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Basic Physics Lecture 19

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But day dollars go and get started I was really pleased
I think for the most part noticed a quiz this week we really was about minus signs with work for the most part because that is a challenge allow students have so mentioned in the quiz even noticed a gay some these I I I do agree from acquitted point of view I some in the quiz put it as a quote was asking about the actual motion and stuff so You do have to read these and and realized Again these won't show up and the exam but these give you an idea of how we describe ocean so work done by the force the but that you let straight up off the table So You're in the process of lifting it is moving there is a displacement that's why it's not 0 As opposed to a book that you're holding Still now and notice in this case right the work done by you it is always 9 0 there is of course there is a displacement now the book might B movie constant velocity It is moving a constant velocity What is total work done on it 0 OK but that because his other forces in this case the force of gravity On which also water balloon when you catch the wound that is right use a during the catching process Only 4 on the balloon is your hand really this Case there might be gravity but is really opposite he knows that in the direction your cat You're the 1 the stopping of arrested changes kinetic energy so close some work Being done and its negative Gujarat decreasing the kinetic energy opposite direction thing is slowing down moving and it's a little tricky the catcher 1 could you may not always rely stuff is moving but when you catch the balloon there's always some most in the bloom compresses it moves a little on your holding your cell phone to your ear the Asuncion is that moving it up and down and that's why this 1 0 this was electric you users I want few more people got Ron Artest there's a clear initial and final day the ball is a whole anorectal the Watkins and there comes out and when it's all done its speed is the same as it was at the beginning and of interacting with a wall here and so forecast of a 0 because its kinetic energy and change so there's some work as a goes against the wall and there's so workers as come away from the wall but The network is 0 in this case the truly really really richer while in a really really involved the reason this happens is there's no displacement of anything to NATO forces you know displays and and there's no 1 is a little different trickier then the water balloon but you want this thing about that have airplane as you fall so you're moving hazardous displays and as gravity of positive this weather Lovell slightly tricky again It was the next 1 with the most runs The minus sign is not the direction you're moving it's not the direction of the forest is the relative to the direction of the forests and what your following what you move into your moving down to pursue gravity is down there in the same direction so workers positive force in this place them in the same direction that the 0 degree angle coastline and zeros plus 1 positive work And displacement opposite each other is when you get a negative work and just think about it and my speed in upper my slowing down gravity speed you up when you go down so it would be increasing your kinetic energy the parachute is slowing you down so the parachute does negative work exactly cancel the network is 0 rout and you fall at a constant speed At the parish it does more work you false Lawrence it gravity does not work you fall faster and faster Yes Do you know what were the ball is hitting in the wall bouncing off gravity is down so that 90 degrees so it does nothing and the forces of the wallets since the speed before Nafta was the same I have no change in kinetic energy I can't have any work The study Asked What But Yet during the whole process right like told the before and after so during that process maybe there's some positive work maybe there's a negative work work slowdown caused watery speeded up so something of a happening during it but the total work work refers to the whole process from ADB so I move from here back here and I had the same speed I ahead before so no work and that that number 4 The most challenging 1 conceptually so you know I think about that when carefully The Boeing you catch so stop their Gaza having some speed another speed so something had to be done to it What is a There's always a little bit displaced you're catching it to slow it down At Other questions as another way to think about that bidders from a physics point of view if there is no displays whatsoever when the boat water balloon hits your hand that is bounce off the same speed if nothing displays his you did no work so fast and up with the same speed you can stop it otherwise Yes there Got a question that came from the quiz But the 2 this is not really A quote just enough people mentioned the direction of taught and how how to identify variables in information necessary to solve the problem that the first one goes back I wanted to say a few words out at the direction of tour all I can say is keep practicing you've got your arm You've got