If two waves are able to transmit energy to each other, then these waves are said to be "coupled." This normally occurs when the waves share a common component. An example of this is two pendulums connected by a spring. If the pendulums are identical, then their equations of motion are given by m x ¨ ¨ = − − m g x l 1 − − k ( x − − y ) m y ¨ ¨ = − − m g y l 2 + k ( x − − y )