hello I’m Lou Bloomfield and this is how things work. Today’s topic skating. When you’re wearing skates, whether they’re ice skates or roller skates or simply something similar to skates, like skateboard, your motion is especially simple. Most of the time if you’re at rest wearing skates and I’m not I’m not wearing skates so I’ll use this can illustrate life for a skater when you’re at rest you tend to remain at rest if you get pushed on you tend to begin moving I’m your hand if you’re already moving you tend to keep moving in a straight line pretty steadily too and in order to change your motion something’s got to push on you so what we’re going to be looking at is the behavior objects that are not being pushed on and the behavior of objects that are being pushed on the object in mind is a skater but it applies to any object so to illustrate to test or examine all this stuff let’s start with the first observation that a skater rest tends to remain at rest so why is that and the answer is because because this is how our universe works that objects that are left alone and that are already motionless at rest is physics jargon for not moving so it’s already not moving it tends to remain not moving it’s going to turn out that you gotta move you got a bucket you got to influence it to cause it to begin moving so this observation that in our universe objects that start motionless at rest can remain at rest is known as inertia it’s a familiar term inertia it’s used in common language this is the real physics meaning of it it’s a it’s a an overarching concept that that observes that objects tend to remain doing what they’re doing in this case there motionless staying motionless that’s the first half of inertia there’s a second half and the second half of inertia is that an object that’s in motion so not yet not yet ready now once it’s already in motion it tends to remain in motion they’re more details coming but that observation that in our universe objects that are already moving tend to keep already moving in a very similar way they do what they were doing already that’s the second half of inertia so inertia is not a physics quantity or anything it’s a big it’s just a big picture view of how things move in the absence of pushes and pulls the influences that cause change so skaters spend a lot of time taking advantage of inertia to keep doing whatever it was they’re doing a lot of little kids may stand around on skates doing not very much it’s a I don’t know it’s a little kid but anybody can be easily just sort of stand around motionless staying motionless once you get going you tend to keep going both of those are reflections of inertia now inertia might sound as I’m as I’m spelling it out here for you like it’s obvious of course things exhibit inertia of course of course but in fact your real life inertia is not so obvious this can you know it’s fine when I tip it on its side so with no role but this can when it’s on its and experiences friction which I’m it’s a dirty little secret I’m trying to ignore for the investor this entire segment for now forget friction it’s a problem in reality this thing does experience friction and so when it’s at rest it tends to remain rest well so ok that’s that’s consistent with inertia but to make it move I gotta keep pushing it if I stop pushing it comes to a stop so inertia doesn’t seem to be right at all what’s what’s going on what’s this claim two things keep doing what they’re doing the problem is that I’m not the only person influencing this can it’s basically not free to do what it wants to do it’s suffering under friction from friction with the table so you got to keep pushing it to keep it moving because friction keeps acting to slow it down get rid of friction however and things keep doing what they’re doing they exhibit inertia okay well can we it nourishes this big picture idea can we sort of pin it down and make a rules out of it or physical laws of it and the answer is yeah sure we can and the first of the the famous physical laws associated with Newton Newton’s first law that makes the following observation in a as yet primitive form on the language’s is oversimplified at the moment I’m gonna whip it down shortly but what Newton’s first law says is that an object that is free of outside influences moves at a steady pace in a straight line so if you don’t bother something and I’m going to get it pretty much free of bother by by tipping this cam outside if you don’t bother it its motion is very simple it moves in a straight line path at a steady speed so right now this is a it it’s exhibiting that law you know it’s moving according that law right now it’s free of outside influences we’ll name we’ll name those influences soon it’s free of those influences but it’s not moving and it’s simply staying not moving so it’s moving in a straight line path at a steady speed of nothing zero boring okay once they get it going and during the getting going part it’s not obey Newton’s first law because it’s not free of outside influences I’m the influence so I’ll get it going now it’s moving a straight line path in a steady speed so that’s the behavior of an object that’s free of outside influences it moves steadily in a straight line okay language is simple doesn’t you know will do better but but that’s Newton’s first law it’s first version we do better well to do better we got