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, y