What's the difference between linear momentum and angular momentum? First of all, the unit of linear momentum is kg m/s while the angular momentum is measured in kg m²rad/s instead; A force is required to change linear momentum but a torque is required to change angular momentum. If other words: If net force on a system is zero then the total linear momentum of the system remains constant; if there is no external torque, the angular momentum of a closed system is conserved. Finally, linear momentum is parallel to motion while angular momentum is normal to motion.