Introduction to Physics - Educational film

we're used to seeing things from a particular point of view that is from a particular frame of reference and things look different to us under different circumstances at the moment you look to queue here you're upside down you're the ones upside down no europe turns out well not he's the one that's upside down his name well let's talk for a hi you lose people want to really up that that you better come into my frame of reference now frame of reference was inverted from what it usually is that view of things would be normal for me if i normally walk to my friends this represents a frame of reference just three route that together so that peace is at right angles to the other two now i'm going to move in this direction if either plane at the same spot on your screen but you know i'm moving that way because you see the wall moving that way behind me but how do you know that i'm not that i think bill and the wall moving workable now the wallet that the pier and you have no way of telling whether imovie or not but now you know that i'm moving the point of it is that all motions it's relative in both cases i was moving relative to the wall and the wall with moving relative to me all motion is relative but we tend to think of one thing is being fixed and the other thing has been moving we usually think of the earth inspect and walls are usually pick to beer so perhaps you were the part of the first time when it was the wallet was moving and not doctor whom a frame of reference text of the earth is the most common frame of reference in which to observe the motion of other things that is the frame of reference that you're used to the framers baton to the table the table is bolted to the floor the florida anchored in the building and the building at firmly attached to the earth of course the reason for having three rods if the position uh... any object such as this fall can be specified using these three reference line this reference line points in the direction which we called up two different directions here than it is on the other side of the air and these two reference lines specify a plane which we call horizontal or level in this film we're going to look at the motion of object in this third frame of reference and other frames of reference moving in different ways relative to the other frame well let's look at a motion this field ball can be held are by the electra mode no i'm going to open the switch and you watch the motions of the ball the ball is accelerated straight down by gravity along the line parallel to this vertical reference blood as you can see the electromagnetic mounted on the cockpit can move i'm going to do exactly the same experiment the doc resume did but this time while a cart is moving at a constant velocity is pulled along by spring which is longer on this photograph turntable and that hold that with a constant velocity when the car passes this line be lol this week that you can see i'm going to start the cart down at the end of the table so that by the time it gets to this point by can be sure it's moving with a constant velocity i want you to watch right here so that you will see the ball falling i think you can see that the ball landed in exactly the same position of the did before when doctor whom did the experiment with the car text but this time the ball could not have fallen straight down let me show you off the ball was released at that point if it had fallen straight down because the cart moves on in the time to take the fall would have landed back here somewhere but it didn't wanted to do the experiment again best time i'm going to let you watch the motion through slow-motion camera which is fixed the cart moves by the ball will fall and you can watch a missile which the camera how true this again first-time there'll be a line on the film so if you can see the fat repeatedly complete but the problem all to the ground all of this has been in a frame of reference fixed to the year all of this motion locked in a frame of reference which was moving along with the car frame of reference like that well so that you can see what it looks like i'm going through solution camera so that it was with the car permittivity experiment again incidently i started and the number to stand here when the ball for all of you will have something which is fixed as a reference point in their moving frame of reference i think you can see but the problem all as a political straight line it looks exactly the same as the day before when doctor whom did the experiment with the car etc if we were moving along in this frame of reference and we couldn't see the surroundings then we wouldn't be able to tell by this experiment that we were moving at a constant velocity as a matter of fact we wouldn't be able to tell by any experiment that we were moving at a constant velocity i'm gonna do the experiment once more and this time i'm not going to stand here behind the ball at the fall so that you won't have any fixed repertoire as far as you're concerned that time the karke wasn't necessarily moving at all that time when you couldn't see the background then i think perhaps it was harder for you to realize that you were in moving frame of reference important thing to realize here all frames of reference moving at constant velocity with respect to one another are equivalent doctor ivy showed you but the motion of the ball that was released from the moving karte looked like in the current frame of reference and in the car frame the motion looks simpler from a car now i want you to watch the motion white spot would probably be the clock moving out circle but that is what it's practiced actually like in the hurt of reference this is your normal frame of reference you father thought moving in the circle because you're high moved