This question arose from Physics, where the force on an object attached on a spring is proportional to the displacement to the equilibrium (that is, the rest position). Also, if the displacement to the equilibrium is positive, the force will be negative, as it tries to pull the object back (i.e. if you pull a string, the force is opposite to your direction of pull).
Therefore, it can be said that:
$$F \propto -x$$ Where $F$ is the force and $x$ is the displacement from equilibrium
Is this the same as: $$F \propto x$$ According to the relation of proportionality, it should be, but my friend says that putting the second one is not correct.
Both are equivalent right?