# What are internal forces and how do you calculate them?

There are many questions similar to the below, where you need to find the internal forces. I find such problems very poorly worded and designed. First, what do they mean by internal forces? Is it the force reaction to the sum of the external forces applied to the object, so that it doesn't move? And how do I calculate that? For example, for the thin rod, is it $F+60=20$ or $F+60=20+120$? Or what? You must choose a system.
In your case it could be $A$, or $B$, or $C$, or $A$ & $B$, or $B$ & $C$, or $A$ & $B$ & $C$.
A system could also include the wall.

Within a system you can identify forces which form a Newton's third law pair of forces which are equal in magnitude and opposite in direction.
Remember that each of the forces must act on a different object and that the forces must be of the same type eg both contact forces of both gravitational forces, etc.

If the two objects are labelled $1$ and $2$ then you are looking for the force on object $1$ due to object $2$ sometimes written $\vec F_{12}$ and its pair the force on object $2$ due to object $1$, $\vec F_{21}$.

An external force to a system does not have such a pairing associated with it.

So assuming $A$ & $B$ & $C$ is the system and that there are only contact forces.
$A$ can act on $B$ and $B$ can act on $C$ but $A$ cannot act on $C$ and $C$ cannot act on $A$.

What is the $60$ kN force acting on?
Does it have a Newton's third law pairing? Yes - internal force; No - external force.

What is the $20$ kN force acting on? Does it have a Newton's third law pairing? Yes - internal force; No - external force.

etc

• Can you tell me please the ΣF for each rod? In my understanding, you need to know the time each force started to apply (the sequence) and also the displacement from original positions. If you take the thin rod, Does it experience both 60 and 20 forces? or only the 60, assuming that the 20 is only acted on the medium rod? – ergon Mar 18 '16 at 18:22