drag force a retarding force, acting opposite to the direction of motion of a body or object. Often caused by air resistance or friction. form drag the resistance force caused by the shape of a body or object which is moving through a fluid medium. surface drag the force, opposing the direction of motion, that is due to the interaction between the surface of an object and the medium through which it is passing (or that is moving past it). propulsive drag force the force used by sportspeople to propel themselves, usually by pushing against a fluid medium (e.g. swimmers or rowers).