Naming vectors in free body diagrams Is there a convention for naming the vectors?
Suppose there is a box on a table. I'm going to draw the forces acting on the box. So I focus on the box and ignore forces acting on the table, the normal vector on the box is named after the box or the table? My doubt is, subscript $B$ to mean "normal vector acting on the box", or subscript $T$ to mean "normal vector, from table to the box"?
 A: There are a few conventions out there. The short answer is: use whatever will make sense to you. I personally don't like single subscripts: It's not as easy to match 3rd law pairs. If you do want to use a single subscript, I'd go with the object that is exerting the force, otherwise every label will be the same, and what's helpful about that since you've presumably labeled the entire free-body diagram with "box"?
The best labeling schemes I've seen indicate both objects participating in the force. So, for a normal force on the box by the table, you could write
$$\vec{N}_{B,T}.$$
The first subscript always refers to the object on which the force is acting (the box). The second one is the object exerting the force (the table). I've heard this called the "on-by" convention. The advantage of a double subscript convention allows easy identification of 3rd law pairs: $\vec{N}_{B,T}=-\vec N_{T,B}$. This way it's easy to "connect" your free-body diagrams for different objects.
Another convention is the "by-on":
$$\vec{N}_{T,B}.$$
It's easy to figure out which one of these two double-subscript conventions is being used: Just look at all the forces on your free-body diagram. All labels will either start or end with the same letter.
A more explicit convention is:
$$\vec{N}_{T\rightarrow B}.$$
The arrow in the subscript sort of shows a cause and effect: The table is causing a force on the box. The arrow points from the object exerting the force to the object experiencing the force, which I'd argue is a natural way for English speakers to think about a force acting on an object. Using this convention is helpful and less ambiguous, but can also become tedious.
