Why does an attractive force always have a negative sign?

[Shafia Zahin]

I've seen in many equations of physics that whenever we demonstrate an attraction force we tend to use a negative sign before it,why is that? In case of coulomb force it makes sense because when the charged particles are of opposite signs a negative sign is sure to be appeared.But why do we necessarily use it for gravitational force?Well,I know that we use the negative sign for gravitational force for vector representation only.That's all right,but I want to know whenever we use a work done by an attraction force we use a negative sign,viz: the gravitational potential.It is written in books that the gravitational potential is negative because the work to bring an object from infinity to the gravitational field is done by the gravitational force itself,no external force is provided.But what's the relation between the gravitational force's doing the work and the sign's becoming negative?
Shafia.

 

[Dale]

In the usual radial coordinate system the positive r direction is outward. So a repulsive force is in the positive r direction and an attractive force is in the negative r direction .

 

[Jashi]

Think of it like this Positive(P) x Negative(N) always gives you a Negative(N), whereas when you times PxP or NxN It always equals to P. you can apply this to Charged Particles Two postives give a Repulsive force ( P) and two non-alike charges give a Negative, therefore you can say that attraction is negative.