Instructions for the Dolphin Rattleback

The dolphins only like to swim forwards. Spin the rattleback in the direction of the dolphins and you will find they spin well and with stability. However, try to spin them the other way, and they really don’t like it. The spin quickly becomes unstable and the speed of rotation about the vertical axis decreases rapidly and the rattleback begins to see-saw. The rotation about the vertical axis ceases and sometimes the rattleback then starts to spin in the opposite direction! The physics is complicated. You can see that the mass of each half is offset from the centre line. In essence, the energy for the contra-rotation comes from the rotational energy in the vertical plane, IE the see-saw motion. Helpful tips: • Strangely enough a roughish surface, like the top of an unpolished wooden table, works best. but try it on an upside down plate. • Make sure the rattle back is horizontal. Try to spin it backwards without giving it any upwards or downwards force. • Try to accelerate it gently. • I find the best results are obtained if you place a finger on the dolphin’s nose and gently push it round in a (horizontal!) circle for about 90 degrees, increasing speed as you go. • Another thing you can do is to tap the rattleback downwards at one of the ends (where the weights are) making it rock. It always starts to rotate in the direction of the dolphins! Have fun!