Basic Physics Lecture 12

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Basic Physics Lecture 12
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I do want As we lead in today's lecture this this was a perfect bunt lead and It came from the wears a couple of people on the short Three-quarters of ask the following I think very reasonable questions
That actually I was going to comment on anyway in my introduction as we move forward from kinematics of forces and it has to do with this idea I love it when a physics problem like you give us more practice problems sewing solved All the problems that we could ask like that I never encountered the last problem on the mid-term now if you act go back and look at the solution of the midterm and this is where physics is so different in so many places in other topics and at the same time so challenging right what we have finished 8 3 weeks of kinematics now what you hopefully not out of those 3 weeks doing the problems is the following important said
There is a class of problems that are related to describe Dean motion and those questions like Where would you land where are you how far did you travel how high did you travel how There also Did you travel And what direction and refocused on this special case of cars the acceleration And basically used fog To derived equations definitions are everyone will look at it for answering these questions and why do we do the different types of problems that the challenges but is actually an infinite number of variations there is unfortunately as much as I would like to know where I could do this there is no way I can achieve by cyanine and doing an example of Every possible Individual problem that comes from the collapse of problems there kinematics Ed The switch you have to make do in physics and only really makes that I said at the beer class but only makes sense really after you've experienced it wants as there was really only 1 problem we ever did We did projectile motion and most 1 demand was projector emotions got near event so there is really only 1 type a problem we did Carson accelerated we did a lot of lots of variations of it if you think of every single 1 of those At a different problem that you need to know how to do that problem It will be very hard to be successful this class but you do enough that you have that realization And you're going into it thinking why is over said wired is all said and you make yourself realize why they're all a single problem just different variations that it will be impossible for me to give you 1 you've never seen because you realize it is 1 of this class of problem and I know how to do it and that becomes really imploring as we add or other 2 classes of problems and what we're working out we started a little bit last week and were now going to spend this week and next week focusing on are the class of problems involving forces so here we were focusing 9 describing the motion asking warehouse Farmhouse how that's what direction warning the type of questions to recognize that was kinematics and that was problem What are these Tight the questions we will get forces In forces were gonna the asking things About the cause of motion so You might have questions like do you start to move for start moving right Start or stop moving this requires an acceleration that's not equal to 0 this requires a cause that acceleration so requires and that Force so it is of course problem and that is very different than athletes have vastly moving where movie it's a do you start to move is callers to your motion is there an acceleration that is generated the more obvious ones or ask the straight for question what is your acceleration right it might just be given the problem that's unlikely you're being asked that you're being asked that Because you'll know the causes of that acceleration of forces and you want a compute what it's the other type question is there does something usually a rope right right that the cause of something happening of course it's too big and the rope operates voters look different questions there were all basically the same physics idea and physics class questioned the other 1 is what force is needed to whatever accelerate avocado accelerated just start move just stop at the end of the day for all of B's you will use the same 3 Newman's lost the 1st lot you won't even realize you're using because it tells you that you have The correct reference frame beds of inertial reference frame of cousin last week we do very very very well aware we have to worry about that that would be in a more advanced but it's there behind every problem we do what we focus on is a secular at people's MA because that tells us what happens to A single object and keep in mind that you stop all the forces for the single object for its acceleration and we will use new blocked because that will give you Paul forces brainer accidents Letter between objects
Annual these These defeated ended here said testicles and so as we go forward The next week or so I will be doing some demos I will be doing a bunch example problems Today layout the big picture and in all of that again learned a lesson from kinematics don't focus that each problem as a separate different problem ask yourself right from the beginning power all these problems basically the same U.S. little details will vary right yourself into something just to move your solving basically an acceleration of 0 that's about to become 0 solving forecast acceleration of solving a number those details matter they do make it a little harder but the way you have to think about it an approach it is we are now doing this class of problems how things move and we can Because we are dealing with accelerations connect this fall away back out to a description emotion right Acceleration is about something that you we used to describe a emotions and it's the thing that caused by forces so now we have to Class of problems that could be connected so the same problem you might need use kinematics for 1 part of the problem and knew it was also the other part and you have now recognize not only 