All right, gang. Today, were 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 were gonna talk about today, momentum. Its got more momentum, more oomph, okay . Oomph. And lets define momentum. Momentum is not just mass, not just inertia, but inertia in motion. So we define momentum to be the inertia of something, its mass, multiplied by how fast its moving, its 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 its 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, couldnt you if v was enormous for the roller skate, isnt 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. Thats what were gonna talk about today. And todays stuff is all common sense, and its an outgrowth of newtons 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. Theres gonna be a difference, isnt there . Its gonna be a lot harder to stop the mack truck. Why . Because youre gonna have to decelerate it. Youre 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 . Thats newtons second law. And this is how you get acceleration, and what is acceleration by definition . If someone asked you, okay, thats how you get it by pushing on a mass, but what is acceleration . You would say check the neighbor. What is acceleration anyway . Change. Its a change. A change in what . A change in velocity over time, over a period of time. Over time, okay . Heres how you get acceleration and heres 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. Ive 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. Im getting tired of writing change in. Aint gonna write change in anymore. Im gonna use a greek symbol for change in. Whats the symbol, gang . Delta. Delta, thats right. So im gonna write that from now on as f multiplied by t equals change in. Get it . Delta mv. This is what were gonna talk about today. So this delta mv, thats delta momentum. We have a name for force times time. Whats 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. Its 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. Whats the name of this . What . Impulse. Impulse. All right. Impulse. Okay. So we write. Impulse equals change in momentum, thats what were gonna talk about today. You wanna change the momentum of something then you have to apply an impulse. Whats an impulse . Thats hitting it over some time interval. Down here i have a see that golf ball on a tee. See this golf club right here . Whats 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, thats 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 were saying here. Lets 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. Whats the t for . Thats why you follow through. When you hit it, you dont justand 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. Lets 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. Isnt that true . So you follow through to get the most momentum possible. Lets 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, whats the reasons, gang . Youre gonna increase the force. When you stretch that elastic way, way back, theres a bigger force acting in that little rock thats gonna be flying out, huh . A bigger force. What else . Thats right. Its 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 barrels 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 theres more force acting on it. True or false . Answer begin with f. False. Its because theres more what . Time. Theres more time. The cannonball is in the barrel for a longer time being pushed, pushed, pushed, pushed, pushed, pushed, pushed. Its gonna go a lot further if the cannon is a lot longer. Can you see that . Okay. And all that stuff makes sense here, doesnt it, huh . So we find out this physics were talking about is really the physics of common sense. How about you youre riding in a car and the brakes have failed and you gotta stop the car, which is gonna require the greater force . Lets 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 youre going at a certain speed and you come to a halt, in both cases, youll 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. Youve 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 dont understand them. You said force, you said impulse, i dont know which is which. [laughter] see, gang. Heres that Critical Thinking of the chorus, see . The impulse is the force multiplied by the time, but the force is not the impulse, its 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, youre not interested in impulse, youre interested in the force. Now, whats gonna be the bigger force . Things like this. Youre 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 . Youre 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 dont know. Either one. [laughter] come on, which one to jump in . Now, if you didnt have any physics, youd probably jump in either one, right . Come on, ill 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, aint gonna be so much. And Everybody Knows that if you jump unto the concrete parking lot, honey, that force is gonna be humongous splat. Thats the end. Thats it, right . Enormous force and some people know why. And those people, is who . Us. Us types. Okay. We knowsuch thing, right . Okay. Heres another thing too. Lets suppose we went down to the football stadium and all the guys are practicing there and we take someone who looks like theyre really in shape, 220pound 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. Hes not so much into fighting, doesnt know the hes a savvy, you know, but hes 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 . Footballtype. And we say to the champ of the world, champ, this guy might not look like much but im 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. Whats 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, hes down. But, you know what . He gets back up. And the second case he gets back up on the first case, likely doesnt get up at all. Whats the difference, gang . Whats the difference . Heres what happens on the first case. The guy isthe guy comes out. Ding, okay, this is the football hero comes out. [makes sounds] boom. Thats it. Thats 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 hes not gonna be smashed. Now, why does riding with the punch make a difference . I used to be into this stuff when i was 17yearold. Scout honor, a silver medalist, new engld states. And i was into this as a teenager. I was really into that stuff and i thought i knew the reason. I didnt know any physics then, but i knew why it was when you ride with the punch. You dont get hurt and if youre 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 youre 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. Thats how fast its coming in. [makes sound] andwait a minute, honey, wait a minute. If this punch come in at 90 miles an hour, you stay there [makes sounds] youre out. But its coming at 90 miles an hour and you move back now, let me ask you guys a question. How farhow fast do i move back . How many Miles Per Hour . [laughter] two. Two at most, right . So if i come back, whats 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 cant be the explanation. It doesnt 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] whats 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. Lets look at the equation, okay . Were saying that this is true. This is true, huh . Impulse equals change in momentum. Now, lets look at this equation with respect to boxers