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## Representing motion

# Frames of reference

NGSS.MS:

MS‑PS2.A.3

, MS‑PS2.A

## 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.