# Invariance of forces

In Newtonian mechanics, the inertial forces depend on the reference frame and the other forces are invariant. Are they still invariant when special or general relativistic effect are involved?

• Why do you think the non-inertial forces are invariant? – G. Smith May 7 at 5:33
• @G.Smith, because mechanical forces depend on relative velocities and accelerations only and those don't change under Galilean transformations? – Jan Hudec May 8 at 8:23
• Consider, for example, the Newtonian gravitational force between two particles. It doesn’t depend on their relative velocity or relative acceleration but rather on their relative position. Now consider a rotation, which is one kind of Galilean transformation. The relative position of the particles, and thus the force on each one, is not invariant under a rotation. – G. Smith May 8 at 15:48
• @G.Smith, oh, right, the direction relative to the coordinate system of the reference frame will somewhat obviously differ. However the magnitudes, and the relative angles, shouldn't, no? – Jan Hudec May 8 at 20:18
• I think that's right, but that's not what "invariance" means in physics. – G. Smith May 8 at 22:05