# How to decide between vectors and it's components?

Suppose we have following situation assume that both are kept over each other.

As $m$ experiences $N$ normal due to $M$ and $mg$ due to gravity. There no motion hence we can write: $mg-N\cos A=0\implies mg=N\cos A$.

Now consider this method, if we broke $mg$ perpendicular to inclined surface will equals to $mg\cos A$, also $M$ is fixed, this should be the force acting between $m$ and $M$, but $N\neq mg\cos A$ from above why?

If we draw FBD of $M$:

And take $N$ to be vector whose component is $mg$ then $N=mg\cos A$, which we got above.

So my question is how to decide which vector is component and which is not?