Basic Physics Lecture 9

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Basic Physics Lecture 9
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Now you you may have noticed Friday at 1 o'clock world tired we ate lunch hour is a dangerous class there there are no Cool demos here But Right now we are just going to be working through 2 very exciting physics problems in preparation for our test next week so I'm in about 15 minutes wasn't a loud noise rock clapped awake you all And there will be a couple quicker questions of you fall asleep now makes the person next year nudges you when there's a quicker questioned the first one we wanna like that was what we started with briefly on Wednesday
And that is The football player trying to make a field goal so we have the basic situation there's the football hero from the side view the field organ ticket yet An angle of 35 degrees With the speed of 25 meters per 2nd and the question is does a clear the 10 meter high field goal That is 35 meters away this is the 1st problem we want solve reverted talk about the question of what is it mean declare it worse is what is it means to just clear a field goal Start So let's Let's look at how this would up we have are given information as we know that it's 35 meters to the the field goal will bar is 10 meters
And we know we not theirs 25 meters per 2nd But At 35 degrees above the horizontal now 1 of our things that we set a lot and trying to figure out how artists Rollins is the best 1st step As always organized are information and a useful way and knowing that this is kinematics the question is do we make it 1st convert this to a physics question right reactor find something that we calculate and what we decided we have to calculate the height at 35 meters away And they're doing height were doing displacement so We will start with our displacement Equation and if this has enough Information for us to get started and we drew our picture Delta x equals 35 meters in a horizontal direction We know really not Where energy but we need we know we know this is 35 degrees and the mean not as our 25 meters per 2nd we don't know how big 1 captured squared is yet because we do not know to what we know it must go straight down and our questionnaires What is that Keegan a mind right that our full The placement as a vector is 3 Yes Point here That's where we are Landing were landing some 35 meters away at the height of the y direction at once worked here having organize these key pieces of information It is where we pause and ask do we have enough to solve for the thing we won a fine which is high and it will like it best We immediately write down 1 useful for equation From a tangent that has fight And The boy And what else in this equation do we not now On I heard people whispering Went on without time once again noticed If you have been listing things from the problem Cut
There is no What a serious problem Where I find tying mentioned at all and that is Again going back to its been said a lot and I know it sounds like a bit a dead horse but were gonna really drivers home his 1st 3 weeks time has not given its not mentioned that meant as an unknown it is this sort of thing you have noticed in doing the physics problem and you may have noticed I'm not doing comeback visited the lecture I'm not doing standard thing you probably learn either high school math physics somewhere which is not listed all my known of unknowns right at the beginning because 80 per cent of the time in this class that won't help you 20 per cent of the time if you do that yes that I'll show you really want the equation is to solved the problem soaring easy problems you could do that right But most of the problems if that's the 1st thing you do it won't be obvious what information is missing after organize the information differently and that is why I like to organize it 1st in my picture because I know the physics concept that's going on I know we're moving under constant acceleration So I know how to represent that as a diagram it will reveal Very quickly that time is something I need to know and I look at my triangle I didn't really see that this site is completely known in this site is only miss in time And so by writing down my sign of 35 right being the 35 meters divided by the being not to be named now done because I have my 2 unknowns and my 2 equations All And so let's look at this this 1 right here because we did not seek times of 35 equals 35 meters now when you get bigger and better at trade But you could take this UBL skipped that step and right right way right away that way because you know what you're looking for is time so having it in the denominator doesn't help you And this will tell me that I is my 35 meters Divided by might be not cosine of 35 An you would get 1 . 7 1 seconds At about appearance I have H Plus 1 have to squared 35 Times by 2 engine of 35 Or age Most 35 The engine of 35 miners one-half GTE squared air when I play better and I will get 9 . 9 meters so might add know what my answer No now I will be out I tried very hard on exams never to make it this close Right because There's a chance it around a really bad joke that the wrong answer around beginners I don't worry too much about significant figures but they are poor and you don't want all around you want but not digits that until the end you know year being pretty accurate and that at the end you around because you have these 2 numbers you're subtracted in their panic close a new around them differently you could get the wrong answer and sometimes to mastering physics isn't super careful with that because is generating random numbers for you so you can get for your individual problem 1 where the numbers are close you do have to be careful so that that's 1 thing as another reason I like to keep the variables as long as possible plugged in that last step up there's less rounding that I do along the way of Likely make areas go on with that any questions on that Re Notice the key thing was we did not set That the 10 meters I cannot tell you how many times in a problem like this to remind what is about to happen is wrong people have uttered idea Right leg being See 1 Hagerty squared up 35 meters and they'll have a H Another something like this Maybe they'll be working and coordinates and components and you've got the 35 degrees they'll have written down maybe for view y direction that they have 8 the height I end up that is being not she signed 35 miners 1 have times the 10 meters per 2nd squared