Rapidly applied or rapidly changing loads have different effects because the deformations cannot so easily keep pace with them. Rapid changes are called for, involving significant accelerations. These bring into being, temporarily, another type of resistance - the inertial resistance to such acceleration that depends both on the acceleration and on the mass that is being accelerated. They may also result in the point of balance being overshot. The whole behaviour can be very complex, but its outcome depends
chiefly on the relationship between the rapidity of application or change in the load and on the natural period of vibration of the element or structure on which the load acts, this period being dependent, in turn, on both the mass and stiffness of the element or structure.