The principle of superposition
What happens when several forces act upon an object? In this case, the action of all the forces acting on a system can be replaced with a single force that produces the same results – this force is called the net force.
The net force is equal to the vector sum of all the forces acting on the object:
\[\mathbf{F}_{\mathrm{net}} = \sum{\mathbf{F}}\]
Newton's second law extended for several forces:
\[\sum{\mathbf{F}} = m \mathbf{a}\]
A useful corollary of this observation is that any force can be replaced by the action of two forces provided their vector sum is equal to the original force.