 Slinkies make great demos of transverse mechanical waves. You can generate a transverse wave by displacing one end of the slinky. A single pulse is generated by displacing your hand up and down. Notice that the up-down pulse travels from left to right. Notice that any particular coil in the slinky goes up and down, but the wave travels from left to right. That is the medium the spring oscillates perpendicular to the direction or propagation of the wave. Also note that to generate a pulse, the person puts energy into the slinky or does work to displace the slinky and in his hand. That energy is transferred progressively along the slinky to displace it and is used in displacing the other end of the slinky. So no matter has been transported down the length of the spring, but energy has been transferred down the length of the spring. Hey, you can create another type of mechanical wave with a slinky. Look at this. Here a single pulse is generated by displacing one end right and left. Notice that the left pulse causes a compression of the spring which travels from left to right. Notice that any particular coil in the slinky goes right and left. The medium, the spring, oscillates parallel to the direction or propagation of the wave. Again, to generate the pulse, the person puts energy into the slinky or does work to displace the slinky and in his hand. The energy is transferred progressively along the slinky to displace it. And at the end of the slinky, some distance from the person's hand, the energy is expended in displacing the opposite end. Again, no matter has been transported down the length of the spring, but energy has been transferred down the length of the slinky. The compressive waves of a slinky spring are longitudinal mechanical waves. Longitudinal because parts of the spring are displaced parallel or longitudinal to the direction of the waves. And mechanical because the wave is a mechanical disturbance of the spring. Let's look at this a little more closely, considering the motion of only a few points on the spring uniformly or evenly spaced on the spring at rest. When we introduce a compressive pulse, you see that some of the points are brought closer together or compressed. But their position in spacing is restored with the compression traveling down the line. The points only oscillates about the resting position in the direction of the wave. They do not travel with the wave. You can see this by following the blue point. Now let me add consecutive pulses. If I freeze the simulation, you see regions of compression where the points are closer together and regions of rarefaction where the points are farther apart. In the compressed regions, the slinky spring force pushes the points away to restore their spacing. In the rarefied regions, the slinky spring force pulls to restore uniform spacing. And if we plot the local force as a function of distance for a time frozen slinky, we can recognize characteristics of a wave. The wavelength is the distance between consecutive compressive regions or consecutive rarefied regions. The amplitude is half the distance between the maximum compressive and maximum rarefaction forces. Unfreeze the spring and we see that the time that it takes for a compressive region to travel one wavelength is the period. The frequency is one on the period and the speed of the wave is the wavelength divided by the period.