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# Basic Physics Lecture 21

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Erkannte Entitäten

Sprachtranskript

00:08

Still it's gonna get started so the unlikely I did what a point at 1 thing was pretty most people did the math with 2 then again urge if I recognize the former for potential energy and identify potential vs. kinetic energies 1 thing that highlight on remember

00:35

The force of frightened times since is called various things in the internal energy or energy dissipated sometimes certain boats and people only refer to it as a work which strictly speaking is incorrect but it's not a potential energy as not a potential energy because it is energy that will never go back into kinetic energy so that 1 way to think of this Potential energies are energies that when you gain potential energy at some point later to be turned back into kinetic energy or to another potential energy that's why it's That enters the potential to move Internal energy energy from friction is just basically trying to see that We did the example their hands together so that the restriction Pritchard is taking either pretend kinetic energy or both Turn into Dissipated or internal energy or something you never knew yet so that's why not a potential so that was With that 1 the other 1 I wonder highlight The other concept because the question asked with single piece of information is sufficient to calculate the height of the job and the forced to leave the ground is not sufficient Right before wires You don't either how long or how far that forces acting but distances acted over how long it's acting for To determine either the acceleration of the war or something else the magnitude of force itself is not enough to tell you how high job what matters is how long that force acts over how much time and that was why we had to introduce work Workers Force times this place and there was a whole thing together the told you about the energy you gain on what you know the speech right will see that you can compare that you're and G 8 3 0 the masses what happened is greater than Jyaistha masses cancel out so you're mass does not comment on why you got All the guns and speed At Questions on that The final thing from the quit

03:03

Or some other general comments and I'd as well

03:06

Let you all There's a lot of you in the same boat at this point in the court's authority figure that we learn a personal implant keeping track of all things we learned still having the struggle deciding language and I could have put a lot of these quotes on and That is 1 of the reasons when I teach the course courses you noticed I trying use 10 week for a little bit of a review and why you have example blasted and we beleaguered artist so that by the time we that final on Wednesday you get that little actually had time to come up with this altogether hopefully that'll help but I really really like this 1 because I could have said it better but again highlighted again I don't really understand what is going on in class until I go back wants the podcast again that perfect you eat it very very hard in a 50 minutes is lecture The understand everything I said that the best you can do this experience kind let it wash over you do your best but between what's in the pot doing the problem going to discuss instead coming office hours going to the tutorial talking to your friends say what happened you know what what would you get that lasted 2 minutes that's overcome together you this is not a class survive in just like coming enlisting the lecture And hoping you could do it on my said at the beginning may be applied and believe me now you do arriving on the crypt cell Their job any questions there so where we going to do today But were kind of going to reproach energy conservation and 3 stages so today I'm going to hold on to what I consider to be simple energy crop problem examples and tell you I think there's simple to discussing you do the hardest of the simple so you are doing much more than what we did today just kind of at the next light level and Friday in lecture I will add a new kind of what I think of as the hottest energy problems then next week we going to collisions but the big thing that you have to just keep in mind now as soon as you recognize that energy might be relevant to the problem you want to start keeping track of what's going on at a great way to do that as the table because our all goal is a Oregon a year's work causes a change in energy and if

