How does relative motion affect speed calculations?

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Discussion Overview

The discussion revolves around the concept of relative motion and how it affects speed calculations, particularly in the context of a train moving at a certain speed while a person moves within it. Participants explore different reference frames and the implications of measuring speed relative to various objects.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions whether their speed should be considered as 2 mph or if it should account for the train's speed of 5 mph, leading to confusion about the correct interpretation.
  • Another participant asserts that speed must be defined with respect to a reference point, suggesting that if the train is moving at 5 mph relative to the track, the person's speed relative to the track would be 3 mph (5 mph - 2 mph).
  • Some participants agree that the statement "you are moving at 2 mph" lacks meaning without specifying the reference frame.
  • One participant humorously suggests that if considering the sun as a reference, the speed could be about 66,660 mph, highlighting the variability of speed based on different reference points.
  • Another participant introduces the idea that if one were a photon, the concept of movement would differ entirely, suggesting a different perspective on speed and motion.

Areas of Agreement / Disagreement

Participants generally agree that speed is relative and must be defined with respect to a reference point. However, there is no consensus on the specific speed calculation in the original scenario, as some participants maintain that the speed is 2 mph while others argue for a different interpretation.

Contextual Notes

The discussion highlights the importance of specifying reference frames in speed calculations, but it does not resolve the specific speed in the original question due to differing interpretations.

gec5741
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OK This maybe a dumb question but we have some debate on this question here at my work. No one here is a physics pro. :) Here's the question.

If a train is moving at 5mph and you are moving tward the back of the train at 2 mph. How fast are you going?

Now I think the answere is 2 mph do to the way it's worded. But how can you not count the speed in which the trian is going? If your on a train that is going 5 mph aren't you also going 5mph?

Thanks to anyone that can help explain this. :smile:
 
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A speed must always be with respect to something to have any meaning. One might assume that the train is moving 5mph with respect to the track. If you move towards the back of the train at 2mph with respect to the train, then to find your speed with respect to the track you would add the speeds like so: +5mph -2mph = +3mph. (Your speed with respect to the train is negative since you are moving towards the back.)
 
there all saying the answere is 2 mph. I agree with what you said! :)
 
gec5741 said:
there all saying the answere is 2 mph. I agree with what you said! :)

Well, if your velocity is measured with respect to the train, then your speed would be 2mph.
 
gec5741 said:
there all saying the answere is 2 mph. I agree with what you said! :)
As long as you realize that "your speed" only has a meaning if you specify what you are measuring against. On a naive verbal level, the problem did say "you are moving at 2mph"-- but that's meaningless by itself. Your speed is also 0mph, 1mph, 3mph or almost anything else--depending on what you are measuring with respect to.
 
So this is what people do at work...
I wish my gas station cashier work were as challenging..
 
Doc Al said:
As long as you realize that "your speed" only has a meaning if you specify what you are measuring against. On a naive verbal level, the problem did say "you are moving at 2mph"-- but that's meaningless by itself. Your speed is also 0mph, 1mph, 3mph or almost anything else--depending on what you are measuring with respect to.

there could be a bird flying by the train at 10 mph and say hey that guys only moving at 7 mph

all of it depends on what you are using as a reference for the speed. ask yourself, it is going that many miles per hour... but (blank) miles per hour past what? the inside of the train? train tracks? neptune? what are you comparing it to
 
gec5741 said:
If a train is moving at 5mph and you are moving tward the back of the train at 2 mph. How fast are you going?

About 66,660 mph, of course! :biggrin:

(with respect to the sun, that is...)

(I say "about" because the exact value depends on the time of day, the train's latitude, and which direction it's traveling.)
 
If i were a photon, you wouldn't be moving at all
 
  • #10
jtbell said:
About 66,660 mph, of course! :biggrin:

(with respect to the sun, that is...)

I prefer to reference the centre of our local galactic cluster, in which case we're moving really damned fast.

Ki Man said:
If i were a photon, you wouldn't be moving at all

Not to imply that you're overweight, but that would be one hell of a diet plan.
 

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