along with the car you put yourself in the frame of reference of the moving truck so if it isn't always true that we view motion from the third frame of reference when the motion is simpler from the moving frame you automatically put yourself and not moving frame now we're going to do another experiment on a relative motion to show how the compare the block today of an object in one printer records to explore all serbian another frame of reference forgiveness drive a spark a certain stark it moves a straight across the table with a speed which is essentially constant because the forces of friction of the made very small this is just the law of inertia an object moves the constant velocity unless an unbalanced force acts on it how you get the same start backwards uh... dougherty and gives it the same started moves back in this direction with the same philosophy now we're on a car here a car which can move in which were you going to move in this direction and we're going to repeat the experiment all right let's go if we were making measurements here then we would observe the same but lots of these that is the same experimental results that we did before and so would you because you are observing this experiment through with the camera which is fast into this car added you were in the moving frame of reference but now we're going to do the experiment again and this time you watched through a camera which is fixed in the poor frame of reference or concentrate on watching the clock to lecture i follow us i think you'll see that in a move faster that way and not so bad this way relative to u_n_ relative to the wall husband here the car which was moving along in this direction with the velocity we were thinking on the card at a table here i am over on the side and uh... doctor doom was on the side and we were pushing back-and-forth on the table well i pushed it and went into this direction with the velocity v when doctor didn't want to do when in this direction the same velocity v but this is the velocity relative to the car what about the velocity relative to an observer on the ground in the picture brain well if it was pushed in this direction it's velocity to you plot being if it's in this direction it will also be used u minded b this is all very reasonable is nothing very hard to understand here the surprising thing about this expression is that it is not actor in all circumstances at very high speed by high speed i mean speeds close to the velocity of light expression breakdown at these very high speed we have to use the ideas about relative motion developed by a albert einstein in his special theory of relativity for all the speed of the we're ever likely to run into this expression you plus or minus the is completely adequate so far we've been talking about frames of reference moving at a constant velocity relative to one another now i'm going to do the experiment with the dropping ball again only that time the karte will be accelerated relative to the earth frame these weight will fall and give the cart a constant acceleration first of all out and then i'll really put the motion is very fast and i want you to watch at the point where the ball is released from the pic camera i don't know whether you thought that are not at the top of the vault with the things that was before for either prime it landed in a different spots this is because the cart kept on accelerating in this direction as the ball was falling now i'm going to let you see it again with the slow-motion camera fixed on to the car this time you saw the bull moving off to one side falling down the vertical reference line as it did in the constant velocity case but suppose you were in the six telerate a frame of reference how could you explain this gravity is the only force acting on this fall so it should fall straight down but at the law of inertia is to hold there must be a farce pushing sideways on the ball in this direction accorded to deviate from the vertical path but what kind of a force is it it is not gravitational or an electric or nuclear force in fact it is in the force at all as we know what story left to conclude but it gets sensor is no force that could be pushing in this direction on the ball but the law governorship just does not hold this is a strange frame of reference we call a frame of reference in which the law of inertia holes and inertial the law how the nurse or holds in the third frame of reference so it is an inertial frame the car moving at constant velocity relative to the earth is an inertial frame but the car which is expel a rated is not an inertial frame because the frame of reference that we're used to living in is one in which the law of inertia holds when we go into a non inertial like the frame of the accelerated car our belief in the lava nurture is so strong but when we feed them acceleration of the ball sideways we think there is a port carpet so we make fiction but they're into force and sometimes we call this fictitious force fictitious forces arise an accelerated frames of reference the frame is accelerated in this direction so you in the frame cn acceleration of the ball in this direction and you say that there is a force cutting it what's happening this time why doesn't the talking straight across the table i did it before how you can see it doesn't if we believe in the law of inertia but we must believe that there is an unbalanced force to change the velocity of the pot but this pope is nearly friction locks so what can be exerting this unbalanced correspondent though that you watched the motion this time through a camera which is fixed in the earth's frame of reference i think if you concentrate on watching dot papa you can't be that it is a warning is a great time and that therefore there is no i'm not quite acting on it now we're going to stop