to start naming stuff we have to identify physical quantities that will work with now and for the rest of the season here and the first physical quantity that I that I’m gonna want to use and I’ve got I got five of them just to give you a heads up five physical quantities to work within the context of skating the first one is a quantity known as position doesn’t sound all that unfamiliar but we’re gonna nail it down and so it has a really well-defined meaning or at least you know as well as I can do in this in this these circumstances position tells you where an object is located in in our three-dimensional world I mean this is a position this is a position okay this is a position you know lots of positions how do we identify those things and to do that we need a couple of things ideas the first is we have to have a reference point to start with you have to have something that identifies position zero or the origin the place where we’re going to start and then everyone agrees on that’s the zero place and I’m gonna find something that’s gonna be it’s going to be the zero it happens to be a baseball why pick something roll a canister so that’s position zero and we now will identify locations positions specifically relative to that reference point to do that we need two pieces of information for for any given position so how do we identify the position of this pan now well it’s about that far away from zero from position zero that happens to be about the metric unit known as the meter actually it’s a little short of a meter there’s a that’s a better meter so that’s a that’s a meter and it’s a little over three feet for people who want to work with the traditional American units so you need a distance distance in this case one meter because well I’m a physicist you can sort of you can live with that okay so the position of the can is one meter from zero I’m not quite that’s not the whole story yet we’re short on some information here there’s something missing does something missing is that’s one meter and so is this and and so is this we’ve got how far away the can is from the reference point but not in which direction so we have to pin that down too so the position of the can relative to the zero point is one meter in this case to the right the direction matters the direction is a part of the physical quantity we know of as position that means that that position has both an amount naming in this case a distance and it has a direction to the right up away from you quantities physical quantities that have both an amount and a direction are known as vector quantities don’t let the word vector scare you it just tells you that it’s a quantity that you can’t simply type into a calculator with with units because you need you need to have the direction part – it’s not it’s not just a number in fact physical quantities are rarely just a number they’re usually a number and the units in which that number to which that number applies so for example this would be what position one inch this is position one foot this is position 1 meter all to the right the units matter 1 1 of what’s one of some classic standard agreed upon amount and for position the amounts that we’re looking for our our all distance amounts a mile a meter pick whatever you like Lightyear all right they’re all measures of distance in this case it’s one of them and we need direction it’s to the right okay so I’ve beaten vector idea to the to death here position is a vector quantity it’s Eric is located you need the reference point and you need the distance and direction from the reference way okay all right that was physical quantity number one this is the quantity number two is how position is changing with time what’s that well it’s called velocity another word that you may well have heard about and used even and it has a very specific meaning in physics it is the speed at which you’re moving and the direction in which that motion is occurring for example here is can I do this that’s it I’ll do one meters brief one meter per second is pretty fast I will disappear from view really quickly so I’m going to go one foot per second and I’m going to do it to the right one foot per second here this is a velocity of 1 foot per second to the right ok and this is velocity of 1 foot per second to the left direction clearly matters velocity and speed are often used interchangeably in common language but that’s not a no that’s not writing physics in physics speed refers to how fast something is moving without any reference to direction who cares this is this is 1 foot per second this is 1 foot per second who cares which way I’m going ok velocity is added information it’s speed and which way you’re going so in other words both velocity is the the speed is the is the amount part of velocity in the same way the distance was the amount part for position incidentally to a to a physicist and mathematician the amount part has the the the fancy name magnitude the magnitude of physic of a position is a distance the magnitude of velocity is speed is how fast you’re moving ok so a scare that is moving like this is moving at 1 foot per second to the right and how do you know where’s the 1 foot per second what does that really mean it means that every second their position shifts by one foot and it shifts in the rightward direction so it’s all self consistent and make me and hopefully