Measuring drag without a force transducer: a terminal velocity assay

1. Organisms live surrounded by air or water, which exert drag on an organism when moving. These forces are significant ecologically because they can affect organisms’ distribution, behaviour or dispersal. 2. Appropriate techniques for measuring or estimating these forces vary greatly depending on the magnitude of the forces and the flow pattern of the moving fluid (air or water; both gases and liquids are fluids). A simple method for estimating the drag in uniform steady flow is described. This technique is particularly well suited for forces of small magnitude (micronewtons) and slow flows (1 < Re < 100), and provides very uniform and well-characterized ambient flow relative to the object. 3. This method capitalizes on the force balance that exists when a falling object reaches terminal velocity. At terminal velocity, all of the forces sum to zero, and therefore the drag may be estimated from the other (known) forces (buoyancy and gravitational force). Orientation during falling may be controlled if necessary.