Newton's second law of motion states that $F = ma$, where $F$ is the net force on some object with mass $m$ and acceleration $a$. This implies that an object with net force $F$ acting on it, may have two individual forces in opposite directions on it, $F\_A$ and $F\_B$, respectively, such that their vector sum is $F$. My question is: how are the values of $F\_A$ and $F\_B$ determined? I know that Newton's second law of motion can only be used to determine the difference between them.

For example, if we apply $2 N$ on an object in a direction and $4 N$ in the opposite direction, and in another case we apply $1000 N$ one a direction and $1002 N$ in the opposite direction, the acceleration of the object is identical in both the cases.

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> How do we distinguish between the two situations? How can we assign
> one pair of values for the first case and another pair of values for
> the second? Is this not arbitrary? How are these values assigned?
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