In mechanics, a couple is a system of forces with a resultant moment of force but no resultant force). A couple consists of two parallel forces that are equal in magnitude, opposite in sense, and do not share a line of action. The simplest kind of couple consists of two equal and opposite forces whose lines of action do not coincide, and it is called a "simple couple"). The effect of a couple is to impart angular momentum but no linear momentum). Couples are free vectors in rigid body dynamics, meaning their effects on a body are independent of the point of application). The moment of a couple is the product of the magnitude of one of the forces and the perpendicular distance between their lines of action. Couples are important in mechanical engineering and the physical sciences, and they are used in various applications such as the forces exerted by ones hand on a screwdriver, the forces exerted by the tip of a screwdriver on the head of a screw, drag forces acting on a spinning propeller, and forces on an electric dipole).