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## Middle school physics - NGSS

### Course: Middle school physics - NGSS>Unit 1

Lesson 1: Representing motion

# Frames of reference

The position and motion of an object is always measured with regard to a frame of reference. Watch this video to learn about the importance of reference frames and how they can affect how we perceive the position and motion of an object. Created by Khan Academy.

## Want to join the conversation?

• The video previous to this stated that we would be using units of SI, International System, so shouldn't this video be using kph/kmph?
• True... but m/s is perfectly valid.
• If I were to be traveling at the speed of light, then from my frame of reference, would everything move backwards in time or would I travel forwards in time?
• I think everything would move back in time because, in the case of you being the reference point, we would be looking at, let's say a plane moving back in time with respect to your reference point moving forward into time at the speed of light.
• We stated that the Yellow truck moved at 40mph speed WRT earth. Also, we can state that Yellows truck also at 40mph speed WRT human. Is that mean both human and the earth have the same frame of reference to Yellow truck ( in the context of the explanation above ) ?

Thanks, you
• I have a question. When I see a car going 45 miles an hour through my car's windshield, it is 5 mph, but when I look at the car door window it is 45 miles an hour. Why is this? Am I like a person resting(not in a car, but on the grass(hypothetically))?
• Speed is relative soo it may look different then its actual speed.
• y does the north america look like a face?
• You are always still if you make your Frame of Reference yourself, right?
• You don't have to be still. You could be running. Please explain more about your question.
• OMG!! I just saw you can turn off notifications now!! Thank you khan academy!!<33
• So a POV from space would see that car traveling at 1,045 MPH? XD
• So to say Earth is a common frame of reference, it is that standing still is a common frame of reference?

Please, I need a better understanding of Earth being the frame of reference.

Thanks in advance for any answers! ^^
(1 vote)
• This depends on the context of the question. Earth is a common frame of reference for our telescopes to view space, to calculate certain orbits, say, the Moon or Mars, even the Sun. But, if we were on a different planet, like Mars, and we were viewing the Earth, we would have a different frame of reference as we are viewing it from the perspective of Mar's movement.

If we are standing still on a sidewalk, and a plane passes overhead, we can estimate that the plane is going a certain MPH, but for the pilot of said plane, we are actually moving backwards. Our perception is the plane is moving forward, past us, and the Pilot's is that we are moving backwards, behind the plane.
I hope this helps!
(1 vote)
• If the position was different would it still work if it wasn't mathematically correct.
(1 vote)

## Video transcript

- [Instructor] When we make new discoveries we need to be able to share them with others. And the first thing we have to do is make sure everyone is on the same page. We do this by using units and frames of reference, which are also called reference frames. We talk about units in another video, so let's look at what a frame of reference is. Let's say this blue box thing is a car, and it's going 45 miles per hour. Someone standing on the side of the road would see it pass at 45 miles per hour. Now, if this yellow truck is going 40 miles per hour someone sitting in the yellow truck would observe the blue traveling at five miles per hour. How could the person on the side of the road see the blue car traveling at 45 miles per hour and a person in the yellow truck see the blue car moving at five miles per hour? This is because both observers are using different frames of reference. So let's go ahead and take a look at that, starting with the speed of the blue car. The person on the side of the road is using their frame of reference of being at rest. So relative to them, the blue car is moving at 45 miles per hour. To the person in this yellow truck, which remember is already going 40 miles per hour, the blue car is going five miles per hour. Now let's do the exact same thing for the speed of the yellow truck. So what is the speed of the yellow truck for the observer on the side of the road? It's 40 miles per hour. And what do you think the speed of the truck is for the person using their blue car as the reference frame? Well, the blue car is moving at 45 miles per hour, and the truck is only moving at 40 miles per hour. So the speed of the yellow truck is actually five miles per hour slower than this reference frame, because the blue car is already moving at 45 miles per hour. Now you might be thinking, "But wait, "the person on the side of the road isn't really at rest. "They're on the earth and the earth is moving." You're completely correct. The person is at rest with respect to the earth. And the earth is the most common frame of reference that we use. To an observer in space who is not rotating with the earth, the blue car is going 45 miles per hour, plus the speed of Earth's rotation. And this is why a frame of reference is so important. We just talked about one blue car having three different velocities depending what the frame of reference is. How would we communicate this to avoid confusion? Well, we state the reference frame we're using. The blue car is moving at five miles per hour with respect to, which I'll write as WRT, the yellow truck. This tells us that the yellow truck is our frame of reference. Or we could say that the yellow truck is moving at 40 miles per hour and the blue car at 45 miles per hour, with respect to the earth. That way everyone is on the same page, a page which, to be clear, is in a book that relative to me is at rest.