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Supersonic Ripples

This image provides an interesting way to visualize supersonic aircraft. In the photo, I dragged a blade of grass across the surface of a lake, slowly at first, then gradually increasing in speed. Near the right of the photo, where the blade of grass is moving slowly, we see circular waves coming from the grass, similar to the kind of spherical pressure waves that come from the nose of an airplane moving at subsonic speeds. After a certain point, when the blade of grass exceeds the speed of ripples in water (~30 cm/s), the waves form into the familiar v-shaped shockwave which supersonic airplanes exhibit. If there were a small creature in the water nearby, it would experience a sudden surge from the collective force of the waves on the front of the shockwave, similar to the sonic boom heard at airshows. This experiment also provides insight into the misconception that sonic booms are only created when aircraft initially break the sound barrier, when in reality they are due to the high-pressure v-shaped shockwave that is continually produced by supersonic aircraft.