A free body diagram uses classical physics, but the effects that we see in classical mechanics come out of deeper truths in relativistic mechanics. Part of it is the speed of light which is the same for everyone, no matter how fast you move. No matter how fast you travel, light always moves at a specific speed.

Time dilates depending on where you are or how fast you travel compared to others, observers can't agree which event preceded another, and objects in orbit around a planet travel in a straight line. A free body diagram uses the assumptions of a flat plane where parallel lines never intersect, but relativity explains the apparent force of gravity using curvature on that plane. In curved space, the sum of all angles on a triangle are not necessarily 180°. It's not just more complicated, it fundamentally throws out one of the foundational geometric postulates. However, it is the second most verified theory in all of physics.