# Genral question about velocity

## Homework Statement

I have just started physics course and I have a kind of philosofical question.

why do we measure velocity in metres per second and not by seconds per metre?.

for instance a man walking 0.5 metre per second, would'nt it be best to describe him as walking for 2 seconds per metre?

SammyS
Staff Emeritus
Homework Helper
Gold Member

## Homework Statement

I have just started physics course and I have a kind of philosofical question.

why do we measure velocity in metres per second and not by seconds per metre?.

for instance a man walking 0.5 metre per second, wouldn't it be best to describe him as walking for 2 seconds per metre?
If we did as you suggest then the velocity of a slower person would be larger than the velocity of a faster person.

tiny-tim
Homework Helper
welcome to pf!

hi eagleye! welcome to pf!
… for instance a man walking 0.5 metre per second, would'nt it be best to describe him as walking for 2 seconds per metre?
yes, we could …

we could define that as his deeps, for example, so that he has a deeps of 2s/m …

but then the faster he goes, the less his deeps is …

wouldn't that be confusing?

(oooh, also: velocity is a vector, so velocities have to add like vectors, and deepses won't add like vectors!)

Yes, I better understand now, but my question is - is there any reason why we shouldn't use my method (s/m - "deeps" like you said) and develop all the kinematics formulas according to it, is there anything wrong with it? because for my understanding, velocity was firstly defined to measure the rate in which distance is changing throughout time, but is there no use at all for knowing how much time passes until an object is completing one unit of a distance?

Again is there any use for seconds per meter in any field of physics?
Because I found somthing that uses this but I wan't to know if there are more uses for this and how to use it.
this is the link to how to determine the distance away a lightning storm from only the sound of thunder that uses seconds per meter.

Last edited by a moderator:
cepheid
Staff Emeritus
Gold Member
All they're saying is that:

time = distance / speed

Hence:

$$t_{sound} - t_{light} = \frac{d}{v_{sound}} - \frac{d}{c}$$

So basically to compute the distance, you take delay between lightning and thunder and divide it by (1/v - 1/c). If you want to take this quantity with dimensions of 1/speed and call it something else, you can, but there is no real added benefit.

Edit: fixed equation

Edit: elaborated, fixed error

Then I understand that, to the best of your knoladge, there is no field in physics that uses a measurement of seconds per meter? (the one we called 'dibs' in this post)

Then I understand that, to the best of your knoladge, there is no field in physics that uses a measurement of seconds per meter? (the one we called 'dibs' in this post)
correct
off of the top of my head I can't think of any reason why using units of seconds per meter would simplify anything

NascentOxygen
Staff Emeritus