Free-Body Diagram: Force Vector Explained

[LM06]

For the following scenario: http://i14.tinypic.com/6jnvi3p.gif
why is the Force vector coming from the left only represented only in two of the three separate cases you can take. That is, te Force vector can only be represented in the case where you are considering a the system as a whole and the forces acting on the first object m1. However, if you do this for object m2 you obtain an incorrect normal force.

 

[Doc Al]

The applied force only acts on m1. Of course, it affects the normal force that m1 exerts on m2.

 

[Moetasim]

The Force vector shown in the diagram is a representation of the external forces acting on the system as a whole, rather than on individual objects within the system. This is why it is only shown in two of the three cases, as the third case (m2) is not considering the system as a whole, but rather the forces acting on that specific object.

Additionally, the normal force shown in the second case (m2) is not necessarily incorrect. It is simply a different representation of the forces acting on that object, taking into account the fact that the object is in contact with a surface. It is important to note that the normal force is a reaction force, meaning it is equal and opposite to the force being exerted on the object by the surface. Therefore, while it may appear different from the external Force vector in the first case, it is still a valid representation of the forces acting on the object.

In summary, the Free-Body Diagram and Force vector are tools used to analyze the external forces acting on a system as a whole, and may not necessarily represent the forces acting on individual objects within the system. It is important to carefully consider the context and purpose of the diagram in order to accurately interpret the forces being represented.