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Why is the work done by the internal forces in a rigid body zero?

Most proof's for that sort of thing seem to assume that the internal forces in the rigid body are in the line attaching the points in between witch they are applied(the internal forces are colinear to the line between the points)

I cannot find why would this assumption be true since these internal forces in objects like thin rigid rods are clearly found to be perpendicular to the rod(for instance while rotating or sifting the rod some parts of it experience only internal forces) whilst they must(apparently) come from the rod's direction thus implying that the forces are perpendicular to the line in between the points.

So how come the internal forces in rigid bodies(including thin rods) add no work to the system?

Qmechanic
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Raphael
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    Action-reaction pairs at any given point inside the body always act on opposite directions (3rd Newton's law), regardless of body shape. so they do work of opposite sign which ends up totaling zero. –  Jun 25 '22 at 19:14
  • Possible duplicates: https://physics.stackexchange.com/q/23097/2451 and links therein. – Qmechanic Jun 25 '22 at 20:17
  • Just to clarify, It is clear to me that the internal forces in the body by definition are opposite, it is not clear to me that they should act upon the line connecting the bodies. the internal force could be perpendicular to that line while still being opposite as in friction for instance or as apparently shown in my example in the question. – Raphael Jun 26 '22 at 01:10

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