impulse. what's an impulse? that's hitting it over some time interval. down here i have a-- see that golf ball on a tee. see this golf club right here? what's the momentum of the golf ball right now? - zero. - zero. watch this. now, the golf ball has momentum. how did it get it? how did it change from zero to something? how did it? 'cause i hit the darn thing, that's how come it did, right?

okay? and if i wanna change the momentum a lot, how do i hit it, a little or a lot? a lot. hit it a lot. bam. and i wanna change the momentum of that golf ball humongously, i hit that thing as hard as i can. you know what we're saying here. let's see if we can read the music. if you wanna get the momentum, the change a lot, then apply the biggest force you can. hit it as hard as you can. what's the t for? that's why you follow through. when you hit it, you don't just--and stop. you hit-- [makes sound] you make that force last as long as possible. and if you can, like, double the time during which that force acts, what will you do to the momentum? double it. let's suppose you can hit so that follow through makes the force-- pretend-- it makes the force last three times as long. three times as long. so 3/10 of a second instead of 1/10 of a second. how much will the momentum change? three times. three times. how much does the mass change?

not at all. so what changes three times? how fast the ball goes. how fast the ball go change tell you how far the ball goes. isn't that true? so you follow through to get the most momentum possible. let's suppose i have a slingshot. i get the rubber band here, you see that? and i take the rock and i pull that way back, okay? the further back i pull it, the more change in momentum i will get for two reasons. see if your neighbor knows what the two reasons are for pulling the elastic band way, way back, you get even more speed when you let go. now, everybody knows that. every child knows that. you pulled it back like this, plop. [makes sound] plop. now... [makes sound] bam. everybody knows that. you guys do, too. now, what are two reasons pulling it all the way back gives you more change in momentum? check your neighbor. okay, what's the reasons, gang? you're gonna increase the force.

when you stretch that elastic way, way back, there's a bigger force acting in that little rock that's gonna be flying out, huh? a bigger force. what else? that's right. it's gonna have a longer distance-- gonna take a longer time to-- have to spend, right? okay. so the longer the time that acts, the greater the force that acts, honey, that slingshot gonna fire faster. i mean, further. yah, yah? and it makes sense, huh? hey, how about a cannon? how about you got a couple of cannons? you got one short stubby cannon and you got the same darn cannon except the barrel's longer. now, you fire the cannonball, same amount of powder. [makes sound] which cannonball goes further? the long barrel? the short barrel? or same, same? short barrel. how about the people say, what? you got the same kind of powder. you got the same cannon. one, so a little bit longer, so what? same cannon. same oomph. hey, they both go the same. what do you guys say? short barrel. long barrel. some say, no, the long barrel. the cannonball go further because there's more force acting on it.

true or false? answer begin with f. false. it's because there's more what? time. there's more time. the cannonball is in the barrel for a longer time being pushed, pushed, pushed, pushed, pushed, pushed, pushed. it's gonna go a lot further if the cannon is a lot longer. can you see that? okay. and all that stuff makes sense here, doesn't it, huh? so we find out this physics we're talking about is really the physics of common sense. how about you-- you're riding in a car and the brakes have failed and you gotta stop the car, which is gonna require the greater force? let's suppose you got your multiple choice. you either drive into a haystack or you can drive into a cement wall. in both cases you gonna-- [makes sound] --come to a halt. in both cases, you are gonna-- if you're going at a certain speed and you come to a halt,

in both cases, you'll have the same change in momentum. true or false? - true. - true. you drive in 60 kilometers per hour, boom. after impact, you go in 0 kilometers per hour. true. you've gone to 60 to 0 in both cases. so in both cases, you have the same change in momentum? yes. do you have the same impulse to stop you? yes. do you have the same force to stop you? i say, what now. i don't understand them. you said force, you said impulse, i don't know which is which. [laughter] see, gang. here's that critical thinking of the chorus, see? the impulse is the force multiplied by the time, but the force is not the impulse, it's part of it. so what happened? what are you gonna hit? the haystack or the cement wall? how many say, well, either ones, both the same. [laughter] same impulse. see, you're not interested in impulse, you're interested in the force. now, what's gonna be the bigger force?