your Athens some direction and the practice to write him rule If you still really have questions I can't opposite hours today or make enough time to see me on this 1 It out As they said the old fashioned simple way when you 1st learn word problems was to go through and find all the variables based on words in the problem and give them a letter so you just take the words of the problem you maps and the variables and then you look for the formula that matches or the equation if you watch how I do it If you want turn this around as you get 2 more more complicated physics follows the way you find the information necessary is you start with your question and now that we have more than 1 way of solving problems trying to show you that I will but in today's example suppose the questions you know asked for the acceleration or ask is it just about to move you know You know you you should have start making use of a system of ways I enter that particular question and you'll find out it might be kinematics it might be forces And it might be energy
Make a new start with the question that with the variables of a big warm problem the situation said if I had defined velocity were Louis I could do it I could do about definition treat OK did they give me an the Fund for the no they didn't that those Mercury while they were Delaware find velocity Is that the projectile motion and that I know use my quit the car's acceleration I checked back and the problem is that a projector motion no it's not Ritalin way find velocities is using conservation of energy doesn't have the part of conservation early yesterday now I can do that all but to do manager needs another height and they give me the height of conniving problems solved I leave this decide how I quite tight US was of the same questions do I had so you actually start with the question and go back and pick out the information as there was every piece of information you have and trying to use that to answer the question that's is my suggestion is you get more complicated problems it led the way you traditionally learn it when you 1st learn more problems But if you look at what I do and classed as I saw problems that's what I'm doing I start with the questions they have not solve this do I have that information have some of that great but of a piece of that I get that information is the process of working backward once I have about all laid out that is just a matter of going through my steps boom boom boom about and as these problems get more more complicated that's really going to be you have to break it apart so watch for that as we move into these final weeks and get some of his more complicated problem 2 20 questions on that Yes No yes in now Right midterm like like it midterm option to my example you seek pretty much just focuses on chapter 4 5 10 and the sector from 10 12 but Occasionally you know there are things you need to remember so right if you look at the Homer problems a standard thing wise What is the forests On our car when it when it goes from 0 meters per 2nd the 20 meters per 2nd in 10 meters but you have to remember had a fine acceleration from its definition to be able a computer Force but Yeah I had to do a complicate a projectile motion problem to do that you just had to use so you have to use some of the stuff from kinematic perhaps by the time it is Final Get It will be Camille did end up given problem will use everything from the course or a lot that I will get some examples of what you had to practice as as we get closer Other questions The good news is the 1st problem I wrote to the exam this weekend turned out not to be solvable so I took it off so you should feel good about that right now that would be fun on Friday will now hear operatic the when exactly on they're not solvable jagged intact and that's when figure buyers with disco so we got a problem like Eurydice offer today that this 1 day I would never Asked the question this way on the exam exactly but there was a point that is poorly ask just to illustrate a point so I might ask you know so right now you need a letter 20 kilogram asked height of 5 meters you can either use a rubber fully to lift a straight up or slider prefers Was rampant it's an angle doesn't make any difference which you choose so white now that is kind of an opinion questioned so let's ask the obvious opinion Clipper question theirs Yes this is an opinion this is why you're not getting great and I'm but I'm curious aspect of lifting an object is of the greatest concern to you the force you must generate further work you must to 8 must in Europe A majority of concern about the work you must do let's think about this for a moment before we do the problem and this actually involves a little bit of biology so this is a Segway biology right irony said but physics quantity of much work do I do on this book when I hold about 0 How higher do I get holding the spoke up For a very long time right to everybody all the arms up like us started the person next year and is deepened there for the rest lecture a about that right adding that this gets Tyree Agassi some your daddy refined doing that right so now that I'm into whatever we give insanity see along until the book up but how is your arm