I was looking through some course notes and realized that I never fully understood this concept. So, looking at the diagram below we can see that there is a 20kN axial force being applied in-between sections (1) and (2).
Looking at the force from section (1)'s point of view, this would be a compressive force, as it it acting inwards towards the section. However, from section (2)'s point of view it is tensile, as it is acting away from the center of the section.
From what I can think, in this case I believe that when drawing the axial force diagram we simply go up (+'ve compressive axial force) when drawing from left to right. Meanwhile, if we were to draw this axial force diagram from right to left we would go down (-'ve tensile force).
Am I interpreting this correctly? Any help/insight would be really appreciated!
Edit: Another way I was interpreting this was, if I were to scan, starting from the leftmost side towards the right. I would initially see a compressive force onto the beam from R, then, I would see compressive forces (by using the section method at point 1). However once I am coming up to the 20kN force, I would, from my perspective, see a 20kN force being applied towards me, and from my section's perspective this would be compressive (-'ve force), so shouldn't I go down in the Force/distance chart? Since -'ve forces are compressive and +'ve ones are tensile.
Edit 2: Looking at some more Youtube videos: https://www.youtube.com/watch?v=Q7kYdDNKE8E, it looks like what happens at the point of force application isn't exactly necessary. Mostly we just calculate the force in-between everything, and just simply connect the lines.