Sign of acceleration I'm developing an application using accelerometer sensor. I'm not good at physics so forgive me if the question is trivial. If I have 3 values of acceleration:  $x$, $y$, $z$, I find acceleration magnitude by taking square root of $x^2+y^2+z^2$, but how do I find it's sign? Example reading: 
x: -0.010020584 
y: 0.010257386 
z: -0.04910469
The magnitude will be around 0.05115, but how do I know if it is deceleration or acceleration?
 A: You measured acceleration in three dimensions, by having measured the x, y, and z components. You need always three parameter to describe it. If you have the magnitude of the acceleration, you need two angular coordinates to define the full vector.
Only in one dimension you can talk about sign, this pointing left or right. In three dimensions, you get more parameters.
A: If your question is: "How do I know if the acceleration is increasing or decreasing the speed?" the answer is:
Calculate $\mathbf{a}\cdot\mathbf{v}$, where $\mathbf{a}$ is the acceleration vector ($(x,y,z)$ in your notation) and $\mathbf{v}$ is the velocity vector, and check its sign. If it is positive the speed is increasing, otherwise it is decreasing.
In your example:
$$
\mathbf{a}=(-0.010020584,0.010257386,-0.04910469)
$$
Let's say for simplicity that $\mathbf{v}=(1,1,1)$. We then have:
$$
\mathbf{a}\cdot\mathbf{v}=(-0.010020584)\cdot1+0.010257386\cdot1+(-0.04910469)\cdot1=-0.048867888
$$
The negative sign of the result means that the speed is decreasing.
