Main content

## Physics library

### Course: Physics library > Unit 1

Lesson 2: Displacement, velocity, and time- Intro to vectors and scalars
- Introduction to reference frames
- What is displacement?
- Calculating average velocity or speed
- Solving for time
- Displacement from time and velocity example
- Instantaneous speed and velocity
- What is velocity?
- Position vs. time graphs
- What are position vs. time graphs?
- Average velocity and average speed from graphs
- Instantaneous velocity and instantaneous speed from graphs

© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

# Introduction to reference frames

How the choice of reference frame is related to speed and velocity measurements.

## Want to join the conversation?

- I know that stationary means to not move at all, but the car and the plane is stationary and does that mean that it does not move or not? I am soo confuse about this?(24 votes)
- A reference frame is a like a fixed point. Properties of other objects such as: position, velocity etc. are measured using the point.

It is so because no point in the universe is stationary or static. Every point is moving depending on another 'so called' static point.

See it like this: you are going to a amusement park in a bus with your friend. When the bus starts moving you see everything outside the bus going backwards. Here you are the reference frame. But for a person standing beside the road who has just missed the bus would 'observe' your bus going onward with you and your friend. So for the pedestrian both you and your friend are moving at a certain speed. But for you, you see that your friend is just sitting beside you, according to you, he is not moving but stationary as you are.

So the summary is when you are the frame of reference you and your friend are stationary and the pedestrian is moving. For the pedestrian it is the vice versa.(84 votes)

- So I notice at6:27Sal says we are taking the view point of this big giant thing called Earth when really the earth is spinning on a axis but we never see it, how does that work? The ground is stationary when you are there but when your perspective is shifted to the plane or the car the ground takes on the velocity of whatever perspective you are at. The Earth itself as a whole spins around 1042 miles per hours so why does the ground not take on an additional 250 or 50? Why is it that the Earth spins yet we cannot see it even though it goes so fast (the earth's mass is pretty large but still if you look at the sky it moves but the earth does not)? If you were in a plane staying in one spot and you looked down at the earth would it move at some point?(12 votes)
- The Earth, car, plane and atmosphere are all moving together with Earth's rotation. Since the motion of Earth's rotation is the same in all those frames of reference, it does not cause any change in perceived motion.(34 votes)

- At5:03wouldn't you feel the G-force because 250 m/s > 9.8 m/s and couldn't you measure that?(5 votes)
- If you're talking about 9.8 as the acceleration due to gravity, you need to remember that its a change in speed or direction (Velocity)

so its actually 9.8m/s/s not just 9.8m/s

The plane is not accelerating, 250m/s to the right, it is maintaining its velocity, you will not feel any G-Force while it is maintaining that speed and direction(Velocity).(6 votes)

- If there is a strong wind blowing, and the leaves and branches of the tree are moving but the trunk of the tree is still,so the tree would be in the state of rest or in the state of motion?(6 votes)
- Depends on your choice of reference frame.(5 votes)

- Can someone please explain to me in depth about what are inertial and non inertial reference frames?

preferably in a way that I can inuitively understand not just a definition(6 votes)- @Andrew M

thankn you for the website link that video explains it SO WELL!(3 votes)

- Why do we call it ' frame '? why don't we call it ' reference point', for example?(2 votes)
- Because it's not just a point, its an entire "space", a set of reference axes with a defined zero point. Things can move around in a frame but they can't move around in a point, right?(9 votes)

- Hii I want to ask that the reference frames examples you've suggested is that the car moving in the same direction of the plane, thus makes the plane seems like it's moving faster for 250+50 m/s but if the car and the plane are moving in the same direction, will the plane seems to move like 250-50m/s?(in example 2) or how fast would the ground seems to move?

Thx.(1 vote)- If the car and the plane are moving in the same direction, the relative velocity between them would be the difference between their individual velocities. Let's consider the examples you mentioned:

Example 1: Car and Plane Moving in the Same Direction (Car Speed = 250 m/s, Plane Speed = 50 m/s)

If the car and the plane are both moving in the same direction, the relative velocity between them would be the difference between their speeds. In this case, the relative velocity would be:

Relative Velocity = Car Speed - Plane Speed

Relative Velocity = 250 m/s - 50 m/s

Relative Velocity = 200 m/s

So, from the reference frame of the car, the plane would appear to be moving with a relative velocity of 200 m/s in the same direction as the car.

