# Homework Help: Genral question about velocity

1. Jan 24, 2012

### eagleye

1. The problem statement, all variables and given/known data
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?

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 24, 2012

### SammyS

Staff Emeritus
If we did as you suggest then the velocity of a slower person would be larger than the velocity of a faster person.

3. Jan 24, 2012

### tiny-tim

welcome to pf!

hi eagleye! welcome to pf!
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!)

4. Jan 24, 2012

### eagleye

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?

5. Jan 28, 2012

### eagleye

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: May 5, 2017
6. Jan 28, 2012

### cepheid

Staff Emeritus
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

7. Jan 30, 2012

### eagleye

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)

8. Jan 30, 2012

### genericusrnme

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

9. Jan 31, 2012

### Staff: Mentor

There may be occasions where it is convenient to describe speed in these terms, but it would mess up other things if you defined velocity in sec/m. For example, acceleration is defined as time-rate-of-change of velocity.