Physics ... I’ll never forget my first Physics class day in college where the professor walked over to a wall and pressed on it with most everything he had. While so doing he asked. “How much force am I applying here?”
Anyone care to answer?
Did the wall move?
None.
CC
ΣFx=0
ΣFy=0
ΣFz=0
ΣM=0
...assuming static equilibrium.
In about my third Physics class, the professor was a small nerdy guy. He was a fine instructor. But the first thing he did, and I will always remember this, was to take the chalk and make a dot on the blackboard. As he did this, he said “Let’s take an electron, and place it here, shall we?”
The question as asked by the physics prof is a double trick question. We are supposed to say there is a force, then he says no because there is no change in momentum, however he is indeed applying a force to the wall, even though there appears to be no movement (change of momentum). The wall deflects ever so slightly like a spring with a very large k and the prof’s fleshy hand distorts under compression. The force would be easier to measure if you placed a bathroom scale against the wall. The magnitude of this (balanced) force could also be measured with strain guages that measure the deformation of the material(s) at the point of contact. The moral is the mechanics of an idealized point mass is very different from macroscopic systems where ‘internal’ forces are very real as any engineer can tell you.