things like this. you're gonna change your momentum. now, you go to multiple choice. if you hit the cement wall, these two numbers will multiply together to give you the same number as this. but if you hit the haystack, take a long time to slowdown. now, which do you want? okay? you're at the top of a cliff and you gotta jump because the fire is coming closer and closer. and you look down and you see a circus net over here, and over here, you see a concrete parking lot. well, i don't know. either one. [laughter] come on, which one to jump in? now, if you didn't have any physics, you'd probably jump in either one, right? come on, i'll put-- everybody knows you jump in the net. everybody knows that. okay? and everybody knows that if you jump on the net the force that acts on you, ain't gonna be so much.

and everybody knows that if you jump unto the concrete parking lot, honey, that force is gonna be humongous splat. that's the end. that's it, right? enormous force and some people know why. and those people, is who? us. us types. okay. we know--such thing, right? okay. here's another thing too. let's suppose we went down to the football stadium and all the guys are practicing there and we take someone who looks like they're really in shape, 220-pound or something like that, and we bring him up and we bring him into the gymnasium. we got the boxing ring there and we put some gloves on. this dude is really tough. he's not so much into fighting, doesn't know the-- he's a savvy, you know, but he's a strong guy. and then we invite the heavyweight champion of the world to come to the university to give a little demonstration. and we put the gloves on both and this is just a sample athlete, okay?

football-type. and we say to the champ of the world, "champ, this guy might not look like much "but i'm telling you, you better be serious. take him out as fast as you can." so the champ means business. now, you ring the bell. [makes sound] they both come out. what's likely to happen? like i said, nothing else. the very good chance our victim might end up with a tag on his toe that night and be stretched out on a slab. because the heavyweight champion of the world-- [makes sound] boom, and hits that guy, good chance of killing the guy. killing, dead. hc. but, but take that same guy and let him workout for a couple of weeks. let him get-- let him just move around, learn how to move, huh, and get a little ringside. and then have the same scenario happen again. ring the bell, they come out--

[makes sounds] boom, he's down. but, you know what? he gets back up. and the second case he gets back up-- on the first case, likely doesn't get up at all. what's the difference, gang? what's the difference? here's what happens on the first case. the guy is--the guy comes out. ding, okay, this is the football hero comes out. [makes sounds] boom. that's it. that's all over. the second case he comes out. sees the punch-- [makes sound] --pulls back a little bit. even a little bit, rides with the punch. when he rides with the punch, boom. go down maybe, but he's not gonna be smashed. now, why does riding with the punch make a difference? i used to be into this stuff when i was 17-year-old. scout honor, a silver medalist, new england states. and i was into this as a teenager. i was really into that stuff and i thought i knew the reason.

i didn't know any physics then, but i knew why it was when you ride with the punch. you don't get hurt and if you're coming in, you really get hurt. now, i had figured it out. and this is my figuring. when that punch comes in-- if you ride with it, come back like this, both are going in the same direction. and the speed of impact is gonna be reduced, see? 'cause if you come in and you stop, boom. you get a little speed. but if you come like this and you move back, it hits with less speed. can you see that? see? and if you're coming in like this, boom, even more speed. and so i knew why and what i knew turned out to be wrong because one day i was looking at these old ring magazines. and i saw an astounding fact that changed my whole theory. it demolished it. and it was a fact that said this, that a fellow by the name of joe louis, who was a heavyweight champ many years ago, they timed his right cross. they timed it. [makes sound] and they timed it to be 90 miles per hour. that's how fast it's coming in. [makes sound]

and--wait a minute, honey, wait a minute. if this punch come in at 90 miles an hour, you stay there-- [makes sounds] --you're out. but it's coming at 90 miles an hour and you move back-- now, let me ask you guys a question. how far--how fast do i move back? how many miles per hour? [laughter] two. two at most, right? so if i come back, what's the velocity of the punch? - 88. - 88. [laughter] and what if i stayed there? 90. 88, 90 is same same. come on. [laughter] so you know what? that can't be the explanation. it doesn't have to do with relative speed. have to do with something else. check your neighbor and see if your neighbor had-- knows what it is. hint. hint. hint. [laughter] what's it have to do with, gang? force. it has to do with the time. see, when you pull back, it takes a longer time for that momentum to spend. let's look at the equation, okay?