Example 2: Car and Plane Moving in the Same Direction (Car Speed = 250 m/s, Plane Speed = 300 m/s)

If the car and the plane are moving in the same direction, but the plane's speed is greater than the car's speed, the relative velocity between them would still be the difference between their speeds. In this case, the relative velocity would be:

Relative Velocity = Car Speed - Plane Speed

Relative Velocity = 250 m/s - 300 m/s

Relative Velocity = -50 m/s

The negative sign indicates that the relative velocity is in the opposite direction to the car's motion. So, from the reference frame of the car, the plane would appear to be moving with a relative velocity of 50 m/s in the opposite direction as the car.

In both examples, the ground would appear to be moving at the same speed as the car since the car is in contact with the ground.(10 votes)

- If we have 2 photons moving away from each other, each at the speed of light relative to their source, and we were to change the frame of reference to either one of the photons, would that mean that the other one is moving at double the speed of light? Or does the photon always travel at the speed of light, no matter the frame of reference?(5 votes)
- Ask yourself this - what happens to time and distances in that frame of reference where you are moving at the speed of light?

https://www.khanacademy.org/science/physics/special-relativity#minkowski-spacetime(1 vote)

- But the earth is always moving(1 vote)
- Now you can see why reference frames our helpful! The earth
*is*moving, but with reference frames, we can explain why we don't notice it, because it moves at a near-constant velocity.(9 votes)

- Can someone please simplify why we should add the velocity of the car (at the first frame of reference) to the plane\ground in the measurement(second frame of reference)?(3 votes)
- In the second frame of reference, the plane is travelling 250 m/s to the right relative to the ground. The ground is moving 50 m/s to the right relative to the car, so in order to calculate the RELATIVE velocity of the plane to the car, you add the relative velocity of the plane to the ground to the relative velocity of the ground to the car (250 m/s to the right + 50 m/s to the right = 300 m/s to the right) to get the relative velocity of the plane to the car. Hopefully this helps : )(5 votes)

## Video transcript

- [Instructor] What 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. There'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 gonna 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 we want
to measure velocities. So from your point of view, since you're stationary
relative to the ground, what does the ground's 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 and 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 1/5 of that length. So let me draw 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 observe
this type of thing happening and it seems completely reasonably. But what if we were to change
our frame of reference, change the point of view from
which we are measuring things. So let's take the frame
of reference 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 physics 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, you're like well the car is stationary, the car has a velocity of zero, the car is stationary and 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
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 case, 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
that 250 meters per second, but relative to you it'd look
like it's going even faster 'cause you're going past it, you are going to the
left from the stationary, from the ground's point of
view at 50 meters per second. So the plane, to you, is gonna look like it's going
250 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 plane's frame of reference? Why don't you pause this video and think about what
the velocities would be of the plane, the car, and the ground from the plane's 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 have our physics instruments out 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 and 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 of the windows, it feels like you are
in a stationary object, that you might as well be in a house. So from the plane's point
of view, you feel like, or from your point of view in the plane, it feels like the plane is stationary. Now the ground, however, looks like it's moving quite quickly. It'll look like it's moving past you at 250 meters per second. Whoops, try and draw a straight line. At 200... At 250. Sometimes my 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 50 meters per second
faster than the ground is. So the car is gonna look
not like it's just going 50 meters per second, it's gonna look like it's going 50 meters plus another 250 meters per second for a total of 300 meters
per second to the left. So this gives you an appreciation for what frames of references are. You can view it for
this introductory video as a point of view from which you're making
your measurements. Now, it's tempting for
a lot of folks to say well there must be one
correct frame of reference and a lot of times in our everyday world you might say well this,
maybe this is the correct frame of reference and these are just, we're just imagining this
or this is just the 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, that they are all valid
frames of reference, not, I shouldn't say they're equivalent, we're obviously getting
different measurements from them, but they're all, from a
physics point of view, equally valid frames of reference.