Fig. 2. The precision and accuracy of force measurements. (A) An example of the measurements of the passive movement of the tether after being pulled and released. These data were used to measure the coefficients of stiffness and damping (see Materials and methods). (B) A free-body diagram illustrates the forces acting on the tether during an experiment. The input force generated by a swimming larva is resisted by a component of the weight of the tether and the stiffness (illustrated by the spring) and damping of the pivot (illustrated by the dashpot). (C,D) Measurements of input force (filled circles) at 1000 Hz from the deflections of a tether (not shown) calculated from the simulated changes in force (red lines). (C) The input force measured from deflection measurements using accurate values for the stiffness and damping coefficients. (D) The input force measured using a damping coefficient that is less than the actual value (by 2x10^-6 Nms rad^-1). (E,F) The time lag between simulated and measured input force for varying degrees of error in the damping (E) and stiffness (F) coefficients. (G,H) The ratio of maximum measured to maximum simulated input force for varying degrees of error in the damping (G) and stiffness (H) coefficients.