we're saying that this is true. this is true, huh? impulse equals change in momentum. now, let's look at this equation with respect to boxers and one hitting the other, okay? this is-- let's suppose this is the force, the force of the punch. [makes sounds] it's coming in, okay? this is the time of impact of the punch, right? what's this change in momentum? what is that? what's that m? if this is the force of the punch, this is the mass of-- mass coming in. in a bar room brawl, this would be the mass of the guy's arm multiplied by the speed. so you saw-- [makes sounds] my arm got momentum. it hits. [makes sounds] the momentum stops, the momentum changes. the momentum changes because there's an impulse acting. that impulse-- that's the impact, huh? that impulse is gonna be force multiplied by time equals a change in the mass of the arm times the speed.

but that's a bar room brawl, the heavyweight champion of the world. you wouldn't put an m for the mass of his arm because the heavyweight champion of the world doesn't punch like that, doesn't punch with his arms. the heavyweight champion of the world punches from about the ankles up. oh. oh. okay? the whole mass of the body is in there-- [makes sounds] --from here up. and you take the mass of the whole person then times the speed. honey, that's a humongous momentum. and that momentum is coming at you. now, you gotta stop it. and someone says to you, "hey, well, "you might as well get it over quick. just stand there." good idea or bad idea? bad. so what do you do? you come back. you roll back, and you make the time of impact, the time during which that momentum spends itself, you make that time as long as possible. and when you make the time long, then what happened to the force of impact? shrink. you kinda see that? and what happens if you come in? boom. it happens so quickly, so quickly.

and very quickly, the force of impact is huge and it's knockout time. here's something i never could understand at that time. and that's this. you're gonna get ready for your tournament coming up next friday. three three-minute rounds. so for three minutes, you're going, boom. and three minutes again and three minutes up, that's it, the fight's over. now, you go in the gym and prepare for this. you gotta be in shape. so in the gym, what you do, you get the great big bag there, and you, bam, bam, bam, bam, bam, bam, you're hitting that bag all day long, bam, bam, and you-- hours on that bag. oh, yeah, you feel good. you feel really good. you're tired, you get energy, ba-bam, you're hitting that bag, hitting bag. and you're saying, "--i hit this bag all day long, tirelessly. "okay, i'm in shape. "and all i gotta do is nine minutes, friday night? whoo, no problem." and friday night come. and at the end of the first three minutes, you go back to your corner and-- [makes panting]

--you are tired. and you can't understand why, because in the gym, you can hit the bag all day long. and first three minutes, boom. what's going on? at the time i thought it was an audience thing, that you're more tense 'cause all these people are looking at you. and that was my feeling. and my thing was then, well, pretend i'm not there and just do the best you can, okay? psych yourself up. and later on when i get into physics, i say, "hey, son of a gun, i know what's going on." do you guys see what's going on? when you're in a gym, you hit it bag with-- boom. you put a lot of momentum-- crunch. that momentum stops. what stops it? the bag. the bag. the bag provides the impulse to stop the momentum of the punch. yes. now, opening night comes and the guy--you throw it out. [makes sounds] he's down here. [makes sounds] he's down here. [makes sounds] he's down here. you missed. and every time you missed, you were-- [makes sounds]

who supplies the momentum to stop that punch? you. myself. i throw another. [makes sounds] who supplies the-- [makes sounds] it's not-- the impulse, i should say. who supplies the impulse? next chapter, we'll learn the energy. where's the energy come from to stop these punches that are thrown? from the person who throws them if he misses. that's why you see good fighters. good fighters won't throw so many. when they throw, they hit. if you're gonna go 12, 15 rounds, you gotta-- when you throw, you hit. you miss, miss, miss, miss, you're not gonna last. you're gonna wear yourself right down. so the laws of physics, very important in sports. you're playing sandlot ball. you're walking across the ball field. these kids are throwing the ball back and forth, huh? you got no glove. "hey, kid, throw me one. throw me one." kid says, "mister, you ain't got a glove. got no glove, honey." "throw me one, anyway. hey, i'm in shape." kid takes a hardball.