for G so that she planned to use squared the or wait I the fine to be so they apply gained meters here because that was given the problem those Salford City them as going I need to know I know how far right there with that You're start and end up like that Somewhere and sometimes they didn't like the team back in here now sulfur H I get back and all just made it So you could see how all sorts of things can go wrong when you plug and events are virtually like didn't I trust you laugh but in the panic of attacks when you're just looking at a numbers of variables in not thinking carefully about your picture back easily happen and that's why I do the problems the way I do for you as an example and not by just listing letters and numbers So Everything I just said It was wrong for the most part Eddie questions Clark art I dead wake up 1 of things you wanna do as you spread Prepared a study for next Friday was trying make up Take the problems at the bank last take the wrong you didn't discuss take the brunt homework and change them just a little bit to a different question so let's suppose I did not tell you how fast was victory We know what 35 degrees We know you're 35 meters away and we know with goal closest 10 meters high and I ask you define the minimum speed to just make a field goal What of these would you jails This is the point of might take the derivative Finding the minimum value your by using 0 and all that sort of this be required at a height of 10 meters a distance 35 meters away its be required to go 35 meters at any given time Or this is not a question you could answer because we didn't do it in class or for some other reason excellent genius I've taught you so well I take full credit for this
Everybody got it right that everybody 88 per cent of you with that Everybody's for by now don't feel bad if you got a I still can't you you're still a person on this well we have to do and notice from a physics for review You've just learned a new useful wired Right you have to know Something about what's going on here unfortunately or fortunately Thereby did Bob
And I have to clear some height H And I'm kicking it are right And Show us managed to clear at like it get any faster you have to know that I'm gonna clearer better thus is the way it works presume be able to be going farther so that's the minimum speed you could do now and That's why you want to just sit so any questions on that deserves a lot of time your got it right So now we move to problem number to stay with our football think that sometimes baseball so I Baseball Cooper the spring but you I last saw thought the cost course years ago on the floor So we got a lot of football examples Blanco about your friend you throw the bonding 30 degrees above the horizontal 3 seconds after friend starts running in a straight line away new at 6 meters per 2nd your friends are right next to The same height with which you throw your exact same Don't jump up and if they run car speed up as was to throw the cats to be made Many of you know beginner's class When I asked what you want to get out of it and that therefore squares wanted some practical real world Useful examples hear you got the Nextel you'd like aquatic out there with you And while the rising value of labor-intensive quickly draw out there you got about 3 seconds to figure out how best to throw it because that's when you're waiting so you could do that right on what do this problem forced out ITT bothered him make could request because I know we've massive so much what is catch me That is a delay in any means we saw offer for while Jack OK similarly together altogether saying That was not all of you Was so that all of you want more time I think comes as the same group louder Solving the Sabres at the same time you may think bets at not trying to say it but transmitted you need to remember that in week are repeated here can help so we need the same place at the same time Let's figure out and draw our situation at what I want you to do it before I drive a Jag assumed likely been doing all along But this is what many years as our main equation keeping in mind that we have 2 things we the ball where the person running drop yourself a useful picture for that By Jedi facts you're throw it added 30 degrees on Thursday 3 2nd delay between when they running you throw it they're running at a constant speed of 6 meters per 2nd a you wanna know how fast the throw it I'm a around drawings causing real havoc for the camera The grounds a remember was I I have my my my friend starts running standard right next to me at 6 meters per 2nd after 3 seconds I tried drove for both of them I throw the football up at an angle of 30 degrees The horizontal and I want to know what's real need the throw it so they just make the cat So I'm throwing it at 30 degrees after a 3 2nd delay their running with a constant speed restarted at the same place Any chances at the same height I threw it from an ally not fast like through now But you are got a lot of work Leger questions right I'll be as they walked around and by this point you really need to build all these diagrams quickly I realize Olmert that until Sunday But you know you work in the skies and said some examples you want to be very very quick with converting them into The diagram and understanding what that really means so what a week How the 1st thing I can easily drop bears the displacement of my friend who's running because they run For 3 seconds So that movie player times the 3 seconds that they run for a run 3 seconds of a cousin speed and then they run for some unknown amount of time at a constant speed and this is the time the balls in the air right that's the motion of the person running I say height start running 3 seconds later I drove ball but they're still running that's all they do And now I know they're doubt that that was the part of the motion that was the easiest once I know that and I know I have a collision I know the ball has start here because we said we started the same place And the ball has the end here they catch it and the ball is only in the air for a time today I want the ball does is it has 1 captured 2 squared because it's in the air So that's gonna end right there adding my 2 vectors of years my one-half GTE squared for the ball at I know 0 4 2 add correctly V not He must go from here to here and what I was doing was this was 30 degrees And what I needed so fine was being not here that's the question there once again I'm in a situation where there was 1 other thing I don't know which is to say that I would not no matter from the problem right the problem never mentioned the time the balls in the air So having said all of that let me ask you this clicker question I forgot I had this could request by made it easy for yes whatever Mason is missing from the problem that is needed the solve that how For the masses how long it is How fast the is going when his car but how long the above the air me but But none of the above the problem did you all the information you need Bait About the answer because I wrote it down and neighbor remembers you might wanna check with them 5 more seconds for 3 days Jeremy What end done excellent be my game Aeolic I gave the answer we 62 per cent that's good but I've actually under the slide with the into nor the not so the answer