make sense and with that concept of velocity now that we we see things moving we can pin down Newton’s first law of motion fairly nicely we’re still we’re still not quite there but we’re better and what Newton’s first law of motion says with the using the word velocity is that an object that is free of outside influences moves at a constant velocity it’s constant velocity mean constant velocity means that how it’s how the position has changed with time never changes he keeps doing the same making the same shift every second so this is constant velocity not yet written over this is constant velocity I’m moving one foot to the right every second ok so that is consistent with how things move into it when they’re inertial when they are following the concept of inertia and they can only do this in the absence of outside influences they move at a steady pace this straight line that’s constant velocity incidentally before I forget to do this to C position you only have to look once this is one of my sticks here is like it is to think about how positional works for position you like to glance once you know flash photograph I’m here flash photograph I’m over here you can measure position in a single burst of light okay you can’t do that for velocity from a lot so you have to look twice you have to look once now and you have to look again now to know that position change of time from there to there it occurred during a period of one second he moved he moved one meter in one second he has a a velocity of one meter per second and it happens to be toward the right so two glances for velocity we’ll come back to that because there’s a physical quantity that requires three glasses coming down the pipe all right so we’ve got two physical quantities so far position and velocity well what about the those outside influences that we’re outlawing in order to have Newton’s first law those are outside influences they are the pushes and pulls of the world so for example this for the second this can sits here motionless when you’re left when left alone if an outside influence comes along things may change and those outside influences which will involve my hand involved pushes or pulls the the fancy physics name for a push or a pull is a force and you know the word force shows up all over the place and movies and such this is the real meaning of the word force it is it is the these influences that take you out of inertial motion the motion described by Newton’s first law they are the influences that cause a different kind of motion and forces are yet another vector quantity Oh what does that mean the a force is both how hard you push or pull and in which direction you push a force to the right is different from a force to the left man left even though they’re equal in strength equal an amount so so you look around you at the world of a force is pushing and pulling on things and how hard you push how hard you pull which direction you push or pull these all matter they all come finally come out forces and so for a skater for a skater to stop being inertial to to to change from being motionless and stay motionless or moving at constant velocity they need something to exert a force on them okay so using the word force to describe to identify the forces the influences that the influences that that show up in Newton’s first law that law can be restated now in its final form then an object that is free of external forces outside forces moves at constant velocity there it is that’s the whole story that is the coasting motion of skaters of so many objects in everyday life that are free of anything pushing on them it’s a little tricky to get free everything pushing you got to get rid of friction you got to get rid of air resistance you have to get rid of gravity or these cancel if there are a lot of things that mess up the the simplicity that you the simple motion that you would see in the absence of those messy forces but if you could get rid of them you’d see lots of examples of things coasting along but the word coasting means exhibiting the motion that that is described by inertia so a coasting object is one that is free of outside influences and specifically free of outside forces now there’s one little detail that I’m sweeping a little under the rug here well I gotta bring it up in any case and that is that if two forces are simultaneously acting on an object like my my my right hand is pushing the can to the right that’s viewed by you two directions MS here my two hands are pushing in opposite directions how’s that the can can’t move be influenced by the individual forces it’s it’s instead it’s influenced by the sum of the forces where direction is taken into account and so two people grabbing hold of a friend who’s on skates and and both the friends pulling on on the skater are pushing on the skater the skater moves according to the sum of the two forces acting on the skater and that summing process it’s not trivial it’s you because it involves amounts of force and in which way the forces are and the forces can act in the same direction or they can act opposite one another and partly cancel it’s a bit messy and we could get lost in it forever but the concept is known as net force the sum of the forces where Direction is taken into account so in Newton’s first law I where bring this back where is that in Newton’s first law an object