1. What happens when bigger mass object (with zero net force) collides into smaller massed object in vacuum, if both are "monolithic" and indestructible? My theory is, that they just bounce off of eachother with equal force. 2. Bigger mass object collides into smaller mass object and both are destructible, in vacuum? My theory is, that higher mass object goes through the smaller mass object because of following reason: Both objects receive same amount of force, which is goes into deforming them. The force vector is mostly sideways, since the objects are getting "flattened" or breaking up and have no way to go but sideways because of inertia (the still object is trying to stay still and the impacting object is trying to move forward). Only part of the force is vectored towards forward in the object getting impacted, thus only part of the reaction force pushes the colliding object backwards. For an example the object may hit something with force of 100N and only gets part-reaction of 50N so it still has 50N forward force which keeps it going.