muscle actually work Anyone now Yes Damn what I want happening in your muscles like at a book like like the tiniest biology level there's little motors in there right there constantly moving and pulling so there is a crime what I'm doing this because now and not be next year But there's a lot of effort that theater my arm that's why I get tired because to maintain a constant force My arm is applying of course through a distance all the time goes little motor muscles are actually moving and so you are actually it's architect weird question you're actually doing some work so bad that DX a near Mosul so in that way you won't care about really at the end of the day is of course you're generating because that determines how much energy it really costs in are now yes you need a way that the total work too Because To doomed say 20 jewels of work lifting something I have to actually spend at least 20 jewels of energy in my body but if I'm doing that with a really really big force I'm an active spend even more energy to generate that force in my muscle then I'm doing it said with a really small course Possibly because I'm I'm maintaining a particular force as well as doing the total amount of macroscopic energy so the energy you expend and there's all the issues of efficiency which muscles are using their using your legs your arms your back in breaking it you know I mean there's a lot of other biology into how tired are you at the end of the process but in general it We are able to do a small force over a longer distance better than we are able to do a large force over a small distance ever for no other reason than most of us have a maximum force we can generate right And so if a large force that needed for the short distance is bigger than a force to generate you're not going to do anything But the was small pores over a larger distant you might start to get tired But at least you can generate that amount of force and that's the difference between biology the biology of lifting ban the physics and track and that's why for this problem I wouldn't actually really ask this question on the test to that more biology detailed what you like to do question
But Type of physics question you again Either how much work is required by you or what force is needed But you know the type of questions you would get asked the problem and antagonist Socrates as we do the problem but I want just set this up so you can see that difference between forests and work and what it's really take to do unless something any questions on that aspect of it Because it really I think point ahead then the back your mind because our intuition of working for tends the come from our own body in how we lifted moves things and we really have to be careful Of the fact that were going to be using the physics definitions because were not to be worried so much about The details of how tired you got generating these things in your muscles make sense sort of At Gama Weiser video few times so let's look at this problem so To remind us of the 2 situations the question is if viewpoint a block of 30 degree ramp Vs. lifted off the ground over poorly What the real difference between these 2 situations but a since you gotta go from age equals 0 And to a height H equals 5 meters so keep that in mind right on the ramp will it in it 5 meters were not moving at 5 meters along their that'll be some different distance reactor work that out and hair were going up straight 5 meters so We will probably is accordance system here Like guests like we usually do it the ramp and hear what is you Why up as positive Ed But we will involve cases have is dealt the age of 5 meters Now West The 1st of the other thing that wasn't super clearing the problem in both cases just so we have something to compare what that 0 There were removed duet at constant velocity so from of physics Neo The next few minutes reviews for Friday's test cell said that because the velocity what else do do I know acceleration is 0 What I want you to do here's your practice for the test drive other free body diagram for this tastes and draw the free body diagram for this case and we are being very nice they were pulling it up a frictionless ramp Further Following the the right the 1 and the right many forces on our block June 2 Where would lose their right directly as energy and intention was the throughout the size of my arrows Rally Where the role the size admirers I may draw them very badly but they must be The same Why does 0 so even the other thing is moving with acceleration is 0 So therefore it's not now yes We approximated a little bit We would have to get the thing the start moving in principle so to get started then we need a attended a little bit bigger get started but were talking about it moving at a height of 5 meters at constant velocity and were not warned about the starting because as soon as it's moving the yet to be And so we would right in this case actually miners energy equal 0 why my solving for FTA What's For I have to apply Ever Right so by look right here my polling forces Equal to attention on the rope that by definition and met the same over here So now this is 1 of those things the question really were asking is how much work and I doing on the