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Scenario:

Two vehicles pass each other on a highway moving at 100km's an hour; from a stationary position beside the road you witness these cars pass each-other directly in front of you: from your perspective these cars are moving at 100km's an hour.

My Question: If you were inside either one of the 2 cars, looking straight out the window to where the other car would cross, would you perceive the vehicle to be moving at 200km's an hour as it passed you- due to both cars moving at 100km's in opposite directions?

I'm unsure if thats right or not- whenever I personally travel by car and bare-witness to another vehicle pass my window in the opposite direction at the same speed, it seems to always be moving much faster than it really is, assuming it was only because of the speed I was going when passing it.

Is this correct?

Qmechanic
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Harry David
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3 Answers3

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Rough impressions can be misleading. The other car really is moving 200 km/h from your point of view.

One thing to keep in mind is that you tend to perceive motion more readily when it is closer to you. This is at least partly due to the fact that you really only can see angular speed across your field of view (like degrees per second). To convert this into an actual speed, you need a sense of distance to the object in question, which you only have for nearby things where for example parallax kicks in.

As a result, if you are moving at 200 km/h, a stopped car right next to you will appear to be moving very fast, whereas the distant scenery off to the side won't, even though both are moving relative to you at the same speed.

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Your understanding is correct. And switching between different perspectives (what physicists would call different inertial frames of reference) like that is a very useful tool in physics, because it turns out that the laws of physics have the same form no matter which inertial frame of reference a problem is described in. For more information, see https://en.wikipedia.org/wiki/Galilean_relativity .

Red Act
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Yes you would actualy see the car with its speed added with your speed.

Chris Ger
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