that obeys Newton’s first law that it’s inertial that moves at constant velocity is one that has a net force of zero acting on it it doesn’t have to be truly free of all outside forces it just has to make sure that if it is experiencing any outside forces they sum to zero they cancel each other out right so that’s the world of inertial motion and that’s the world a skater lives in much of the time the skater actually is experiencing gravity down and we haven’t gotten there yet but they’re being pulled downward by the force of force due to gravity it’s it’s it’s secretly known as their weight and that by itself would cause trouble however the ice or the sidewalk is pushing up on the skater just as hard and is making sure the skater experiences net force zero the scanner then moves according to Newton’s first law at constant velocity all right well what if you don’t move at constant velocity well if an object that is experiencing a net force that is not zero doesn’t move a constant velocity they do something else they accelerate acceleration is the fourth physical quantity I’m bringing up today and it’s sorry about this it’s another vector quantity it describes the change of velocity with time how your speed and direction of travel are changing as time passes so it illustrates acceleration let me show you make myself accelerated I’m going to accelerate towards your right and I’m going to start at rest and if I were not accelerating I would continue at rest velocity zero velocity was zero velocity zero it’s constant boring but constant instead I am going to sell it to your right and I’m going to do this with the help of the floor it’s it’s gonna push on me with friction so that’s the secret underlying this how I’m doing this but I’m going to begin to accelerate to the right and I’m gonna do it very gently because otherwise I’m gonna disappear from view quicker so I’m an accelerate to the right and that means that I start to develop a rightward velocity and it gets bigger and bigger and bigger and bigger so that was that was I was excited the whole time once I got started and as a result my velocity is initially small and it’s bigger than right and bigger the right and bigger the right and bigger than if I go out of you so that’s acceleration it has an amount which magnitude which is how rapidly the velocity is changing with time how come much the velocity now is different from the velocity now it also has a direction acceleration to the right like this is quite different from an acceleration to the left right another interesting observation is that if I am moving to the right which is to say I have a velocity the right but I accelerate to the left watch what happens I’m going to accelerate to the left means I’m going to acquire more and more leftward velocity but I’m going to start with rightward velocity so before the story starts let me get going here okay I’m already going I’m now gonna accelerate to the left which means that I lose my rightward velocity and I momentarily come to a stop and then I start gaining leftward velocity and often I go out of sight so acceleration and velocity don’t have to be in the same direction directions matter and if you are moving in one direction which is to say your velocities is that direction and your acceleration is in a different direction you are gonna evolve your motions gonna make a transition of some sort in order for you to start excel to begin they have more and more velocity in the direction you are accelerating right now I promise like that there were a three glimpse physical quantity and that three glimpse physical quantity is acceleration to see to see acceleration you need three glances you need only one for position here right you need only two for velocity here and here but for acceleration you need three I’m here I’m here and I’m here so between the first two first pair of lenses you saw how fast I was moving in which direction so you could figure out my velocity and between the second two glances you could see which direction and how fast I was moving later on after the acceleration head had had time to cause things to change so three glances for acceleration well what causes accelerations the answer is forces again the fancy name for pushes and pulls if you push or pull on something and nothing and there’s nothing else involved you will cause that thing to accelerate so forces are the cause of acceleration there is however any resistance to acceleration it’s got a name that’s also familiar to you and it is our fifth and final physical quantity and happy happy it’s not a vector it’s just an amount the quantity I have in mind is called mass it is an object mass measures its it well I should say it’s the measure of an object’s inertia or I also think of it as an object’s resistance to acceleration how hard is to make the object accelerate so this object this can has a certain mass and to measure that mass but conceptually I’m not gonna make it kneel get it to three digits huh the way I do it I make it accelerate I’m gonna make it go from heading to your right to your left that’s a bigger that involves big accelerations so it’s velocity goes it’s right where velocity leftward velocity right with the shaking process which I’m doing now shaking something is effectively observing the mass how hard is it to get it to change its velocity to accelerate and the