Robin wrote:
On 12/09/2019 09:00, Tim Streater wrote:
In article , NY wrote:

"Tim Streater" wrote in message
.. .
In article ,
David Paste wrote:

Hello,

When writing, for example, "metres per second per second" for
acceleration, it is noted as ms^-2 (where the caret symbolises
the -2 is in superscript).

I understand that, and why, "per second per second" is "seconds
squared", but in the notation, why is it superscript minus 2? Why
not just superscript 2?

Because its "per", and so you are dividing. which gives you a negative
coefficient.

E.g.:

Miles/hour is speed, and if I say I go at a certain miles/hour for a
certain time, the answer must be miles. So, 30 miles/hour for two hours
is 60 miles. That is:

30 m/h x 2 h = 60 m

So the /h and the h cancel, meaning that as the h is really h^1, the /h
must be h^-1 in order for the coefficients to add up to zero (meaning
there's no h left in the final expression). When you multiply things
you add their coefficients, as in 10^2 x 10^3 - 10^5.

The notation m s^-2 and kg m^-3 always strike me as perverse: what was
wrong with m/s^2 and kg/m^3? I remember being taught m/s^2, and then
when I changed school to one that did a Nuffield physics syllabus, the
notation changed to kg m^-2 which it claimed to be "better" in some
unspecified way.

I'm inclined to agree.


I disagree. Mainly as the superscripts reduce the risk of mistakes
where there are multiple terms. Eg

kg.m^ˆ’1.s^ˆ’2
kg/m.s^2



Certainly if you are analysing a problem and checking the dimensions ( a
useful technique, not always taught these days although I made a point of
teaching it), indice notation is probably far easier to use.

I suspect it is a question of what you are used to. A physics teacher when
I was a pupil first introduced indice notation for units, so it is the one
Ive used most. I have used others but much prefer indices.