hävarm

Hävarm, in mechanics, is the lever arm or moment arm. It is the perpendicular distance from the axis of rotation to the line of action of a force. The torque (or moment) produced by a force F is given by τ = F × r_perp, where r_perp is the moment arm. Equivalently, if r is the distance from the axis to the point of force application and θ is the angle between the force and the lever, then τ = F × r × sin(θ). If the force is perpendicular to the lever (θ = 90°), r_perp equals r and the torque is simply F × r. If the force runs along the lever, θ = 0 and the torque is zero.

The concept is central to the functioning of levers and rotating bodies. The moment arm can change with geometry, such as joint angle in a limb, which alters the torque produced by a given force. In biomechanics, muscles generate joint torque through their attachment points; the length of the moment arm affects how effectively a muscle can rotate a joint. A longer moment arm increases torque for a given muscle force but can influence range of motion and speed of movement.

In practical terms, hävarm is used to describe the effectiveness of forces in systems ranging from simple machines to biological joints. Common examples include the lever in a door (hinge as axis and door handle as force application point) or a wheelbarrow, where the grip location determines the lever arm and resulting torque. The Swedish term hävarm is synonymous with the English term moment arm.