answer in this case not too bad you know it’s got a certain mass and the classic unit of mass that we even use in the United States is called the kilogram a kilogram it’s not a weight we’ll get to wait later weight has to do with gravity this has nothing to do with gravity we could get rid of gravity altogether in our universe and you’d still have mass round it’s how hard it is pick a bunch of skaters a little kid you shake them back and forth I mean of course you know you can’t arrest it but you shake it back and forth and and they’re easy to make the mixer rate get some big burly person whoa oh really hard okay it’s a little bit like okay I don’t have a big burly person to shake round but I do have a lead brick and I don’t know why physicists love let brick so I’ve got a lead brick and this sucker has a lot of mass when I tried to shake it it shakes me which is also an interesting observation it makes me accelerate quite a bit so so this has got a huge mass not so much okay so given the idea that you have forces which you can measure and you have mass which you can measure and you have acceleration which you can measure there’s a relationship between them and it’s an exact relationship and it’s called Newton’s second law of motion and in my preferred version of Newton’s second law it reads is this the force exerted on an object and actually it’s it’s the sum of all the forces so it’s the net force on the object divided by that object’s mass so net force divided by mass is equal to the acceleration of the object the object accelerates in response to the force opposed by the mass so the force forces the cause the mass is the resistance to that cause and the acceleration is the consequence the result so the you double the net force on an object you double the acceleration you double the mass of the object which is you can’t do like you have to change the mat the object you make a pick an object it’s got twice mass yep you have the acceleration so the identical pushes on a bunch of skaters the low mass skaters look solid dramatically the high mass skaters look sorry much less so that’s Newton’s second law of motion and it applies everywhere – everything you push on stuff if you exert a net force on them they accelerate in accordance to the force you you exerted the cause and they’re all mass the resistance now Newton’s second law is often rearranged algebraically to get rid of the division so nothing divided there’s no no division sign unfortunately I don’t like this form of Newton’s second law because if it mangles the it puts cause and effect and and jumbled together so the F equals MA force equals mass times acceleration I mean that relationship is still true but what causes what it’s kind of a mess so the point is that forces cause acceleration the acceleration is exactly proportional to the force and the constant of proportionality is is associated with the mass that’s how much the the object resists acceleration so when you watch skaters zipping around the ice or on the skatepark when they’re going straight the net force on a zero and they’re going straight in constant velocity their their net force is zero when they are turning or when they’re speeding up or slowing down something’s pushing on them they’re experiencing net forces other than zero and they’re accelerating as a result last bits and pieces of things to mention one is the idea of a frame of reference I’ve talked about all this motion here from the point of view of people standing still you are effectively standing still because you’re watching this video and presumably the computer whatever you’re watching it on is not moving but if we are all working in a train we would all see the same story but someone’s standing outside the Train would look in and watch and see they would see them everything happening but even the things that I claim are dressed like this suppose that a video on the train you’re sitting on the train watching the video and the person looks in and this stuff is all moving so even the can that’s arrest isn’t it rest it’s moving well how do you deal with this well it turns out that there are many in fact an infinite number of possible frames of reference point of points of view from which to watch the world and as long as those points of views are ones that are themselves inertial they are themselves coasting they are they are not undergoing acceleration their views of the world are just fine and everything they see about the world moving the motions the world it’s also consistent with the with Newton’s first law second law it all works out so how do you do this let me try to illustrate this with you that what I just said is clearer for you guys this can is motionless I’m motionless everything is very simple here that’s an object arrest an arrest but now from my point of view while I’m doing that I’m moving along it’s still an object at rest stay at rest right you can you can believe that I look over there it’s not moving relative to me looks the same do-do-do-do-do-do right there there right there right there so my from my inertial frame reference and I was moving at constant velocity I was not accelerating well during during the me at the story from my point of view it’s an officer s velocity zero from your point of view and you’re also sitting still presumably in an inertial frame of reference it’s a can that’s in motion