this rotation but i can talk to you about what is happening here i don't know about you but i'm busy in d fixed fame of reference there was no one balanced force within the frame of reference for taking in this turntable there was and unbalanced because the velocity kept changing was with the fictitious part rotating plane is unknown inertial or accelerated fame just as the accelerated frame of the karte the doctor humes showed you you know that every object which is moving in a circle has an acceleration toward the center of the circle this is the exploration of has a special namely and capital acceleration now you will this puck for a while workstudy while the turntable is rotating i'll get off theoretic i'm ready but the rotation you concede that now the park isn't moving in a circle dr humans exerting a force to keep it moving in the circle and you can see this from the fact that the rubber ring is extended heat is exerting the center of the billboards and this is the only horizontal force acting on the part but not let's look at it again from his point of view in the rotating system he is exerting a force towards the center of the table and yet the puck is standing still produced more or less taylor some vibration how he believes in a lot of inertia so he thinks there's and equal parts on the part away from the center of the table so that there is no unbalanced boring this outward force in the pocket the fictitious sports in this case sometimes it's called the centrifugal force in the pics reference frame there is no award boris on the path now suppose the doctor doom stops exerting a barge watch the pot in the picture frame of reference the puck moves off in a straight line there's no no unbalanced boris acting on it now let's look at it again from his point of view in the rotating system when he releases the proc which to him was everest it mood the force away from the sender is now on unbalanced part on the talked to him the outward course on the part is fictitious in our food frame of reference it doesn't think that's but the doctor whom in the accelerated frame of reference it's a perfectly real force i hope by now doctor i do not have convinced you the rotating frame of reference is not in their shel frame for you've all been told that the hurt it's rotating abode of taxes and that also it travels in a nearly circular orbit around the time why then do we find in a frame of reference attached securely to the earth but the law of inertia seems to hold why don't we observed fictitious forces the five of the fictitious forces which we have to introduce in a non initial frame depends upon the acceleration of the frame these smaller the acceleration is v smaller the fictitious forces that we introduced here's a frame of reference attached to the equator of the car the acceleration of this frame is really very small because bearded spinning about it axis it has an acceleration directly inward app three d_ one hundred of a major per second square on they one kilogram at the equator their is the fictitious for us directly upwards out three one hundred but this is not by gravity which is a port downward of nine point eight new so the net downward force is smaller than that of gravity alone if i've dropped av one kilogram at the equator the acceleration would be slightly smaller than that due to gravity alone very much now the acceleration of the car in its orbit is even smaller still and produces even smaller affect in our frame of reference mike said that the earth was rotating abode faxes how do we know but this is slow well if you take a time exposure photograph of the stars they seem to be moving in circles a both p pole star but all motion is relative is there any way of telling which is moving or the stars the fact that it is because which is rotating can be demonstrated by means of the pendulum if i've got a pendulum swinging its wings back-and-forth in a plane though it turns out is this pendulum with the north pole of the art the plane of swing would remain fixed relative to the stars but would rotate relative now i'll have to show you what i need this pendulum is epicenter how this turntable which will represent europe no i was about the cable turning around in this direction i'll put a black arrow on so that you'll remember but the rotation the pendulum is that the north pole of the earth annuals motion as you ordinarily do standing on the earth the playing of swing rotates in the opposite direction from the rotation of the turntable and that exactly the same rate now look at it from the fixed cabral which will represent the frame of the stars the turntable the alert rotate but the plane of the pendulum remain specs a pendulum used for this purpose because approval pendulum use tommy dot wong at the beginning of this film let's look back again now the focal pendulum dropped slammed as its wings i think you can see the paint line where the planned trail began the amplitude upswing is decreasing the famed trail is not long now but the important thing to speak is that the plane upswing has been rotating during the half hour that we've been talking to you an inertial frame of reference is one in which the alarm inner cities without all frames of reference moving at a constant velocity with respect to an inertial frame are also inertial frame we use the word as an inertial frame of reference but it is only approximately one a small acceleration with respect to the stars for example the frame of reference other stars is the best we can do when we look for a frame of reference which is for all practical purposes bextra accelerated frame of reference is not a member shall frame and when we are in an accelerated frame we have to introduce forces which we called fictitious forces in order that the law of inertia and the other laws of physics don't change