05:47

Work 0 the change in energy will be 0 And the change in energy is final minus initial so what we need Now is the initial and the final energy and in this class there only for energies we will deal with kinetic energy potential energy dealer gravity potential energy spraying an energy dissipated For what you might call that Internal energy that's from friction and you add them all up you had been total energy initial You can do that Any work in the problem and that of equal the energy final so this is why I say at some level energy problems are the easiest in this class The concept of physically what is potential energy is very hard But to use it If you just believe in it is very easy and is really has a point where I take this as the starting point is to recognize that if you think energy being used thirst for energy Jack role apply he initial plus work was EU final you will get 1 equation it will have 1 or 2 unknowns has 1 unknown your done sold for it If it has to figure out what else you need to do defined what that's the processor in a practice after practicing enough you'll start to get a better understanding of what these for energies are an outright help you with that but this is a very mechanical part of the course is very much work through it now Let's review re-equip with what those for energies are right kinetic energy any moving object has it But how Editors one-half and squared that's pretty easy potential energy do gravity only occurs if you have a system with the earth class and object each object will have its own potential energy is it with the Earth you need to select a point where the height is 0 and then the potential energy of gravity is given by G 8 now in all of these problems alternately the only thing that ever matters It is the change in energy because notice we always have an initial and final and the Fundamental formula As for the change in energy but by picking up point rates equal 0 we can use of formula For the initial and the final energy so that works potential energy doodle spraying is whenever you have a spring don't have a spring you don't have that now sometimes your brain has erupted stretches of the problems as acts like a spring some years as a mattress that you're sitting on top of the problems acts like a spring just rating is above a border of that stretches acts like a spring and they will have Always give you a spring constant Check but The potential energy of the spraying as 1 have JX squared using equilibrium as 0 So you have to know where the equilibrium position was Rania's and said people 0 again this is because The change of would join spring as 1 after X final squared minus 1 half but for kayaks initial square And the final 1 It was our internal energy this It is always 0 initially I mean you set it not to to be 0 because the changes all that matters to you said Some arbitrary number things still work out but The easiest thing matters is the change is 0 initially and the final energy dissipated as the force of friction times the distance to move so it only exists friction and this case kinetic friction is in your system so I would recommend keeping this and this is on that bridge for acres with you at all times during all over the next week or 2 and make sure This makes perfect sense to look And you've got all of that Speaker Any questions on our for energy Yes I've got a few or at For not So will force is negative yes that matters that do the forces are possibly light potential energy is not negative 1 half its work the potential energy it 1 has carrots where There is no might decide your back a lot more work done by spraying has a mind side of the force has a mind but the potential energy is the opposite of the works so it doesn't There's not a potential I this is up more I mean the name as well as relevant all you really need to know that you got for energies where there word engineer 1 after its threat force tidy whether you call it a potential Or not is an important concept that doesn't come up much in solving the problem but the real reason is exactly It never energy goods around that you can use to generate 1 of these others it's always showing up only in the final answer as in your objects as he or vibrations or stuff great and so what merchandise is Any other initial energies potential energy kinetic energy some of that always get used up by in the fortune in the final and you'll see that we do the problems you never get it back out But questions But There but Final piece of his remember isn't an urgent initial

12:41

Work because energy final always Didn't Chapter 7 the learn about work becomes relevant what were not did anymore however really is worry what work done by work by gravity because we're always put in our system and have the potential energy of the spring gravity did you ever find in a problem for the same spreading of the same object you do both work due to gravity of potential energy gravity you that it will stop but occasionally there are external forces Right so pieces the force has external and Jo Durie member work is F . dealt the acts so if force perpendicular to the displacement Does no work so there are 2 ways in which we will have no work done on system the 1st Way Is that additional forces the sad way is our forces A perpendicular tour displacements set If I walk and will come back to this But my most general expression for an urgent matter at all for energies in mud Oh a problem that's what I would have been noticed right it's all about not only with a vector It's about speed about height about Position as a bad distance travel right none of those are vectors author also sailors End of fundamental clue that you're doing energy is that you know exactly the path you know the path the particle talk so of you want all of our energy ones they'll be things like something swinging on a rope notice the rope is setting the path of so we know the distance traveled to know the path we know its position writer speed at various points because its 6th on the path the other Keep thing is we might be doing roller coaster is in hills of various types and again You're told that the object is on track the whole time and so your emotions all now typically by go up a ramp and then I'm flying through the air Once I'm in the air Probably switch to using projectile motion because now not tied to a particular 6 path tracker think so It is likely that point to using new thing even I've initial file and will reiterate his Israel later but I wanted you to recognize that in all these problems Oregon and that with just 1 equation for each motion and it will have Steelers said At a cost on them But Excellent were another couple of example problems now and demos But I'm going to show you another technique for organizing this concept as we do it so well going do you remember yesterday not yesterday Monday I had an object at a rout that I Swan And came back and luckily did not hit A reader like at his motion I wore it again said are The Kind of 3 points in the motion and yogurt waited our where it is that angle theta and has some high above its lowest point which will call a Chico 0 and this length here because we endure geometry is l cosigned And that's our general motion are put up a particular problem a moment but what I wanted to do this for any motion say we go from 1 to 2 or 2 to 3 what we're gonna set up is an energy initial plus any work A gross Energy final ever and how much kinetic energy is there How much potential energy from gravity how much potential energy from a spraying and how much energy dissipated ever remain bar graphs So diverse cylinders SQA could your question What will see what happens the next 1 and I may not count warned that if There were still being re-elected his era that fears of the anyone tell me why it 0 Army she was Start The bought the there was wasn't a radius our other language we call that The forces what well-to-do this place That Perpendicular to look for as this way and the displacement that way there perpendicular so The voice of changing his perpendicular doubt that so work is 0 So right here I know my work is 0 I don't know why that erased Now when I start it would just like how much kinetic energy do I have just letting go of it is not moving 0 but I have some potential energy gravity collect 12 that you go 0 so I have some MGH as there's in the problem So there's none of that is there any energy dissipated the beginning now is a frictionless pendulum state Went down at the bottom Do I have to never They are moving its I mean I might not broad product that many potential in 0 gravity at the bottom now Drive in its ranks now Man shot up dissipated No no friction sought by my role that initial class W has he final Melino this bar has people that are so there's 1 have envy square is equal to that A graphically I've got this Now I'm their makers like really messy and do what it would that I'm going our way Covered a point Ray The assert Right this is starting at 1 going over . 4 rating I'm not sure whether this is what I know . 3 is the same height so might