and one hitting the other, okay . This is lets suppose this is the force, the force of the punch. [makes sounds] its coming in, okay . This is the time of impact of the punch, right . Whats this change in momentum . What is that . Whats 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 guys 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 theres an impulse acting. That impulse thats 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 thats a bar room brawl, the heavyweight champion of the world. You wouldnt put an m for the mass of his arm because the heavyweight champion of the world doesnt punch like that, doesnt 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, thats 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 its knockout time. Heres something i never could understand at that time. And thats this. Youre gonna get ready for your tournament coming up next friday. Three threeminute rounds. So for three minutes, youre going, boom. And three minutes again and three minutes up, thats it, the fights 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, youre hitting that bag all day long, bam, bam, and you hours on that bag. Oh, yeah, you feel good. You feel really good. Youre tired, you get energy, babam, youre hitting that bag, hitting bag. And youre saying, i hit this bag all day long, tirelessly. Okay, im 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 cant understand why, because in the gym, you can hit the bag all day long. And first three minutes, boom. Whats going on . At the time i thought it was an audience thing, that youre more tense cause all these people are looking at you. And that was my feeling. And my thing was then, well, pretend im 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 whats going on. Do you guys see whats going on . When youre 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 guyyou throw it out. [makes sounds] hes down here. [makes sounds] hes down here. [makes sounds] hes 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] its not the impulse, i should say. Who supplies the impulse . Next chapter, well learn the energy. Wheres the energy come from to stop these punches that are thrown . From the person who throws them if he misses. Thats why you see good fighters. Good fighters wont throw so many. When they throw, they hit. If youre gonna go 12, 15 rounds, you gotta when you throw, you hit. You miss, miss, miss, miss, youre not gonna last. Youre gonna wear yourself right down. So the laws of physics, very important in sports. Youre playing sandlot ball. Youre 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 aint got a glove. Got no glove, honey. Throw me one, anyway. Hey, im in shape. Kid takes a hardball. [makes sounds] now, you catch the ball. How do you catch the ball . Man, that ball coming dont you put your hand way out here . [makes sounds] and then you throw it back, right . Kid said, didnt hurt your hand, kid, i mean oh, no, no. Im in shape. It hurt your hand, kinda, did it, all right . And the kid throws another ball. Dont 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. Youre 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, aint that right . And so the kid says, gee, howd you do that. I said, my hand, im in shape, kid. Kid says, you got a strong hand, huh . Put your hand up against the board, man. Now, we canwhat happen if you catch the ball now . Huh . Oh. Then itd be a short time. Short time, large forces. Lets 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 . Im 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 im taking this piece of cloth and putting around my hand, protect it a little bit . You see that . Okay . How many people dont see it . We all see it. Were imaginative types, arent we, okay . Now, watch this. Im gonna hit it and im 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. Isnt 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 dont 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. Youre walking along the street. Theres a plant pot up above. The plant pot comes down, hits you on the head. Boom. It sticks to your head. Youre in trouble. But the plant pot comes down andbaboom bounces off your head. Honey, you are really in trouble, because the bouncing gives more impulse. A lot of people have trouble understanding that. Lets see if we can understand it with this idea. Lets suppose youre standing on a skateboard right here. Youre 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. Lets 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 im 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 . Youre gonna recoil. Do you see that . Do you see that if you throw the ball just as fast as you caught it, youll recoil the same . Heres why. If you change the momentum of the ball the same amount each time, then youll have on you the same impulse each time. Do you see that . Does this stuff mean anything to you . Some peopleand whats that, a greek letter d, a m, a v, a equal sign. I dont know what it mean. Can you guys see that everything im talking about ties into this rule . Okay . Now, heres the thing. Lets suppose youre 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 bacout again that therell be more impulse on you than if you only catch it or youre only throwing it . Or that when something bounces off you, its 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, didnt 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 uturn 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. Aint that neat . Any questions to this time . Okay. Here, we have a little device called an air cart. And right now, theres a lot of friction with these things. See, its a they kinda slow down very quickly. A body at rest doesnt remain at rest very long. I mean, a body in motion doesnt remain in motion because ive got a opposing force. And a force of friction, of course, is slowing these things down. What im gonna do, though, is im gonna this is like a vacuum cleaner in reverse. Im gonna throw on the switch and its gonna blow air jets. Air is gonna cut them. You cant see that, but theres 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 its pretty nearly frictionfree. Let me show you what i mean. Riding almost frictionfree. Now, lets see what happens here. Watch this. How come it moved . Lets try it again. And how come this stopped . This is common sense. But lets 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, thats 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. Lets look at only object b, only this one. And well focus all our intention on that. Im 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 its 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 theres a change in momentum, there has to betrue 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 were 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 whats the force that acts on a . B. B is hitting it, all right . Is that right . So see that arrow, so thats what stops a. Lets 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 theres a force poking in there, theres 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 theres a force from the outside that acts on it. The force of b pokes in and stops it. Now, lets concern our system to be both together. Then this acts on this, this acts on this. The action and reaction are within the system. Theres