if body A exerts a force on body B, body B will exert an equal and opposite force on body A
newtons second law
force is equal to the rate of change of momentum
elastic collision
kinetic energy before colliding is equal to the kinetic energy after
inelastic collision
kinetic energy before the collision is not equal to the kinetic energy after after a collision
conservation of momentum
the vector sum of the momenta of bodies in a system stays constant even if forces act between the bodies, provided there is no external resultant force