How refreshing to read C. D. Collinson’s [1] article in the March 1986 Gazette on the importance of stressing approximations in teaching mechanics. Once students get over their initial scepticism that mechanics has anything to do with the real world, I too find an enthusiastic response from classes invited to contribute their ideas on the sort of approximations and assumptions which have to be made to model a physical situation. Approximations and assumptions are fundamental to the art of mathematical modelling in mechanics and cause more difficulties to students than is often realised; class discussions reveal that many of the problems which students have with mechanics are related to approximations, and such discussions can also bring out some unsuspected fundamental misconceptions. One such misconception, related to approximations, is discussed here and the proposed method of clarification includes developing a little more Collinson’s use of “the linear approximation”.