# Directions of velocity and acceleration

nrqed
Homework Helper
Gold Member
MeJennifer said:
Yes, but how is that relevant to physics? Math and physics are two entirely difference things. One simply uses math as a tool in physics.
Yes, but one has to be clear with the maths concepts before using math terminology in physics. So it's not a waste of time to get the math right.
And apparently some do not even understand the suitability of selection of scale in cases of temperature.
So you would tell your students that negative temperature are *impossible*?

Btw, have you ever heard of spin systems in which the thermodynamic temperature may be negative (because putting in more energy may decrease the number of microstates available)? If not, you should look this up.

Really now? 0 Celcius is "well defined" while 0 Kelvin is not?
If the criterion is to be able to measure something experimentally, then the O Celsius point *is* well defined experimentally. Tell mw how to go in the lab and obtain a system at 0 Kelvin?
You are the one who keep insisting that math is secondary and that physics comes first. Then what is so horrendous about a scale which has a zero that is accesible experimentally? The zeroth of the Kelvin scale is a theoretical construct.

0 Kelvin is very well defined in classic theory.
Of course the uncertainty principle will disallow making something 0 Kelvin for any lenght of time but the same goes here for 0 Celsius. But presumably that would go for mass as well.

Anyway there is no point in arguing with you on this.
Keep telling students that temperature, speed, mass etc can be negative because one can make a scale in "almighty" math that would make it so.
That's so incredibly childish...to put words in my mouth. As if I had argued that I wanted to allow for negative speed and negative mass. Please be more mature and don't put words in my mouth.
By the way, your comparison with mass (and doubling the mass of the Everest using 2 x 0 ) is inappropriate because this mass and temperature are not both extensive quantities.

But if I am to join the ranks of people on this site you consider stupid, I am in good company (I resisted the temptation to give links to some of your other arrogant posts).

Have a good day

nrqed said:
So you would tell your students that negative temperature are *impossible*?
Yes! Just like I would tell them that negative mass and negative speed are impossible. Furthermore I would explain them that in science cold is not the opposite of hot. Everything is hot, only in different degrees.

I would tell them as well that unfortunately some people in history have developed inappropriate scales that map certain temperatures as negative.

As if I had argued that I wanted to allow for negative speed and negative mass. Please be more mature and don't put words in my mouth.
Feel free to demonstrate the logic in finding the idea of negative speed and negative mass inappropriate while seeing no problem with negative temperature.

But if I am to join the ranks of people on this site you consider stupid, I am in good company.
I do not consider you stupid.
But it seems you are a bit indifferent to the phenomenon of temperature, it seems you rather focus on the math.
Let me include some quotations to explain what I am refering to:

What is the mathematical definition of something being "absurd"? Either something is correct or it's incorrect (or it's unprovable :-)). There is no place for "absurd". Absurd refers to a question of personal taste so it's irrelevant if someone finds something absurd or not. There is nothing wrong with the Celsius scale.
Here you are clearly disconcerned about the suitability of a particular scale. Imagine we would do the same thing for mass or speed!

There is no mathematical problem with assigning the zero scale of mass to any value.
Here again you are only concerned with "if the math works". I am fully aware that there is no mathematical problem. That is until (hypothetically of course) we would have a true negative temperature in nature, then unfortunately we would have used up the negative sign already.

Your argument goes like: "Sure temperature can be negative, here let me make a scale for you to demonstrate it! The math works, so who cares!".
My point is: "Just the fact that we can model it as such in math does make it so for the phenomenon in nature. And some models are simply more appropriate than others as is the case with the Kelvin scale".

Ultimatelty, in nature temperature cannot be negative, the why would you want to use a scale that would use negatives for temperature?

By the way, you are entitled to your opinion and I respect it!
As far as I am concerend there is no need for name calling and so, we simply have a friendly discussion about a disagreement.

Last edited:
Kurdt
Staff Emeritus
Gold Member
MeJennifer:

Scales are arbitrary anyway. There is no point saying that everything should be set at the what YOU consider the correct scale. There are many situations where it is advantagous to change the zero point of a particular scale. For instance in energy levels of a hydrogen atom it is advantageous to set the lowest energy level to zero. The physics isn't any different you just get different numbers out of it. I don't understand your grievance and I pity your students. Personally I think this argument is going nowhere. If you believe in using set scales when its easier to change them then fair enough but it is not wrong for anyone else to do otherwise.

Speed or velocity?
speed is magnitude of the velocity vector. So speed is always a positive quantity.
Now along a line we define the acceleration as a =(vf-vi)/t, (or delta t).
Along a line you define a positive and a negative sens.

if along this line a > 0 then the object is spreeding up along this line and the acceleration is directed along the positive sens.