Using the venerable vector model, we develop an expression for the change in the polar angle of the angular momentum of a rotator caused by collisions in a cell-type experiment. For an initial j precessing with polar angle theta, and a given distribution of 'tipping angles', we derive the distribution of final polar angles theta'. Final m' levels are also determined. The results agree well with exact quantum calculations for thermal collisions of He or Ar with NaK. We also identify a special case where the distribution of theta' has a simple Lorentzian form. (C) 2015 Elsevier B.V. All rights reserved.