If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:6:55
INT‑3.A (EU)
INT‑3.A.1 (EK)
INT‑3.A.1.1 (LO)
INT‑3.A.1.2 (LO)
INT‑3.A.1.3 (LO)

Video transcript

I'd like to do in this video is talk about the notion of a frame of reference and this is an introductory video in future videos we'll go into a lot more depth but a frame of reference is really the idea it's a point of view from which you are measuring things and as we'll see many of the quantities that we might measure in physics like velocity or displacement they could be different depending on our point of view depending on which frame of reference we are measuring from and to get this an intuitive grasp of it I'm going to draw the exact same scenario from three different frames of reference that's the first one this is the second one and this is the third one so in this first frame of reference this first scenario we're going to talk about the frame of reference of the ground so if you are a stationary observer on the ground so you could imagine this is you here and you're the person doing the measuring of let's say let's say we want to measure velocities so from your point of view it's since you're stationary relative to the ground what does the ground velocity look like well you and the ground appear to be stationary appear to not be moving now what if you take out your instruments for measuring velocity or you see a change in you see what the displacement is over a certain time for the plane in the car and you're able to see okay look this plane has a velocity to the right of 250 meters per second 250 meters per second and let's say this car that is moving quite fast by car standards is moving to the left at 50 meters per second so this should be one-fifth of that length let me draw it a little bit so let's say this is moving to the left at 50 meters per second well none of this seems crazy you might be able to go outside next to the highway and see well 50 meters per second would be quite fast but anyway you could you could observe this type of thing happening it seems completely reasonable but what if we were to change our frame of reference the point of view from which we are measuring things so let's take the frame of reference from of the car well in this frame of reference let's say you're sitting in this car and I don't recommend you doing this while driving let's say someone else is driving or it's an autonomous vehicle of some kind and you take out your your physic instruments with the stopwatch and you see what the displacement is of the ground and the plane over say a second and you are able to first say from your point of view like well the car is stationary the car has a velocity of zero the car is stationary and you from your point of view you would actually measure the ground to be moving you would see the trees move past you to the right or behind you if you're moving to the left and so from your point of view the ground would actually be look like it's moving in this direction in that direction at 50 meters per second it would look like it's moving behind you or in this cave the way we're looking at it to the right at 50 meters per second now what would the plane look like well the plane not only would it look like it's moving to the right at 250 meters per second not only would it be just at 250 meters per second but relative to you it looked like it's going even faster because you're going past it you're you're you are going to the left from the stationary from the grounds point of your 50 meters per second so the plane to you is going to look like it's going 250 plus plus 50 meters per second so the vector would look like this and so it would look like it's going to the right at 300 let me write that in that orange color at 300 meters per second now what about from the point of view of the plane what if we're talking about the planes frame of reference why you pause this video and think about what the velocities would be of the plane the car and the ground from the planes point of view all right now let's work through this together so now we're sitting in the plane and once again we shouldn't be flying the plane we're letting someone else do that we are physics instruments and we're trying to measure the velocities of these other things from my frame of reference well the plane first of all is going to appear to be stationary and that might seem counterintuitive but if you've ever sat in a plane especially when there's no turbulence and the plane is already at altitude it's not taking off or landing oftentimes if you close your eyes you don't know if you are moving in fact if you close all the windows it feels like you are in a stationary object that you're you might as well be in a house so from the planes point of view you feel like or from your point of you in the plane it feels like the plane is stationary now the ground however looks like it's moving quite quickly it looks it'll look like it's moving past you at 250 at 250 meters per second whoops try and draw a straight line at 200 at 250 s and those mic tools act funny so at 250 meters per second to the left and the car well it's moving to the left even faster it's going to be moving to the left fifty meters per second faster than the ground is so the car is going to look not like it's just going 50 meters per second it's going to look like it's going 50 meters plus another 250 meters per second for a total for total of 300 meters per second to the left so this gives you an appreciation for what frames of references are that you can view it for this introductory video is a point of view from which you're making your measurements now it's tempting for a lot of folks say well there must be one correct frame of reference and a lot of times in our everyday world you might say well this may be this is the correct frame of reference and these are just we're just imagining this so this is just a mistake and the reason why we do that is because we're using the frame of reference of this big giant thing called the earth but it actually turns out that none of these frames of reference are more valid than the other ones that they are all equivalent they are all valid frames of reference not I shouldn't say they're equivalent we're obviously measuring getting different measurements from them but they're all physically they're all from a physics point of view equally valid frames of reference