2 Classes of problems but when a single problem involves 2 types of problem and by the end of the course a 3rd authorities energy conservation as basically rivers 3 classes of problems kinematics describing motion forces cause emotion and energy which is kind of in between the 2 involves both causes and description and making a relate with each other and all 3 of those in relay or probably just 1 of those and that's where you wanna be approaching this class so hopefully that makes that I I really was looking for but I was really glad I got to cause because it gave me a chance was anyway 2 really emphasize now had this experience what it's really about is it only gets harder As we go but that the bad news any questions on that Although it real This is an interesting set of chapters because really what I'm going to do As kind of 2 things today I am going to briefly mentioned all the key ideas in chapters 4 5 and 6 . 1 day you're going need over the next week or 2 do homework you look at the homework assignment is due Sunday at 1 of our longer ones and it really covers stuff All of these places fighter jet before all of Chapter 5 Insurgents export 1 male why does it do this because we need to learn to recognize the types of acceleration And the types of forces But at the end of the day All the problems we do we use the exact same method and technique independent of what types of forces and acceleration show up in it so it's kind of weird thing so what I wanna do is right off the bat they tell you about all the forces accelerations me Over the next week so you have them in front of you don't look forward to do a little better and show you how we would solve any given force Problem in general with a particular Demel example and then do bunches Of examples and demos for the next few days that combine these different forces in different ways so a very good idea Mr. to start reading the chapters and start the homework kind of in order for the does build it do earlier start is only 1 of 2 types of forces as you get a lighter stuff It could involve any of the forces we've learned at that point I gave you wait to the weekend to do this homework assignment it will probably be too light seed everyone build With For the week At the homework and with the Class AAA questions So let's meet these next 2 parts of the lecture will be very very useful to come back to because they're going be the 2 key lists that really have to start with a whole new piece of paper analyst wine It is acceleration and the reason I do this has led strong briefly tour online quiz that you're gonna have to go back to the snow open up and Our ot reopen it tonight we 2nd version with the same questions but I'm gonna removed like It showing you the answers they need a 2nd version will be due Friday At the end when we've covered all the stuff energy that is We got thrown their some key ideas to look for what's this whole friction saying movie because speed or going ramp this question number 1 is about the fact that there is a from going up a ramp those friction and there's gravity and those were pushing At a constant velocity is acceleration 0 So what's all forces due at 2 0 If this question that the fact that I only mention to look forces I'm missing gravity I have to push with a greater force friction because I'm also canceling gravity so 1 of the key Things is practicing finding every single force a problem you always have to do that 1st summer in a practice that was that Lend so The only weird for Sweeney is of course a static friction because the forces static stretching could change its value And that there may be something but that it would be the hardest conceptual challenge this week and so this question get that back into your wanna look into how that works The most common state is something that is a problem 3 and 5 I use the word here on purpose triple forces do not exist centripetal acceleration exists and you have the centripetal acceleration there must be a bunch of forces That add up to call that 2 cars that some people will like to refer to a total net Force that causes the center of acceleration the centripetal force which is OK but very very dangerous because I can guarantee you if you are Thinking in terms a centripetal force you will put it in your free body diagram you will write and be squared over are you will put the acceleration as an eastward of our those 2 will cancel out and you get the problem because there is no no force and eastward over there's no physical for the physical forces are always the normal course of a wall friction A rope calling on you gravity there's always some other real force that Causing the centripetal acceleration and if you if you think of the centripetal force as a force in your list of forces you will get an extra 4 kinds as very important so that was a hero the road once again shows up here in the local loop so your honor look at them and then his last 1 again has to do with it Keep in mind breaking things in the components what's happening in all the directions what cancels because a lot of problems early on where the normal force Does it all the way And so people start to think of the normal force always equals weight because its people the way The last 3 times and on the problem but it does best is what happens in some of our initial warmer problem so you have to be aware that the normal force is not actually able to await the storm force and you have to do with it that way so These are things you wanna luck Or as I made my list as I got the example so that's why for this week I wanted to put that frequent
