Mutual action between objects where each object exerts an equal and opposite force on the other.
Newton's third law
Whenever one object exerts a force on another, the second object exerts an equal and opposite force on the first. Or put another way, "To every action there is always an opposed equal reaction."
law of conservation of momentum
in the absence of an external force, the momentum of a system remains unchanged. (Net momentum before a collision = net momentum after a collision.)
a collision in which colliding objects rebound without lasting damage or the release of heat