In an experiment, Mary compares a collision of two 1-kilogram steel balls with a collision of two 1-kilogram foam rubber balls. She makes sure that, except for the material, the two collisions are the same. Mary finds that the force of impact of the steel ball collision is greater than the force of impact of the foam rubber ball collision. From her force measurements, Mary concludes that the foam rubber balls’ acceleration during the collision was the steel balls’ acceleration. She concludes that the collision between the foam rubber balls is an collision.