object the 1st thing I should have done right is figure out was a lot like the door And blocked the door and how much force pushed door open wide you realize that the pushing for it was equal and opposite the the normal force from the board you never worried about it again and you solve the problem for what's happening to the board thing here once they realized that I have some polling Forrest that Your work based on the distance it moves through the same as a distance the rug moves right of the rope was dealt to X are Delta Why up that my hand is also going Delta why this is whether going to be the work I do So the work I do as good a mail same as the force attention Times whatever displacement does over here It's amid tension times dealt to act because that's the variable amusing so that was relayed back there it's that 1 day it was recognizing this feature and that's why I went right to the free body diagram now On the other 1 Yemeni forces do I have And what are they And down ship normal force what ways that up and to the left the right or left good at White House center would try to that Under the right he adds a little harder to draw the arrows Accurately right because what has to cancel well not quite energy in Athens I have to look at the components operator canceling right so the energy of recall the suspended a my IMG coastline Guy gentle ever end as my energy signed later but a gentle out So my equation hair
Will be at Might as and Signed Equal 0 my MA over there and Miami over here So we see the 1st ever between the 2 situations in this case but tents and I have to apply it is only of 100 million When I plugged in my mass which was 20 kilograms GE or my 10 meters per 2nd squared and idea the sign of 30 Where's over here the sentimental plight is the full 200 new so I cut my attention and half I cut of course slapped applied in half that would make me very happy if I was Lifting a 10 kilogrammes mass instead of a 20 kilograms We got on that A mail I most sides right the dealt the acts of the rope in this case is the dealt the X I need an over here the dealt the wife of the road is the Delta why I need we Ortiz said that It is the same that I pull away at tension is the same that I pull away So I can get the the work done by taking in this case the detention times dealt the X and hear the work the changing times Delta why this is the easy ones because I just go straight up 5 meters and I do a thousand jewels of work now in this Of em I had redraw that diagram for the ramp and what I say that I wanna go up 5 meters at an angle of 30 degrees there what I'm really doing Delta acts along here and when I see a sign of 30 equals 5 meters over doubt X so in fact my Delta X is 10 meters I the full 10 meters a ariled for pull for a distance of 10 meters and now my work becomes 800 times 10 meters wet equals a thousand jewels which is exactly the same so from work Point in year I wanna let that object 5 meters There is nothing I can do to avoid doing my Thousand jewels work I will find that later as we talk about other types of energy basically you could think of moving up the gravitational against gravity as doing a change of potential energy and work changed energy and so there's no way around meeting that amount of energy however By being clever I get a force to be a lot less and as a bunch ways to do it 1 way or the force of course a lot less 1 way is using a ramp I can also use mobile pulley system the key idea if I can make the distance longer then I make no force West at its heart this is a lot of what goes into from your jeers and a bicycle right When you got a lower gear to go up the hill is it easier harder a paddle Easier you're doing look you're applying a smaller force that's what feels easier but you have the pedal more last you pedal more state your your change in the ratio of your years so the number of times you have the panel to get the bike remove the same distance just became more so you got a smaller force to achieve the same change and high so to achieve the same amount of work by peddling over a longer distance and sometimes as really key because sometimes with a steep enough Hill into high gear you can generate the forest rangers like roll backwards and bite so it's really useful so once again for those of you who wanted useful applications physics you now know why go into the lower during case to spend your whole life stuck in high gear wondered what was hard to peddle yes Yeah At the height of the work What debut right at this Point Since you know that they have to do the same work you would that is twice as long notice basically Here are We have signed they don't here we don't here we have a science data that if you look at it is really the bottom and here there's no sign that so what's happening is that signed it is canceling out Yes Notice Relocatable it's not actually going any higher about 25 meters it's it's going for a longer distance or you're wrote going on But yes I their questions on this problem OK Let's go to problem number Now this is a very common thing what to start work in energy It is to use our brains to launch staff and the spring constant But you remember we had the equation was minus can change at this is The spring