The place where miss seeing as how long the ball is in the air noticed What We do don't diagram that's what becomes obvious that's what I do the diagram right now I don't need to know how long the past
Right now there's no point maybe solving for this distance write this This is given by this expression so let's write that even bigger what I really have And a nice big triangle as I have V the speak my players running at times T plus 3 seconds as the time in the air And that's the delay time and that's equal to my one-half GTE squared and I know as 10 meters per 2nd square and that Angola's 35 30 degrees in this problem sorry that angles 30 degrees and IV not at any time I brought 1 of these triangles I know I can always get at least 2 independent equations and so I checked and sure enough I only have 2 unknowns the thing I want a flying and time so I not and once again it was something that was never meant that the problem that provides the key step in connection in solving the problem Under review of asked you have trouble recognizing the oppose measure what's missing the artist nobody is really good at recognizing it just reading the problem done time that these so I sort of had but what I'm really doing even abound as do it really fast is in my head of drawing picture Former dried out it's only once you sit down and translate the information you would get into a clear diagram does the missing information you need show up right by making this diagram I forced myself To write things with key it and that it becomes clear that He was missing a doubt that acts down here was the thing that was missing that would become clear and that is why we trolleys diagram of the same blood was for citizens in the lab with energy ways of diagramming the problem that reveal the missing information Any questions on that so now we actually solve it But don't really bear on the screen anymore But What we noticed But different ways we can do this 1 I'm going to go where the show high um let's 1st take our coastline equation from here so coastline of 30 would equal BP times T plus 3 seconds over being not right so that tells me That might I want my player plans by 3 seconds plus the losses player times to be peoples Avino coastline 30 times t so if I can find see since them given that that Icahn's offer me not so I like here I see that in this case If the sign of sorry if the attentive once again want When my hero for Donna didn't go for the sign In this case the turns out to be nice I went 1st sign of 30 people's 1 have G G squared over Now you might have I do that as that of the Tanjung could done either 1 given minutiae as well This 1 I can see my canceled and I have a nice expression for me not really not equals I'm sorry I'm doing it Slightly back for you guys As what happens on Friday I am trying it was to be not sign 30 over GE but I can plug in here years and then get a nice funding math equation Trevino Where I know all of this and I know Odess The reason I did this for you I guess said the problem with doing me out Gerard We want ways to do it I love this 1 I want to show you that sometimes we we're only getting quadratic equations for tee up to now I want to show you something to get quadratic acquaintance for other things I'd be not And you can't do it where you get weird coefficient and then you plug in the numbers I will actually this 1 is a fun 1 because there are 2 or 3 of ways to do the algebra Song make a little bit for you have different ways out of receiving see which 1 makes the most sense to you but here I just plug into this number in this number And I get 0 . 4 0 8 6 6 v not squared minus 0 . Sigsbee not minus 18 people 0 And so I would get a V 18 . 3 meters per 2nd and a minus 13 . 8 5 meters per 2nd and again I wanted to show you that as to remind you that we often that But not too numbers would be the quadratic will make a decision which makes sense and this is of course for a friendly cat which 1 must like that The 18th are they get a Casanova throw backwards Not OK so that That is A related to a completely different situation that would give the same equation so this is the 1 we allocated based on the visit Many questions on that I apologize for the glitch there in the middle but again the process here was if I had I knew I could sell for not so I got expression Verdi even tho it had been ordered This will happen occasionally notice I could not get a number for today at this point and that is something that often will make you Yes What am I They up the picture that disappeared So the and 30 was the adjacent over the high partners Nor would I will do that a lot better shop a lot Bet Another reason in addition to showing you that you get these frontier grazing sometimes notice All you like to do is have numbers deployed which is very common you get hearing alike away I can remember for 2 yet because I don't know being But that's OK because knows what I want solved for I plug it You're anyway And I get my Where for being or and I saw that you might apply with signed a 30 10 and multiplied by 2 so you might have 1 number times not it don't like keeping variables around and then you would apply in annually gone straight to this equation here So depending where you did hear you either what I did to there were straight to them but the main point is do not hand act if you don't have a number yet for tea I asked over the kind of all asking for is the velocity you're good at this point about 40 out really need to recognize any questions on that Where up thank you yes Just Look out I just would enforce this foursome was given a 6 meters per 2nd the I notes we don't have the equator about A question on the board at the same time I keep telling her right foot through screens at every lecture hall So I put the question here and solve a problem there would you like that that's right and tell them we need to screen actually may LIB that erasure tuition have choice do streams more money or just suffer like this that the worries and even minor mentality IPOs the problems at a time And so you can pointed out and just have that ready for you to keep track of what the question is I really encouraging do that now and other questions this time you look up when I ask that so let's just have a follow-up question how long is the tax are dead and used do that right here