[makes sounds] now, you catch the ball. how do you catch the ball? man, that ball coming-- don't you put your hand way out here? [makes sounds] and then you throw it back, right? --kid said, "didn't hurt your hand, kid, i mean--" "oh, no, no. i'm in shape." it hurt your hand, kinda, did it, all right? and the kid throws another ball. don't you put your hand way out here? why do you put your hand out here when you catch it? why do you do that? check your neighbor. you hold your hand out there so that when you catch the ball, what are you making bigger, gang? time. you're making the time during which the momentum of that ball cuts down to zero as long as possible. so you grab way out here and-- [makes sounds] take a lot, a lot of time, a lot of time, a lot of time, a little force, ain't that right? and so the kid says, "gee, how'd you do that." i said, "my hand, i'm in shape, kid." kid says, "you got a strong hand, huh? put your hand up against the board, man." now, we can--what happen if you catch the ball now? huh? oh. then it'd be a short time. short time, large forces.

let's suppose you want to get the largest force possible in stopping the momentum of something. how would you like to see a karate demonstration right now? i'm up to my white belt. [laughter] see this four by four oak? can you see it? yeah. see it setting on these two little things right here? can you guys see it? see the-- i'm taking this piece of cloth and putting around my hand, protect it a little bit? you see that? okay? how many people don't see it? we all see it. we're imaginative types, aren't we, okay? now, watch this. i'm gonna hit it and i'm gonna break that four by four oak. okay, watch this. hitman. you see that? [makes sounds] done. done. look at it, splat. now, how was i able to break that? because i changed the momentum of my hand in a short time or a long time? so short you hardly saw it.

isn't that true? how about i do it like this, gang? [makes sounds] no way. no way. [makes sounds] done. and, furthermore, if i do it in such a way, i don't pull my hand back. but if i do it in such a way that my hand bounces off there, whoo, honey, that is gonna break. that is gonna break, because bouncing gives a lot more impulse than just hitting. you're walking along the street. there's a plant pot up above. the plant pot comes down, hits you on the head. boom. it sticks to your head. you're in trouble. but the plant pot comes down and--ba-boom-- bounces off your head. honey, you are really in trouble, because the bouncing gives more impulse. a lot of people have trouble understanding that. let's see if we can understand it with this idea.

let's suppose you're standing on a skateboard right here. you're in the skateboard. and on the skateboard, someone throws you a ball. okay? the ball is coming. the ball has momentum. you catch the ball. boom. any impulse on you? yes. and that impulse does what? [makes sound] pushes you along, right? okay. let's repeat the experiment. this time, you stand on the ball-- stand on the skateboard, and you throw the ball. you give it the same momentum that you stopped it coming in, that it had when it came in and you stopped it. you get the idea of what i'm saying? when you throw the ball out like that, do you supply an impulse on the ball? can you supply an impulse on the ball without the ball supplying an equal and opposite impulse on you? before, we said force. we can say impulse, that force times time, force times time. so when you throw the ball, what are you gonna do? you're gonna recoil. do you see that? do you see that if you throw the ball just as fast as you caught it, you'll recoil the same?

here's why. if you change the momentum of the ball the same amount each time, then you'll have on you the same impulse each time. do you see that? does this stuff mean anything to you? some people--and what's that, a greek letter d, a m, a v, a equal sign. i don't know what it mean. can you guys see that everything i'm talking about ties into this rule? okay? now, here's the thing. let's suppose you're on a skateboard and you catch the ball. and then you throw it back again. more impulse on you or the same as if you just caught it? check your neighbor. can you see if you catch the ball and throw it bacout again that there'll be more impulse on you than if you only catch it or you're only throwing it? or that when something bounces off you, it's in effect the same as catching it and throwing it, and a bouncing collision is gonna give more oomph than just a sticky collision.