constant noticed its units are new and Premier because we multiplied by Sunday with meters and we get something with you and Now this 1 just to do the picture we have a spring here that's compressed it's going to launch a massive have a slide along on a frictionless surface Others ring over here it's going to hit this spring and then the whole thing will compress and I'll end up in some final state notice we have an initial and we have a final That was our key there were going to be doing something with work in energy you're just energy conservation is a problem the review now with worked out after the next chapter we would just all conservation energy never use work but I wanted to show you the 2 steps is how we get there Now I noticed This is all about velocity is positioned so depending on how you look at it you might think all kinematics I could use kinematics but as will say that it was of course from a spring constant
Talks Let's let Clipper question vote on that forests of spraying think its constant or not constant day somebody who thinks this constant wiser constant You can't why would you think it's can't say what a bad it might imply that it's a constant force What is a formula for the force from a spring negative Delta ax Does it change the venue where you are so it a constant no this this is very important you have to start recognizing What's the 1st duty wait Your 1st Friday and Jay is that constant or not less White is any media that changes any time you will wrap up your ass stays the same status saying this Is constant dialog This is the only forest acceleration you got would be constant and that I use my equations Of the forces must then be acceleration as France then I knew my equations Acceleration that You gotta keep keepers on your tips not months then of course the acceleration is not And I don't use 1 Because exonerated equations but I get a problem like the 1st day I can use I can use our we went backwards I can use work and I can use energy but I can't do it with my initial and final I don't really have to worry about the fact that every change in between because my rolled have to do with the beginning and and this is where Robles 2 equations failed miserably you would have these adult that's is probably have asked Andrew F. you calculate an egg and you might do yourself and students do get a problem like this but Athlete was kayaks the operators that icons that the user For an 8 they put aid to equations of cars acceleration which is not allowed to do and offer positions velocities a wrong So do not do that that is 1 of the key Things you have to pay attention to it now There was another keeping by now we should be used to the mask Is released meaning it leaves a sprained notice is not physically attacked as is being pushed by the rank and then elites at some point so because her question for you what does it mean to to leave the spray does this happen when the spring to abstract anymore Remember spring from Monday Does it have a when the velocity of the block is greater than the velocity of the spring doesn't happen when the force of the spray on the block is 0 Or does it depend on more than these things you have to worry about real details of the situation Batteries wet The antagonists Exactly like stuff we talked about the last 3 weeks so sorta question your and on Friday and since there is not a majority that got it right Bob some of you were right about the people pretty talk to your neighbor and vote again let's get this right European noted based on Sunday we talked about having to do with the contact between 2 objects and when we Moon's contract reported the find what it means to lose contact that and talk about a remote had done so See is correct remember contact in physics is all about whether or not you have a force between the 2 objects so Even with a sprained right knee forests of the ring on the block going to 0 that means you lost contact so if I go back to my picture If Whatever I must bring pushing an object there's a forest between these 2 what happens here It is weak and hour earlier language we would have drawn the normal force on the block from The spring we now know what that is at the Spring Forest so we just We draw As a force of this know the formula for it But that really is a contact forces they contact between this spring the block it is ignore what type of normal force So when the force of this ringing 0 0 that means we lose contact now in your mind you your intuition might sell you are well when I get to exit row 0 block now keeps moving ever went 2 0 so I lose contact away the spring keeps moving the thing McGee reminders In physics Remember robes were we are they had no mastodon stretchers anything they just carry between bars brings all have massed equal 0 and so that gives them weird behavior that when they get that equal 0 and has not been attached to many more than just moving as was aware they so the spring doesn't keep going and that's why I ask this problem and I really have to make it clear This is something you have a big every Macera really light rain Josie that's true as soon as you had actually was Nova overbought goes away the spring basically stopped but my spring of along path that it's going to keep moving and the block in it might stay together a past that