20:38

Pretty Potential merger gravity is NGH derive any moving from the body of a kinetic energy about It's all in my potential so this has to be 0 so for Chase 380 I Olomouc we found out that as I swing all the way to the same high 5 0 velocity of my swing back to where I started what my velocity 0 So did the demonstration on Monday when I 1st to widen the can't get knocked over what I did not get here by had pushed right The only way for that award was actually been some initial kinetic energy and there would still be some when it comes back And another way another tool tools bars can help you to do things like that any questions on that so again now you see why we call a potential energy because it switches to kinetic and then back the potential evidence Dissipated as gone for good can come back Any questions so now let's actually would that a problem Late so let's oppose You start with the pendulum 45 degrees from the vertical you leg Dow and swings down so only 30 degrees from the vertical and you wanna What is it speed at that point with what I call a very simple quick Conservation measure brought this is actually a type of problem maybe not this 1 because I have angles in it but That's when I could Something like this could be a short answer questions on the exam and the basic thing you're gonna but what we just did in this case are dead But try Just your initial Said at your final I'm actually still called when it's at the bottom age equal 0 I could Call the 2nd place in the swing grew 0 you'll see why For this case calling the bottom so I Miller get kind of the general situation of years held that swinging where there's an angle theta so for any given angle theta my height from the bottom Right It is whole fight L minus this Peace signed data So At the beginning of what makes this 1 just slightly harder than the easiest energy conservation is I have to figure out I was given a goal of use NGA charity wrote that in the previous life so I need to know what the height in terms of being so every now and then You're you're gonna have to do that where you're given 1 of the things you know you're wanted to as the potential gravity and you're given age in terms of data so you need a formula for embattled come during your trade usually will be at the top of a ramp at the height of the ramp that you're done through the top of the rampant digital labor the rampantly angle lineages trade defying the height having done that I will pick my sister as The Earth and the object and that I recognize my free body diagram only has tension on the rope but Engineers perpendicular displacement so I know my work is 0 But The other possibility As I could put The rout in the system now my freebie diagram has no forces so workers 0 because the external forces 0 And I have to make sure that as an internal object in my system the Rock doesn't do anything weird like Aden energy or do something like that so there is no unfortunately 1 right way to do these problems as we discovered with everything we've done this quarter I prefer when those around And the rug does no work as an external force I like it that way then afterward at the rope anymore It's outside the system that's done no work now we can make our little bars we know the only thing we have potential energy from gravity with some amount at the beginning we have work is 0 Kinetic energy started 0 because I just let go And then be final energy that we saw amount genetic and some amount of potential We can turn this into an equation Yes was ever Initial calls one-half and squared And G H final this was that 45 degrees this was that 30 degrees and we Ortiz that 8 0 minus Cosigned state so we have Angie claims L. myself Because sign of 45 Equals 1 half and these were she claims L minus cosine of 30 and we were given L notice is a great case were and cancels out And if you plug in the numbers which leave to you do you should get me equals 2 . 5 meters per 2nd samplers here this says like I said except for the added staff There we had to do trade defined 8 this is about as easy as it energy conservation problem that because you're given everything your equations except the 1 I've known you want I wanted the speed and I found the speak with 1 equation and I'm done I decide to keep track and make sure there were no Springs there was no friction and saw I was left with those I picked my sister and I picked my age equal 0 And I was going to go And I got my 1 equation And I found VAT mildly and not any questions on that A key issue Yes the late baby I'm sorry it was given the problems which no wasn't given the problem I like I did in tight belt is due meters sorry when I post is our re correct the PowerPoint

28:41

There was a very good question it up Let's try not to oppose the young and on to the next quicker questionnaire that see what we can do so outlets of Rogers Serena pens and final angle is greater than the initial angle a message situation which is the final angles greater than initial you must removing out direction because subtracted to process the negative You can never have a final angle bigger than the initial because it violates energy conservation or there must be some initial speed or none of the above Most people got it right there must be some initial speed and left for dead in time an your way If I'm dealing with Bears Braboy 8 Noticed what was Speed in this case in energy is always square right so let's suppose we finally 0 then what this tells me Right is a H final was greater than its initial there must have been some initial speed that had to be positive notice that missed Phase right final speed is A rout visit case where But this job Can't had been bigger than H final because I would be telling me I'm at the square root of a negative number For By For us Yes I'm emerges with no other forces that I should include request yet there were other ways to do it Like outside where where they've been a spring in the problems other the things like that But in general when I want to get at least square can never be less than 0 you get there was less than 0 Eva the question is asking you could this happen with syringe for you made a mistake so Those are the things you wanna watch for How Betty other questions on that So let's look at the front of the next is a long a worry