We already saw that doubt the acts of the player was 6 meters per 2nd times time plus 6 meters per 2nd times of 3 seconds there was delayed And we had an expression for time we got this as part of solving the problem and now that we need nobody not we just pull again and at 1 . 8 3 seconds plug-in here we we find that we ran 29 meters so we have roughly a 30 meter pass so a lot of times you'll see this in the homer in the master in physics the funny thing is Often ask you things like they might How long was the past how long was the ball the air and so things that you solve forest part of part might just be the answer departed so you wanna watch for that a little bit Any questions on that Now when I posters solutions but also show the components and like I said I when a show couple a different ways to do the algebra on that 1 does is a little review for your now we have a very important work activity Preparations For the exam next week where I would like you to do is make a less 4 years out of other parts of an object's motion you would like to know when solving a problem aspects of Ivanov motion so like not its colors you that whether the ball or car a boat or a plain white Things about its motion argues In solving a problem that's about emotion that is about things being somewhere at a certain time And that is 1 idea you really wanna keep in mind of the problem is about being somewhere at some time is probably I kinematics problem And that through the rest of the Court of at some point in the course of a problem you need to know that something is a certain place at a certain time you're probably going have to do kinematics at some point in the problem you may have do other stuff as well but at least part of it will be that so you wanna know that what sort of things should you As you write your latest feel shot a few of them out now was initiating time unusual our city final losses Acceleration wanna know about the acceleration Nite Other negative something even more basic The 1st question you might ask about the acceleration As it Is it possible not 0 9 0 job Here my worst not as I did not But tying the less because I don't consider time to be part of its motion that were using to describe its motion but you but time Lewis is perfectly fine art another problem with that of the Maine Things about it Our its initial velocity finalize acceleration of his final visit with which wanted did And be able to draw on C plus 1 MGG Gigi squared equals dealt the ax correctly you're going away unveiled rugby not G T V final correctly and often drawn on like that but of course it could be something Launched out and is still going up right so that would be not Gypsy because we find it could be a lot launched down That land somewhere so that the not GTE squared with a one-half And that's still So there are all sorts of variations On these types of things notice You need to be very alert Many things happening to the acceleration We had a lot of problems where I was moving with a constant velocity so I 0 acceleration and then I started to accelerate the cats the object so I had a break that in the 2 pieces The break it and to be 0 acceleration and new accelerate or I might and driving along an isolated the break so I went from 0 acceleration through a negative acceleration or maybe as accelerating realize there was The person and not to accelerate faster so I had to different positive acceleration due lot that and We call it we want for objects in the way Do I make it over the field goal but make it over the wall the make it over the tennis net do I clear whatever it is I'm trying to clear that would be an object in the light of what other objects in my interacting with At a questions on that Now This is just opposes where they were not gonna do it now But I These are 5 he thing Journal under review as you do the homework this weekend and before the test you wanna make sure you know what type of Flight means you wanna make sure you know what it means to just clear the object you want and make sure you know what minimum speed to clear an obstacle means you really wanna know what captured a hidden me so those of 5 big main ideas from the course work at and Collapsed The door A very important thing I've said a couple of times but I wanna say debt so let's suppose we're doing that football problem but we just If I did my standard saying what I wanna find is really not a did then velocity of a person the delay time so as velocity of my friend best time and I always know G acceleration due to gravity and I was there the angle of the throw a But Having done this less his wanted to re-emphasize our last point that is nowhere in hair the time for the road show up easily But But once I dropped my picture There's no way to draw my picture without including the time of the rock band so I immediately realize I've 2 1 the danger appear with leaders and again you might laugh when you see it You're the calmness of class but you right down there was not Plus 1 have 80 square how great I have time and I just plug in 3 seconds everywhere Could I taught was given to me that to my variables equation it's the most it is a good way to initially learn about solving problems that Europe Variables year equation but for most of the problems we did will Matthew so The standard was your employer a match equation solve the problem doesn't really work right where you really need to do everything we've been practicing what you take you through the rest of the class is identified the physics identified the motion is a cousin acceleration are there to accelerate and are there to objects were 1 it's amazing several instances of Dover Vanua walk for the around the information and the problem and then you solve it from the equations that apparently lectures all of our
So I will see you next week would ever adversaries about 205 over a Frederick right