you know who made a fortune on this? back in the 1849, in the gold rush time in california, a lot of people made a lot of money. but one of the people to make the most amount of money was a fellow by the name of lester pelton. and what lester pelton did was he redesigned water wheels and had the good sense to patent his design. water wheels at that time had these blades. and, you know, didn't have electricity and power like we have today, so they would have a water wheel to turn the wheels of the gold mills, huh? and the water would come down, hit these paddles, kinda come to a stop and kinda go along with it. what lester pelton did was this: he redesigned the paddles. he made the paddles like that. so the water would come down, make a u-turn and bounce back out again. and he made the paddles such-- in such a way to make the water bounce. and when the water bounced off the paddle wheel, more oomph.

and he made more money on this than most of the gold miners did on their claims simply using the laws of physics. you apply more impulse to that spinning wheel by bouncing the water than just simply catching it or stopping it. ain't that neat? any questions to this time? okay. here, we have a little device called an air cart. and right now, there's a lot of friction with these things. see, it's a-- they kinda slow down very quickly. a body at rest doesn't remain at rest very long. i mean, a body in motion doesn't remain in motion because i've got a opposing force. and a force of friction, of course, is slowing these things down. what i'm gonna do, though, is i'm gonna-- this is like a vacuum cleaner in reverse. i'm gonna throw on the switch and it's gonna blow air jets. air is gonna cut them. you can't see that, but there's a hole out of little holes there. and air is gonna blow out. and these things are gonna ride on a cushion of air. and it's pretty nearly friction-free.

let me show you what i mean. riding almost friction-free. now, let's see what happens here. watch this. how come it moved? let's try it again. and how come this stopped? this is common sense. but let's look at it with some care, and we might see more than we would see otherwise. this a coming in and it hits b. when it hits b, what did we see?

we saw this one stop and this one continue. and i ask, why did this one start to move? and you would say, it started to move because it was belted, right? this one come in, a come in, bam, and belted it, that's why it moved. when i say belted, more specifically, what happened? because there was an impulse. there was an impulse on this. right now, momentum is zero? this come in and hit it and an impulse was applied to this. let's look at only object b, only this one. and we'll focus all our intention on that. i'm gonna put a dotted line around only object b. and i can answer the question why object b started to move, because, bam, there was a force of impact of a on b, and that made b move. and so you guys saw b takes off.

so it's easy to see why b started moving, okay? an impulse acted on it, its momentum changed. but did you notice also that the momentum of this changed? let me show you again. why did this one-- why did object a slow down to a dead stop? what slowed this down? -- to say it slowed down is to say there was a change in momentum. true or false? true. and if there's a change in momentum, there has to be--true or false, an impulse. true. and did you see an impulse act on this? yes. yes, you did. because if we look at this again-- okay, right here. if we're only interested in a, we let object a be our system.

is there any force, any impulse acting on a when they collide? yeah. yes. and what's the force that acts on a? b. b is hitting it, all right? is that right? so see that arrow, so that's what stops a. let's suppose we focus on both together. momentum, momentum. how big was the momentum of this compared to the momentum of this after collision? did you see that? if we look at both, if we let our system, our concern will be both. this acts on this, this acts on this, how much force is acting on our system? none. see? if our system is only b, and there's a force

poking in there, there's a force acting on it, which provides an impulse-- away it goes, b, huh? and over here, if we let our system be a, we can see there's a force from the outside that acts on it. the force of b pokes in and stops it. now, let's concern our system to be both together. then this acts on this, this acts on this. the action and reaction are within the system. there's nothing poking from the outside. let me ask you a question. for this being my system, what's the net force acting on it? zero. zero. what's the net impulse acting on it? zero. not so many people said the same thing. you guys see the net force acting on a system is zero, what's the net impulse then? zero. it's the net force multiplied by the time. and it's -- gonna be zero. so what's the change in my momentum if there's a zero impulse? zero. zero. and the momentum of my system doesn't change. this one stops and this one continues.