31:39

But The next fiber fairly straightforward problem with just along the gotta give you a lot of numbers We got a block as lodged horizontally on a surface bias spring that almost the rest of this is numbers until you get to the question how fast the movie when it leaves the spraying And how far from its release point the travel of the questions everything in here It is numbers that Information but that's really the 1st sentence is the physics and the end is the question and what we're looking at it as a block of them moved to exit row 0 where will leave the spraying and that'll will travel some distance did at our redraw that picture but that's the fundamental problem that we're having an again the reason this is relatively straightforward and it is all we have to did It is DOS A few of our energies right so we have us rang only the potential energy of the ranked we have French and so on to do our internal energy and of course will have Connecticut Energy noticed a week picked on me said because is this accord precautions let that system Croat India that so in this place as a said You could actually Any of these what'll are happening for some other choices yield do work by certain forces external forces for some of them you want to work and some will be a little redundant if I look it I go back to my solution of the problem right if I picked as my sister will block the spraying and before that really all I need because yes My free body diagram there is Wait but wait is perpendicular to doubt the act so workers still 0 I don't have to worry about it I put the honored my sister and I would ask about any changes in potential energy and I would see a height is always constant there's no change of potential energy so again it would turn out to be wrong relevant so in this case no matter which I do I get the same result I don't have to worry program both wars Find a key because the floors with giving me friction and I'd rather deal with that batters internal energy personally that Because otherwise rebel force friction and do all that this 1 is a step up in complications because it to relate the question has broken the 2 motions motion Eddie And motion be right what is our speed here is what it's asking matters All I had was how far do I go then this would be my initial and this will be my final and I wouldn't have to worry about with speed I had in the middle of the waiter think about this with our bars right But I consider this point 1 this point 2 and this . There's never any work done so I have The energy at 1 equals the energy at 2 equals the energy of parade and initially so I might have Connecticut Potential energy from the spraying energy dissipated And here all I have is this ring when I get the point I remember spring anymore but I'm moving and I dissipated some energy when I'm done All I've done is put everything into dissipated energy and that's what people back because work He was 0 everywhere so there's an initial equals this final And I started here this initial equals this final Oregon think of it that as . 1 being initial and that being the final And this is what makes the energy problems sometimes slightly more complicated is you will have to motions you will connect using energy conservation Now what I wanna do with this 1 and I leave this for you OK After work at the algebra for the 2 but I will pass the solution tonight This afternoon so you could look at it and thought that got me and I got all they want to do we say up 5 minutes just to the leaders for you to do designating mean rather time of day Questions on this 1 None of what I did unjust identifying with my for energies I have no potential energy from gravity so I just goes through it Now let's go to a fine or 1

37:38

Our Jagr Let's Ceballos I Wannamaker looked a little Today Jagr So rail right supply 1 asked why I joined start my roller-coaster so I just make it through the local real quick with the review you got 5 minutes and the clicker now 5 minutes 3 seconds and the quicker question What is the condition of the ball the Jazz make it through the lawyer It is not what they did it is not the velocity of 0 of the velocity 0 we fall long before we get here we fall somewhere over here you're just make road when you just lose contact and you just lose contact with the normal forces 0 no way way way

38:35

While you were still got like 3 minutes here that you What we have to did we won a figure out what's going on in this problem Right the question was how why I do we have to start to just make it through none

38:56

We need velocity here Because we Hey these where over are accelerates velocities in 1 hand and be square so we have is that the start above year show that we could make it Starkey

39:13

Then He said Would be a velocity of 0 they're right

39:17

Because It 1 have and square players NGA business they might not as fell off here

39:27

Are so this is a clear evidence that velocity was 0 If not recognition from the top of the normal course being 0 there's a problem that will be a bonus parliament discusses so that otherwise and think about that

00:00

Tag

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03:02

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04:45

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20:36

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28:32

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37:30

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### Metadaten

#### Formale Metadaten

Titel | Basic Physics Lecture 21 |

Teil | 21 |

Anzahl der Teile | 26 |

Autor | Dennin, Michael |

Lizenz |
CC-Namensnennung - Weitergabe unter gleichen Bedingungen 3.0 Unported: Sie dürfen das Werk bzw. den Inhalt zu jedem legalen und nicht-kommerziellen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen und das Werk bzw. diesen Inhalt auch in veränderter Form nur unter den Bedingungen dieser Lizenz weitergeben. |

DOI | 10.5446/12932 |

Herausgeber | University of California Irvine (UCI) |

Erscheinungsjahr | 2013 |

Sprache | Englisch |

#### Inhaltliche Metadaten

Fachgebiet | Physik |