To avoid instabilities often associated with penalty forces, such as high-frequency bouncing of the objects in contact, we adjust spring constants when the spring breaks using a method similar to [Chang et al. 2002] but with some additions. We dynamically increase the spring stiffness during initial contact while using full damping to avoid a large spring impulse. As the spring breaks, we first decrease the stiffness to zero followed by the damping. We allow the spring constants to reengage if the particle’s distance decreases before thespring breaks, so that the contacts do not pass through one another.