this one continues with the same momentum this one had. let's suppose at the beginning when-- let's suppose this had five units of momentum. comes to a halt. how much momentum does this move with? five. so what's the momentum before and after the collision? five fives, no change, which turns out to be a very, very neat idea. and the idea is this. if there's no net force acting on your system, and when you say system depend-- what do you mean by a system, gang? see, i've got three systems here. but if i have a system such that the net force is zero, then the impulse is zero. that means that change in momentum is zero. zero. careful. it doesn't mean the momentum is zero. it means the change is zero. so whatever momentum you have before something interesting happens, you will have the same momentum after. that's a very powerful idea and underlies a lot of physics.

take a spinning system. it's got momentum. let the system blow up to whatever you want. i don't care what they-- what it does. check out momentum afterwards and guess what you got, the same momentum. when you study astronomy, that will be a pillar from which you will build many other ideas. that the momentum that a system has, if you don't mess with it from the outside, whatever momentum it has, it's gonna stay. it can't change unless there's an outside force. outside force means outside impulse. you know that, you're sitting in your car and the car baery is dead and you gotta get out and push the car, but it's raining. so you say, "well, i think i'll just sit here and push in the dashboard." you push--does any-- did the momentum of the car change? so you push harder on the dashboard. what are you doing? you're pushing in a dashboard, dashboard pushing back on you.

what's the net force acting on the car? zero. zero. so what's the change in the momentum of the car? zero. so that's why you gotta get outside. you get outside, now you push. you're pushing the ground, the ground push on you. you push against the car, there's a force pushing on the car, the car changes its momentum. you gotta have an outside force. in the absence of an outside force, what's the change in the momentum of any system? zero. zero. no change. it will simply transfer from one to another. i can show you that with another case of the collision. this time, i'm gonna have-- my colliding cars are gonna stick to one another, okay? and so let's see what we got now. and i move this car with momentum, it's gonna hit here. let's suppose 10 units of momentum. ten, just twice its masses. one more time.

from what you know about physics, i moved body a in with 10 units of momentum. that means its mass times its speed multiplied together be 10. ten units, 10 whatever, okay? it hit. boom. afterwards, you saw so both of them moving. how much momentum for both? five. some people say five. where the other five momentum go? let me say this, gang. here's your system right here. when these things hit, was there an outside force acting on the system? no. was there? no. no. so is there a change in the momentum of the system? no. no. so if this got 10 and this got zero,

add them together, what do you got? 10. 10. after collision, what do you got? 10. 10. you got the same momentum before and after. now here's the question you're asking about-- velocity. the speed. if the momentum is 10 before and after, what's the speed? we can assume that it's-- they have the same mass. in this case they have the same mass. you've got one car coming in with the speed. let's suppose the mass is 1 kilogram, so it's coming in at 10 meters per second, let's just make that up. okay? and it has a mass of 1 kilogram, so what's the momentum of the object? 10. it's 1 times 10 is 10, 10 units of momentum. okay? now, it hits another car, but this time

instead of bouncing, both cars move together. what's the momentum before and after collision? ss. same, same. same, same. so what's the momentum after collision? but now i'm gonna ask, what's the speed after collision? see, why some people get mixed up? "they got speed, momentum, mass-- "there's all these ideas. i can't handle them. i'm gonna drop the course." no, no, no, no. get some tutoring, okay? you see the different ideas? so if you have twice as much mass moving with the same momentum, they must be moving with how much speed? 10. oh, not 10. 'cause now this is one, this is one, i get--look at this. to begin with, the momentum before is gonna be equal to, after. before i've got mass of 1 times 10 mv plus--