People 0 But were not gonna worry about that this is what we're going to do for all of our spring problem China won't leave is by taps the spring physically the my object then of course it just keeps moving back and forth and oscillate so you have to know is not the rest of the race in which case it leaves when a guess X equals 0 Or is attached ring with case that are back for any questions on that So let's look at how we would solve the problem and key step to remember Right black going from here it's being compressed from Mexico 0 But . 1 meters And there's gotta come over here and another spring doesn't use a new quarantine system allowed to do that but the separate interaction and Ghanem of some distance acts that we have to figure out on this spring has a spring constant of 20 new premier this spring has a spring concert at 15 new and formula Andy mass massive my object is 10 kilograms The key thing to remember With ratings is another work Done by this rating we can either do or in a role exit initialed acts of final of minus kayaks times the acts are in a role of F D. acts But we've done that before we note When adapted squared and we remember how did do are definite a minus 1 after X final squared minus X initial square and noticed this is employing Final squared minus X initials where does not equal X final minus exponential squares right for ever numbers always positive this number be greater than 0 or less than 0 depending on the relation between X final antics initial And physically what will happen as the work is less than 0 meaning you slowed the object down X final is greater than exponential coup that means you stretch or compress this ring more than it was before so it slowed things down because its forces always opposite its displacement the work is greater than 0 x final is less than X initial meeting you been speeding it up there And at the magnitude thing so noticed Going from here to here X-Files less than existential so we speed up a we start moving going from here to here Sorry And this magnitude X final is greater than X initial and so we slowed down and that the 2 pieces to this problem very quickly Alvaro I wanna make sure all the concepts and set up as is clear any questions this because I know in the next 2 miniature Oregon a pack up and leave Even the with 4 minutes left OK so let's let's go ahead and do it We did 2 steps Subpart while we are here and we moved from X initial 2 x final equals 0 so that means are work and is 1 have minus 1 FJX final squared plus one-half they X initial square that 0 final a 0 by again going get 0 . 1 jewels that work would people might change and kinetic energy which is 1 have Visa Final squared minus 1 have an initial square that was 0 because he initially 0 and I said this sequel the . 1 jewels And I play again I will find that final is 0 . 1 4 meters per 2nd now X final and initial that I was maybe 0 sometimes you will actually have to plug into the subtraction but this is kind of the most standard often that will happen often starting at rest any arrest starting equilibrium arranging illegal immigrants any questions on that part so for part till we have we had our spraying we have somebody initial that is 0 . 1 4 meters per 2nd and this is a big step to watch for a V final at once that often becomes an the initial at the next at our People 0 RX final is what we want a flying It will be a little bit clever we know are changing kinetic energy which is 1 have and v final squared minus 1 half and the initial squared It is now just minus 0 0 . 1 jewels but exactly what it was before can't be anything else we're going from the initial Anthony final it a 0 and the same value we had before and so now our work which is mine is one-half X final squared plus one-half X initial squared equals er minus 0 . 1 jewels that was 0 so we cancel or minus signs and we saw 4 x final And we get 0 . 1 2 meters so
Fairly straightforward that's kind of the most basic problem you'll be doing on the Hallmark but it covers pretty much everything you need to do
Magnetisches Dipolmoment
Closed Loop Identification
Anzeige <Technik>
A6M Zero-Sen
Satz <Drucktechnik>
Elektronisches Bauelement
Angeregtes Atom
Ford Mercury
Masse <Physik>
Relativistische Mechanik
Mutter <Technik>
Platzierung <Mikroelektronik>
Trajektorie <Meteorologie>
Sänger <Raumtransporter>
Innere Reibung
Initiator <Steuerungstechnik>
Vega <Raumsonde>
Hydraulischer Aufzug
Kaliber <Walzwerk>
Zelle <Mikroelektronik>
Maske <Halbleitertechnologie>
Negativ <Photographie>
Elektronische Medien
Satz <Drucktechnik>
A6M Zero-Sen
Direkte Messung
Hulk <Schiff>
Direkte Messung
Avro Arrow
Airbus 320
Steppen <Nähen>
Innere Reibung
Proof <Graphische Technik>
Scheinbare Helligkeit
Masse <Physik>


Formale Metadaten

Titel Basic Physics Lecture 19
Teil 19
Anzahl der Teile 26
Autor Dennin, Michael
Lizenz CC-Namensnennung - Weitergabe unter gleichen Bedingungen 3.0 Unported:
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DOI 10.5446/12930
Herausgeber University of California Irvine (UCI)
Erscheinungsjahr 2013
Sprache Englisch

Inhaltliche Metadaten

Fachgebiet Physik

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