Hi, The attachment below is about strain rate in fluids*. It shows how the strain rate [tex]d\phi/dt[/tex] is related to the velocity field derivative [tex]du/dx[/tex] when you stretch the element in x (i.e. longitudinal strain). It has no intermediate steps, and I can't see how the angle has been related to the velocity field (or where the 0.5 factor comes from). Thanks very much. *From website: http://www.see.ed.ac.uk/~johnc/teaching/fluidmechanics4/2003-04/fluids5/stress.html [Broken] Sorry about the small picture - that's how it is on the website.