Newton's third law of motion
when one object exerts a force on a second object, the second object exerts an equal force in the opposite direction on the first object.
the forces two objects apply to each other.
is a measure of how hard it is to stop a moving object
law of conservation of momentum
the total momentum of a group of objects stays the same unless outside forces act on the objects
action and reaction
for every action force there is a reaction that is equal in strength and opposite in direction