The following comes from Landau's Mechanics, pages 97 - 98. For a particle in a rigid body, v = V + W x r -- (1) where for some origin O of the moving body measured in the "fixed" system of co-ordinates, v = particle's velocity in the "fixed" system, V = velocity of O in "fixed" system , W is the particle's and body's angular velocity in O, x is a cross product and r the particle's radial vector in O. For another origin, O' distance a from O, r = r' + a, and substituting in (1) gives: v = V + W x a + W x r'. The definition of V' and W' shows that v = V' + W' x r' and so V' = V + W x a, W' = W -- (2) ???But the earlier substitution r = r' +a is only correct if the axes of O an O' are parallel and not for some arbitary orientation of O', yes? The first part of (2) shows that if V and W are perpendicular for a choice of origin O, then V' and W' are also perpendicular for O'. Formula (1) shows that in this case the velocities v of all points in the body are perpendicular to W. This is easy to prove by taking the dot product of both sides with W and using the fact that V.W=0 if perpendicular. Next he says it then follows from (1) that in this case the velocities v of all points in the body are perpendicular to W. This is easy to show since v.W = V.W + (Wxr).W = 0. He says it is then possible to choose an origin O' such that V' is zero so that the motion of the body at the instant considered is pure rotation about an axis through O'. How do you show this to be true? Thanks.