if this was a zero at the beginning, remember the zero? boom, they hit. and then after they hit, i've got how much mass moving? two. two. two. both cars are moving. so two times, and here's what i got to figure out. 1 times 10 is 10 equals 2 times-- there are people what can calculate what this must be. is there anyone in here who can't? stand up, i wanna see what you look like. what's this number here, again? five. and so we see, yeah. the two cars do move at half the speed. so they'll have the same momentum. i used to live out in colorado and i lived near a place where the railroad cars would get together. and it had like a lot of mining out there,

it have, sometimes, like 80 freight cars gonna be pulled to gunnison or maybe pulled to pueblo or somewhere like that. and boy, it used to be tough at nighttime because at nighttime, these cars would all clank together like the freight car would come up and they wanna get them all clanked together. and it turns out that the coupling between the cars was loose. so instead of them coming up and, boom, one great big clank, there'll be clank and then clank, clank, clank, clank, clank, clank, clank, 78, 79, 80, oh, hey, pull, 80 clanks. and, whew, now, i can go to sleep. but then what happened is they just connected together. now, they're gonna pull this way. and what happens is clank, clank, clank, clank and that's it 78, 79, 80. and i used to think to myself, "why don't they take those couplings "and tighten them up so they don't all clank? why do they do that?" who--i know why, i know why, i know why--begins with f. physics. physics. physics. yey, all right. it's because of the physics.

let me ask you, guys, a question. you wanna put-- you wanna set into motion 80 freight cars? 80? let's suppose you tie them all together so there's no loose coupling. so when you start to pull, you pull all 80 at once. fat chance. you know what's gonna happen with that locomotive? those wheels are gonna spin. you ain't gonna pull 80 cars, no way. you can't get that much force on your truck, you can't do it. but you can do this, you can pull one. and once you got one going then you can pull another one. and now you got two going, now you can pull another one, and then another, and then another, and you can get all those 80 cars moving by pulling them how? stretch out the what? begin with t, end with ime. time.

by stretching out the time. you'll have enough force of friction between that big wheel and the track to get all those cars in motion. that's right. ain't that neat? it's like taking your courses in school. did you ever take 80 units at once? [laughter] honey, you can't pull 80, 80 units at once, right? it's like taking a summer course and maybe, like, three calculus courses in the chemistry lab and then in 10 weeks-- you can't do that. you need more--time. you like to see something neat? it's called the swinging wonder. watch this, gang. i gonna be lifting one of this balls up. that's strange. only one ball came out the other side. i'm gonna be lifting two balls up. that's strange. two came out the other side.

i'm gonna be lifting four balls up. no. i lifted three, i said four. i thought maybe it might be what i'm saying, okay? and it turns out it wasn't what i was saying. let's see what happens if i do lift four balls up. but there's only 1, 2, 3, 4, 5, 6, 7. let's try it. that would be wild if four balls came out the other side now. if that were the case, it would seem that there must be some underlying rules applied here, huh? let's try four. huh? and we got four. let's try five. wow. let's try six. woo. let's try seven. [laughter] yeah. ain't that neat? i wonder if there's a reason for that. i wonder if there's a reason. let me ask you a question. when one comes in--

it's got a certain momentum, right? that momentum-- [makes sounds] --and--how much momentum? the same. the same or more? the same. i say, "oh, well, probably more." come on, free lunch, no way. okay? when one comes in, boom, you get one out; two in, two out. now, let me ask you a question, how do the balls know? [laughter] you wonder about things like that, ain't that right? see? the momentum on one side, the momentum on the other side: the same. we say the momentum is conserved. it just transfer it through the system with none being lost and none being gained. so one, one, two, two and like that, but i'll tell you what, gang. how would it be if i had two of these come down, two, and one come out with twice the speed? if that happened,

would the momentum before and after be the same? think. two balls, certain speed, one ball pop out with twice the speed. check the numbers, can you do that? and when you do that, you find out that there's nothing wrong. but you know what? you could do this for eons and eons, and you never lift up two and see one come out with twice the speed. no can happen. why no can happen? because there's something else that has to be conserved besides momentum. and that something else is what we gonna be talking about next time. begins with a e, gang, what's it gonna be? energy. energy, next time. yay. [music]

captioning performed by aegis rapidtext