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### 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. For example, a car's speed looks different if you're still, moving, or in space. To avoid mix-ups, it's important to state the frame of reference. This concept helps us understand and communicate about motion accurately. 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 think it isn't about the object that is observing, it is about the observer's position.
• 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))?
• That is an interesting question! When you see a car going 45 miles per hour through your car's windshield, it may appear to be moving slower, around 5 mph, due to the effect of parallax. Parallax is the apparent shift in position or motion of an object when viewed from different angles. When you look through the windshield, you are viewing the car at an angle, which can make it seem slower. However, when you look at the car door window, you are viewing it more directly, resulting in a more accurate perception of its speed. As for your second question, if you are resting on the grass and observing the car, you would have a stationary frame of reference, which means you would perceive the car's speed as it actually is, without any parallax effect.
• What's the difference between frame of reference and reference point?
• the frame of reference is the overall view or background, while the reference point is a specific object or location that you use to compare or measure things within that view. So, if I am looking at the cars the view I have or what I am seeing is the frame of reference, while I, myself am the reference point.
Hope this helps!
• Video makes a claim at near the -minute mark, that the truck is necessarily rolling in the direction of the Earth's rotation. The truck's direction of motion is not given, such that we don't know what direction the truck is rolling, so how can we say the truck in rolling with the rotation of Earth, against the rotation of Earth, or rolling north/south??
• The earth rotates on an axis and spins around the sun. We, humans, are on a specific place on this planet(for me, Boston, MA). So why don't we get dizzy if the earth factually moves ay around 1000 miles p/hour? We don't feel dizzy because we ourselves are not resisting the motion we have the same speed as the earth has for spinning. Dizziness is felt when everything spins around you fast and you are getting thrown out of the spinning object but on earth its gravity keeps us in place.
(1 vote)
• y does the north america look like a face?
• If i was moving like the speed of lighting would time go backward?
• What this reminds me of is,
Y'all know how like, when you're moving in a car and it's driving fast.. then you look out the window to see another car but it looks like it is going in slow-motion?? Is that basically what this person is talking about? ( Ha-ha! Am I making sense? This is my best way of explaining this common experience that I have )
• Yes, this is exactly like that! This is the best way to learn. Relating the concept to something you've experienced in your life. It makes learning so much easier!
• Fun facts you guys might want to know about in physics
(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.