and maybe a one foot per second to the left so your opinion about this it’s fine if you do do all the mathematical calculations of motion whatever you like using your frame of reference and you’re you’re gonna want a point zero and all that stuff it’s all good in mind I gotta keep them all moving okay so if I do all my little calculations and measurements all to make self consistent to what we compare between the two of us we have to be careful but in your frame of reference everything works out in my frame runs everything works out when we talk about our different observations we’re gonna have some things that are complicated but that we can you know you don’t need to worry about that the point of this bringing this up is that that they’re often very simple frames of reference in which to observe motion that make everything very simple for example if we want to watch a ball going up and down up and down up and down the motion is very simple if if I’m not moving you’re not moving we’re all we’re in the same inertial frame of reference but if I’m moving relative to you to me the ball is going straight up and down but to you it’s strictly in arcs so you can see there are times when picking the right frame of reference and thinking and you know working in that frame reference makes life simpler and we’ll will do that from time to time okay so phrase reference the last thing to mention in this section is to take a peek at units units are very important because mo the physical quantities in our world have units involved in there are maps to them that other than just numbers the number for if you are cooking for example and the recipe calls for this much flour they can describe it in various ways they cannot describe it with a number alone they can’t say seventeen use seventeen flour what’s that instead they have to say well use two pounds the pound is a conventional unit of flour of or of anything so if you use two two of those pounds of flour you you’ve fulfilled the goal but there are other units that describe amounts yeah i i’m chuckling because i pick one this it’s fraught with peril because of the pound is a complicated sometimes it refers to wait sometimes it refers to mass where we’ll get messed it what was a safer unit this i think yeah this is this is this is a 32 ounce container you fill with water i will contain 32 ounces that would also be 32 ounces but 600 as it sits it’s several hundred cubic centimeters it’s a measure of volume basically and you can measure volume with many different units cubic meters is a unit volume a leader is a cube as a unit of volume a quart is a unit of volume but anyway the point is that that once we have agreed on a set of units if i say four four quarts you know what i’m talking you know the world around will know will know what I’m talking about they’ll be able to come up with four quarts worth of something and that’s true for the five physical quantities I introduced today position has units there they happen to be units of length of distance things like meters inches miles velocity also has units they have the units of dis as per time that is m/s miles per hour that’s how–that’s for the speed part of velocity acceleration also has units they’re a little lower more unfamiliar the units of acceleration are meters per second per second or simply meters per second squared that’s how the velocities changing with time the units of force also have have standard units in the conventional system we’re so familiar with pounds pounds is sort of the American unit of force in the metric or SI system Internacional system the the classic unit of force is what’s known as the Newton the Newton is a force that’s it’s less than a pound it’s about how hard a small Apple to make it memorable Newton Apple ha ha ha it’s about the force a small apple exerts on your hand when you hold the Apple up so that’s about a Newton of push Newton of force and the last unit thinking the physical quantity mass has units the classic unit of the mass the SI or metric unit is the kilogram there are other units of mass but let’s leave them all in the dustbin of history the only mass that really matters mass unit that really matters in modern life is the kilogram and that’s a certain amount of resistance to acceleration so with that then you look back at skaters skaters spent a lot of time moving inertially that is moving according to Newton’s first law of motion because they are experiencing zero net force when they are moving inertia like that they are coasting it’s the st. you know more words for the same idea on the occasions when they are not moving according to Newton’s first law of motion that is when they are experiencing a net force now they accelerate in the accelerated accordance with Newton’s second law of motion their acceleration is equal to the force exerted on them of the net force if by their mass and so they they speed up they slow down they turn and all that and that about does it for the topic of skating

Section 1 1 Skating
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One thought on “Section 1 1 Skating

  • February 1, 2019 at 6:54 am

    Forgot my book and ran across these videos trying to find summaries. Thank you for uploading these! I now watch them in addition to your book to make sure I get the concepts. Appreciate it!


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