Motkrafter

Motkrafter is a Swedish term used in physics and engineering to describe forces that oppose motion or counteract applied forces. The word combines mot (against) and kraft (force). It refers to any force that resists or counters a motion tendency, rather than to the driving or propulsive forces themselves.

In classical mechanics, motkrafter are the forces included in the net force that act opposite to the intended motion. Common examples are friction between contacting surfaces, air resistance and fluid drag, elastic restoring forces in springs, and magnetic or electric counter-forces in certain systems. In a free-body diagram, motkrafter appear as vectors opposite to the direction of motion or the applied force.

The concept is important in design and analysis of machines, vehicles, and structures. Engineers account for motkrafter when sizing actuators, designing braking systems, or calculating energy consumption and wear. In robotics and control theory, motkrafter are treated as disturbances or resistive forces that actuators must overcome to achieve a commanded motion.

In biomechanics, opposing muscle forces and external loads are described as motkrafter that must be countered to produce movement or maintain posture. While the term is common in Swedish-language literature, the underlying concept is universal in physics and engineering, often expressed as negative contributions to the net force or as opposing torques in rotational systems.