I am doing this problem:
The upward normal force exerted by the floor is 620 N on an elevator passenger who weighs 650 N.
What is the magnitude of the acceleration?
This is how I solved it:
$$\sum F = F_1+F_2=N+(-W)=ma$$
I am saying here that the sum of all forces is equal to the $Normal \ Force + Weight$ and that all of this should equal $ma$.
I then re-arrange my equation:
Now here is where I am making a huge assumption, and where I am having problems. I decided to solve for $m$ by imagining that $m$ is the mass of the person. So in a way I am saying that the whole system in this problem is really the person. Is this a logical way to solve this kind of problem or will I run into problems by thinking this way?
If it is not clear what I did here is in equation format:
In other words: What is the proper way of thinking about the mass when applying $F=ma$? At the moment I am thinking of it as the mass of the objects that affect the problem. So if it was two people in the problem I would add up their masses and use that.