# If a train is moving 5mph?

1. May 24, 2007

### gec5741

OK This maybe a dumb question but we have some debate on this question here at my work. No one here is a physic 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 wich 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. :rofl:

2. May 24, 2007

### Staff: Mentor

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.)

3. May 24, 2007

### gec5741

there all saying the answere is 2 mph. I agree with what you said! :)

4. May 24, 2007

### neutrino

Well, if your velocity is measured with respect to the train, then your speed would be 2mph.

5. May 24, 2007

### Staff: Mentor

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.

6. May 24, 2007

### KingNothing

So this is what people do at work....
I wish my gas station cashier work were as challenging..

7. May 24, 2007

### Ki Man

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

8. May 24, 2007

### Staff: Mentor

(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.)

9. May 25, 2007

### Ki Man

If i were a photon, you wouldn't be moving at all

10. May 25, 2007

### Danger

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

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