And they give you saying quiz later such important concepts I want image as that Experience ahead 5 without warning that game right and then try to go forward with that questions Jack But how might you know it the acceleration of arbiters That might you out or some way you find out in physics columns about acceleration so far Obama given velocity is definition take derivative But there's a more common way Let that was really is your way I heard it Aids Dilbert 1st as a mouse Common way to know the acceleration If it 1 of the things that is least likely to use a formula And it'll be given Gedo usually as some constant that's not equal to 0 4 0 matters a very common situation that now there's a subtle way Be dividend yield either told you're just about but to actually start moving while you meet the bureau arrest at the next considered and removing what is that mean you have You have an acceleration of just sitting there master sometime What is your acceleration 0 so just about will to go from able 0 2 egg that equaled 0 BILLING number we can solve for you you there's no number we can agree on that is bigger than zeroed off for so physics trick If you're always gonna be solving for April 0 with Juergen be solving for a situation where there's some forests that can change a value and it wants to reach the value so that exactly become 0 so Simple situation right is among them list something similar that right I can apply lots of little forces where what left but there'll be some force that is the biggest forest that was still have 0 acceleration but may make it any bigger now accelerate and so that the sense of solving drew 0 Rates and they will as we do more problems but that the trick you want and then the final line a section 5 to the but centripetal acceleration to ways to approach this wine you can memorize the formula And not think about it too much But is very helpful to have a picture was going chervil acceleration dribble acceleration workers anytime you terms It particularly useful for constant speed in a circle and is what happens ever movie with you want here at some later time moving with me to over here But I wanna ask for my Delta which is viewed to minus V Y Villagers like that it might 1 goes in the opposite direction So Delta V as basically pointing radially inward right as I Garland Circle No matter what circle I move my velocity by definition is changing toward the center by velocity points more toward the center that I did before there's a little hard to picture because the velocity itself is always change into the circle Right so the velocity actually never points toward the center is a change in velocity the point sources center because I'm turning in the direction of the center and that's what you have the pitcher Now to get the to derive it I'm not going to write this formula that it's in the book it uses the basic triangles and stuff where you could Buying this change lasting sometimes and you see that has to do with the radius of the circle and the speed you go at so The smaller the radius the bigger the acceleration and this is a teacher behind designing a coaster I could make a really sharp turns was small rainy I of curvature small circles then you will feel a bit accelerated in the roller-coaster and it will be more exciting even if I'm going at the same speed as another rollercoaster because we really feel the roller-coaster the accelerator and that's why the were 20 more turns the more lives The more exciting roller-coaster any questions and that of income we do the problems but This is that these are the 3 ways we get acceleration so when you're reading a problem you're asking yourself Do I know the acceleration a ready and I'm Now was 1 of these 3 things happen either give it to me they tell me just about to move Or they tell me my speed move a circle that they don't tell you whether they things ratifying that losing that was made something union the problem any questions For that Are Force less as a tad longer over forces says that actually mention the 1 in Chapter 6 1 1st that gravity that the famous Newton's laws of gravity by the mass 1 and amassed to separated some distance are the the magnitude of the forest is new and gravitational constant G Justin number and 1 2 over or squared The direction is attractive meaning but force
On mass and 1 storage and too And I am to restore them It's an attractive force equal and opposite now when you'll notice can't like numerical constant in that I think the only 1 we get I will give you the test of the ever a new and La gravitation I respected have G memorized now there's a special case of gravity which is section 4 sets This is gravity near the earth's surface duty Earth We call that weight and its magnitude comply Since the mass of the Earth the radius of the earth and GE don't change we plug then and that we get our Messier William through what is number it is in this turns out to be little G which is 10 meters per 2nd square so The weight is given by energy do not confuse us with 2nd lot a looks like it What we can find out that the acceleration due to gravity near their surface is which we already used But the very different idea this is just a calculation of the course From the definition of the 4th due to gravity But it's about of course we have and we Do very little with the 1st version of his or we don't do many of those funds most of our problems take place near the service the I work going away that this version of the way I just use energy for the weight of an object We next 1 is related to one-fourth that we break in the 2 What I can't spell as contact forces 50 luggage contact there is an equal and opposite force between them So object there's object being an entering club We can't break everything integer directions we could make 1 that Perpendicular to surface between them at a direct parallel to the surface and when we did that we get to forces be perpendicular contact forests we call the normal force there's no formula for base just went to object 1st together There's of course perpendicular the service we call the normal course it always exists For anything in contact with a parallel course and that is searching for sex but parallel force parallel component is friction and that section 5 want it does not always exist in physics we imagine frictionless services and there's something that approach that ice moving on the table is basic research analyst plot That an Iraqi table like a little bit of gives you a frictionless surface so you can actually get real close to that friction and by far I think is the hardest Board That you will DOS And the reason