nothing poking from the outside. Let me ask you a question. For this being my system, whats the net force acting on it . Zero. Zero. Whats 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, whats the net impulse then . Zero. Its the net force multiplied by the time. And its gonna be zero. So whats the change in my momentum if theres a zero impulse . Zero. Zero. And the momentum of my system doesnt change. This one stops and this one continues. This one continues with the same momentum this one had. Lets suppose at the beginning when lets suppose this had five units of momentum. Comes to a halt. How much momentum does this move with . Five. So whats 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 theres no net force acting on your system, and when you say system depend what do you mean by a system, gang . See, ive 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 doesnt 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. Thats a very powerful idea and underlies a lot of physics. Take a spinning system. Its got momentum. Let the system blow up to whatever you want. I dont 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 dont mess with it from the outside, whatever momentum it has, its gonna stay. It cant change unless theres an outside force. Outside force means outside impulse. You know that, youre sitting in your car and the car battery is dead and you gotta get out and push the car, but its raining. So you say, well, i think ill just sit here and push in the dashboard. You pushdoes any did the momentum of the car change . So you push harder on the dashboard. What are you doing . Youre pushing in a dashboard, dashboard pushing back on you. Whats the net force acting on the car . Zero. Zero. So whats the change in the momentum of the car . Zero. So thats why you gotta get outside. You get outside, now you push. Youre pushing the ground, the ground push on you. You push against the car, theres a force pushing on the car, the car changes its momentum. You gotta have an outside force. In the absence of an outside force, whats 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, im gonna have my colliding cars are gonna stick to one another, okay . And so lets see what we got now. And i move this car with momentum, its gonna hit here. Lets 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. Heres 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 heres the question youre asking about velocity. The speed. If the momentum is 10 before and after, whats the speed . We can assume that its they have the same mass. In this case they have the same mass. Youve got one car coming in with the speed. Lets suppose the mass is 1 kilogram, so its coming in at 10 meters per second, lets just make that up. Okay . And it has a mass of 1 kilogram, so whats the momentum of the object . 10. Its 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. Whats the momentum before and after collision . Ss. Same, same. Same, same. So whats the momentum after collision . But now im gonna ask, whats the speed after collision . See, why some people get mixed up . They got speed, momentum, mass theres all these ideas. I cant handle them. Im 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 getlook at this. To begin with, the momentum before is gonna be equal to, after. Before ive 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, ive got how much mass moving . Two. Two. Two. Both cars are moving. So two times, and heres 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 cant . Stand up, i wanna see what you look like. Whats this number here, again . Five. And so we see, yeah. The two cars do move at half the speed. So theyll 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, therell 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, theyre gonna pull this way. And what happens is clank, clank, clank, clank and thats it 78, 79, 80. And i used to think to myself, why dont they take those couplings and tighten them up so they dont all clank . Why do they do that . Whoi know why, i know why, i know whybegins with f. Physics. Physics. Physics. Yey, all right. Its because of the physics. Let me ask you, guys, a question. You wanna put you wanna set into motion 80 freight cars . 80 . Lets suppose you tie them all together so theres no loose coupling. So when you start to pull, you pull all 80 at once. Fat chance. You know whats gonna happen with that locomotive . Those wheels are gonna spin. You aint gonna pull 80 cars, no way. You cant get that much force on your truck, you cant 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. Youll have enough force of friction between that big wheel and the track to get all those cars in motion. Thats right. Aint that neat . Its like taking your courses in school. Did you ever take 80 units at once . [laughter] honey, you cant pull 80, 80 units at once, right . Its like taking a summer course and maybe, like, three calculus courses in the Chemistry Lab and then in 10 weeks you cant do that. You need moretime. You like to see something neat . Its called the swinging wonder. Watch this, gang. I gonna be lifting one of this balls up. Thats strange. Only one ball came out the other side. Im gonna be lifting two balls up. Thats strange. Two came out the other side. Im gonna be lifting four balls up. No. I lifted three, i said four. I thought maybe it might be what im saying, okay . And it turns out it wasnt what i was saying. Lets see what happens if i do lift four balls up. But theres only 1, 2, 3, 4, 5, 6, 7. Lets 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 . Lets try four. Huh . And we got four. Lets try five. Wow. Lets try six. Woo. Lets try seven. [laughter] yeah. Aint that neat . I wonder if theres a reason for that. I wonder if theres a reason. Let me ask you a question. When one comes in its got a certain momentum, right . That momentum [makes sounds] andhow 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, aint 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 ill 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 theres 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 theres 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, whats it gonna be . Energy. Energy, next time. Yay. [music] captioning performed by aegis rapidtext travel advisories to Small Business loans. Retirement savings to medicare coverage. Id theft protection to contacting elected officials. Student loans to taxes online. Whether you have information to get or ideas to give, usa. Gov is the official place to connect with your government. From surplus car auctions to finding a new job, our new mobile apps will keep you updated on the go. So from marriage records to passport applications, Veterans Benefits to birth certificates, Patent Applications to Energy Saving ideas, product recalls to home buying tips, check out usa. Gov. Because the country runs better when we stay connected. Patty filing for Social Security online, 9 out of 10 experts agree, its groovier than a brooklyn hot dog cathy or a crepe suzette when cousins are two of a kind welcome to another session of beliefs and believersin fact, our second session. I think our first class went extremely well and we began to explore a number of very important class themes, pervasiveness of religion, identity and relationship, and well be looking at th