The other type of wave is created by a force that moves in the same direction as the wave that it creates. These are longitudinal waves. In longitudinal waves, energy moves in a motion of expansion and contraction. If the medium is moving left and right, the wave is moving parallel with it, in the direction of the original force (either to the left or right). Imagine a slinky. If you lay it on the ground in a straight line and push on one end, what happens? One part of the slinky will compress - the coils will momentarily get very close to each other. Then the same place will expand - the coils will momentarily get far apart from each other. This is a longitudinal wave. We will learn more about longitudinal waves in the next lesson when we learn how sound moves.
