Talk:Q181175

vector quantity (Q2672914) = opposite of (P461) scalar quantity (Q181175)

vector quantity (Q2672914) is not really opposite of (P461) scalar quantity (Q181175): A scalar is just a one-dimensional vector. — Finn Årup Nielsen (fnielsen) (talk) 13:20, 4 February 2014 (UTC)

Not really, they're two different concepts. Think about a one-dimensional system where a point mass moves with velocity and acceleration different from zero. Even if acceleration and velocity are "1 by 1 matrix" (the easiest system I can image), they are still described as vectors: ${\underline {v}}=v{\underline {i}}$ and ${\underline {a}}=a{\underline {i}}$. On the contrary, mass can never be described as a vector, as a matter of fact you write ${\underline {F}}=m\cdot {\underline {a}}$ and not ${\underline {F}}={\underline {m}}\cdot {\underline {a}}$.
Hope the explanation is clear enough. Let me know if I'm wrong! :-) — TintoMeches, 15:11, 4 February 2014 (UTC)

Ok, I see your point. But an acceleration in one dimension would be one-dimensional. To me that would be a scalar. — Finn Årup Nielsen (fnielsen) (talk) 15:39, 4 February 2014 (UTC)

Velocity in a one-dimensional system.

I couldn't find an image showing what I want to say, but basically a vector can always be represented as a linear combination of unit vectors (usually ${\underline {i}}$, ${\underline {j}}$ and ${\underline {k}}$,which are arrows whose lenght is one), so that also a random vector is itself an arrow.
Mass, temperature and other things cannot be represented as arrows, because they don't have a "preferred direction" in the space, but they're are still a peculiarity of the physical body. — TintoMeches, 16:04, 4 February 2014 (UTC)

Eureka! Maybe I understood what you mean. You probably mixed up speed (a scalar quantity) and velocity (a vector which is speed in a given direction). Check out on the English Wikipedia if you understand the difference... — TintoMeches, 16:10, 4 February 2014 (UTC)