1. The problem statement, all variables and given/known data Show that if the vector function r(t) is continuously differentiable for t on an interval I and |r(t)| = c, a constant for all t [itex]\in I[/itex], then r'(t) is orthorgonal to r(t) for all t [itex]\in I[/itex] What would the curve described by r(t) look like? 3. The attempt at a solution The solution is given as: |r(t)|2 = r(t) . r(t) r(t).r(t) = c2 ( I can see the algebraic reasoning for this but what's the conceptual reasoning behind?) then, r'(t).r(t) + r(t).r'(t) = 0 that is, 2r'(t) .r(t) = 0. Hence r'(t) is orthorgonal to r(t) for all t. What is the question I should be asking myself?