Mrs B bought me a balloon powered boat. It has holes in the bottom and two at the back. The air through the bottom is supposed to make a sort of hovercraft effect and the two at the back will propel it along. I haven't yet had the best from it but the bottom picture showed that it can demonstrate the wave pattern associated with the Doppler Effect. The whole system seems to create circular ripples but the boat was moving from right to left. It moves away from the ripples on the right, increasing the wavelength and thus decreasing the frequency. It moves into the ripples on the left, decreasing the wavelength and increasing the frequency.
