> On page 3 of Landau & Lifschitz's Mechanics it is stated that > > delta q-dot = d(delta q)/dt.
> This fact is not demonstrated, it is asserted. q-dot is a vector /tangent to/ > the particle trajectory. delta-q is a displacement /of/ that trajectory. I > see no a priori reason to believe the two are interchangeable.
pho = position vector = q tau = tangent vector = q-dot tau = dpho/dt