is this to retire their static and there's genetic so you 1st have to know what you're using some key pieces static has a less Our people too It actually changes its value and they're both related to the normal course Which does not have a formula for and might change and the problem depending what's going on so these are 2 pieces that can cause you problems the other piece Is that it opposes The yet Dead marches And every now and then the attempted motion is not obvious and that's why you have to think about little bit waste so I'm going to start moving my car which directed and the this way we start moving my car Which direction is the force of friction Back on me As a moving backwards or forwards they trying start my car What efforts in the back with how the heck am I gonna move forwards Bowie Don't think This is where it's horrible what is your real doing you turns like that to move you forward do we Illustrate slight backwards on the ground reflect To move forward is trying to move backwards might attempted motion is actually backwards when I won a move forward so Friction acts forward and move forward because the attendees want focus on is the attempt most of the 2 objects in contact right offered to know that my foot touching it my foot tries to move backwards Ferdinand forward now when I start my clock what most in Is being attempted by the wheels or my feet on the ground a slide forward so which way is friction a work backwards and the start and that's why friction By far Could be the thing the cost you the most pain and literally right a rope never mind so you've got all of these issues referred the final last worse we need as British straightforward that's tension in stirrings at any time I push poll now pushing and pulling you could often think up As a normal force its 2 things in contact we often think of it separately is just pushing and pulling but they They come together it is a contact I realize that was rather you know the most boring 30 minutes of our class but it needs to be done I really feel it if you just try If we did to these wonder time and you had read the book through it would just be a little overwhelming as each 1 got thrown at you I think starting with your at the beginning maybe make your own list printed out headed for you as you go through the next week or 2 just haven't there and be checking in bodies all forces that I find all forces are any these nite during these years there's not going to be some other magical force that shows up at a problem if you've got through the problem and every single 1 of these is there and you have a freebie learned the really can't be anything else now There might be 6 normal courses through the object you care about might be touching 6 different things out of every 6 normal forces I was pathetic you but there could be more than 1 normal course might be tempted to a 3 object there might be to friction forces should you might be tucked into objects there might be rejected as you might be on held by 2 ropes so there to be more of each of these but there can't be any different type of force
I'm ready for quicker questioned ready wake up project I gotta do masses year runner-up allies started quicker questioned about that as as a writer work well I lumber demo spell that I can deal with my Inc Inability to use powerpoint is more embarrassing each of these has a massive triangular but as the mass that will give you a 10 new imports now That in a row and is a scale in the middle of their boat pulling on the scale and the question is What is going to be the force in new in that the reads given mind we saw this last week Inzer kilogram-meter per 2nd squared their uniforms and so it is a trend 20 years of 5 so basically asking by having the strain do I get the a force of 1 of these both of their murders somehow cut it in half The diet or bothering him 5 more seconds 3 2 1 And at as you use off said so we can turn our still around And it's it's 10 new and map 1 of the fund is a that of Virginia such as late they get stuck above hearings but I knocked around that suntan you now what I'm going to do so take this off A look at the scale so that the scale It is only having A poll from this matter so the situation is like it's connected to a long and is being pulled American Newton Mass above give a force of 10 new ones The question is this That is the question now with 1 maps and series that Lewis the title of change that so now when I take off The 1 Masson duel with want what is the scale than a read with 1 mass while there must be a day for total PowerPoint failure And very interesting were not sure nicely spread out now what I wanted to know before I show you the answer to this But to I can make this work harder we approach the the way you gonna do all of these problems at and so important that it you ever as history related roughly but it yes I live on the response question so you're right now do not draw the freebie you will lose points I don't care to get the problem right the reason being I want to know that you could show me all the forces and that's the way the best way to do it so we Joe was called a free body diagram This is our key tool is the main thing we do the way you do it as you take your abject and you're basically now using the 3rd block and you ask the question What does it Interact with The salt are massed made in Europe On this train I will tell you right now has 2 interact What are the 2 things in his interacting with It was not really at the ground you have to be careful we use that phrase because actually touching the ground What is it interacted with Recalled as Big Bang there were steady on They are rather actually the owner of the were interacted with just Helped us to use that same common language so it has interaction with the urine so it draw that and be interact with the earth is always straight down And the vertical direction and we like to call the weight so as W weight duos A lot of different notation SOW just a capital LOW Object a with the earth there's lots of ways will be ready now has 1 other interactive What is it The Roper string