Force is an agent capable of producing acceleration and/or deformation of a body.

The Different Types of Forces

The free body diagram, FBD , is a diagram representing the object of interest and the forces acting on it. To describe the mechanical behavior of an object is necessary to know and understand the forces acting on it. The figure illustrates the FBD of a brick on the ground, and being pushed forwards.

Typical forces that appear in mechanical problems can be understood as follows:

Net force \((\vec{F}_{net})\)
When several forces act simultaneously on a particle, they can be replaced by a single force, which alone will have the same effect of all others together. In mathematical form, it is written: \begin{align} \vec{F}_{net} &= \sum_i^n \vec{F}_i = \\ &= \vec{F}_1+ \vec{F}_2+...+\vec{F}_j+...+\vec{F}_n \end{align}
Weight Force or Gravitacional Force \((\vec{W})\)
It is the force of the Earth that attracts bodies to its center. Near the surface of the Earth, the module of this force is: $$W = mg,$$ where \(m\) is the mass of the body and \(g\) is the constant known as the Earth's gravity.
Elastic Force \((\vec{F}_e)\)
It is the force which arises due to elastic deformation of the bodies.
(See section Elastic Forces )
Normal force \((\vec{N})\)
It is the force that acts between two contact surfaces. Its direction is always orthogonal, to the contact surface.
(See section Contact Force )
Friction Force \((\vec{F}_{fric})\)
It is the force that acts between two contact surfaces. Its direction is always parallel to the surfaces. It opposes the displacement of the surfaces, one against another.
(See section Contact Force )
Tractive Force \((\vec{T})\)
It is the force that is exerted by ropes or rods.
External Force \((\vec{F}_{ext})\)
The forces of external agents that are not part of the system of interest. In general, the forces that do not fit the above definitions are called external forces, and have different sources: motors, people, animals, etc.
Internal Forces \((\vec{F}_{int})\)
They are forces of internal components that are part of the system of interest. Since they form an action-reaction pair, they do not contribute to the resulting force, since they cancel each other out.
The \(IS\) unit of force is Newton, \([F] = N\) .