Triangle law:

If the two forces acting simultaneously on a body are represented by the sides of a triangle taken in order, then their resultant is represented by the closing side of the triangle in the opposite order.

To understand this , see the previous parallelogram. If we represent force F₂ by OA and F₁ by AC , the resultant will be represented by OC. Thus, you can add two forces either by parallelogram law or by tringle law.

Fundamentals of Engineering Mechanics

Law of gravitational attraction : Two particles will be attracted towards each other along their connecting line with a force whose magnitude is directly proportional to the product of the masses and inversely proportional to the distance squared between the particles.

where G is called the universal gravitational constant.

Parallelogram law: Stevinus (1548-1620) was the first to demonstrate that forces could be combined by representing them by arrows to some suitable scale, and then forming a parallelogram in which the diagonal represents the sum of the two forces. In fact, all vectors must combine in this manner.

For example if F₁ andF₂ are two forces, the resultant can be found by constructing the parallelogram.