pulling it out I most often use and my last comment as set retention so the but sometimes there will be problems were that's not quite as clear and you will use a are for a rope on there will be giveaways you do it Some people orders right that is a capital T. so there are these 2 interactions once we know all our interactions with applied to add equals MA the object is not move it on this hanging out there and is Velocity is not changing what is In this case it was Iraq we direction the deposit So very common right is now is just their magnitude and put the minus side in my hand so that my assigned is the directions Further weight I reply get 0 for acceleration and we have 0 now by drawn heroes we have at the quest to my next asked you about The virus to draw that as a picture is An object going across a pulley there's a scale but basically attached or wall and I ask you what Force as the stalemate Measuring some basically asking you For the tension In the row right that was pulling on the scale so now Having seen that and the way we think about these problems vote again why you expect for the attention when I bought a really know their authors traces you talk to people
Top And Where world emotions torts said millions let's see what we get look at that Techno and thank him for and you know it's really sad new inside and lock their does move wife Is that 10 will let Go back so now I get to click on the answer new and it
Let's go back to our pictures And What we call everybody diagrams right They deem our place Let's look at our 3 pieces right there is our scale And the scale goes to something else Whether I have amassed over here or not I wanted draw the free body diagram for each of these objects with free body diagram for Eddie Had Out weighed down the free body diagram for beat Just as Jude tensions and equal opposite directions And 1 of the things that happens there's a little bit of a trick here that you're never not skills work and the minute but that had an acceleration of 0 so what that's been the first one but to redraw those of AA we had at tension at W. amid acceleration it was their out so that tensions were taking out to be positive miners at W. equal 0 out and I told you the way it was such that Ng was equaled the 10 new ends Evidences 10 new routes Forestdale weather was attached to a wall or the other man's The only choice here again a equal 0 But pick that would be the right eye at tension mind as ever Engine equal 0 let's suppose that and think they were the same to begin with I'd have found out that they're the same to you when he was to and so what you need to know the the way this scalp works right it is If you look at it this isn't doing anything and the only hold on 1 side and that's all scales measure their forces in the road so it's not like the scale of measuring the 10 plus attack was their religious measures the tenants being poll with it measures detention in the there's a single tension the road and it is equal to or new and because this See 1 was wet connected to that by the wrote this Is the heart features are erupts in physics problems so let me just show it to you again in another picture let's look at the picture Without a scale in there over to objects on the polling in physics misses 1 single wrote and I said is very quickly if underclass last Wednesday The key feature of ropes in physics because they always have a single tension and it can attention than the Roque can change anywhere it's a single object that has whatever tension that has so when I drop free body diagram for object a an object B I have the start out With a single tension because I have a single rogue states now if I had to differ wrote I would have 2 different tensions and this is filed views to objects are connected and that's 1 of the things you're gonna wanna look for our objects connected because their touching or objects connected to there's a rope between them And that will give you a connection between them where there is a similar force In both diagrams just the opposite directions now They may have different ways of their different objects and so in principle I have 2 separate equations now I take up to be positive I wait 1 minus F T equals mass 1 times 1 at F of weight 2 miners at center people's mask to science 8 And now we have this is rule number 1 of the ropes the other role In visit Easy wonder remember if 2 objects are connected In any fashion Like this moon together they always have have the same acceleration If this wine has a different acceleration than this 1 then the rope either has the break or collapse because of the new moon different amounts they different displacements and the rope only moving the state Do you wanna have a mental pitcher of it 2 things are connected in their moving there is no other choice Then same accelerated You may have started the problem Virginia to have an acceleration want you realize they're connected than we apply Rule number to an 81 people's aid to rural ones and rolled to end this case we're Jim M-1 equals and 2 and so therefore
We end up with a a single Wait a single tension and a single acceleration and things are much simpler and from there we can go I did What we did when it was 0 and find out that that single tents and passed it will await dozen people twice the weight people's away now After Of course of I take this and do that And now from over here and saying the mass right here in the same place I now do indeed double will force the goes into the 20 because I now have doubled await being balanced by I still have weighed down at To not but now ever weight is twice as big as I have 20 new Sarasota so Betty questions on that So let them 1 final quicker question When suppose You a block on a mass with friction how many for actually or redraw right here cerebral block a mass on a ramp and there is friction how many forces when you draw the free body diagram for this block How forces are going to be in your free body diagram 63 per cent of your there there will be break battery star without Wednesday restored doing problems like that start like that Wal-Mart was Sea-Land