How do we know the Earth spins around its axis? How can we be sure that it is not the sun moving around the Earth, causing sunrises and sunsets, day and night? Like acceleration, rotation is absolute and can be measured without looking at the surroundings, since a motion in a rotating system needs a sideways force to follow a straight line – the Coriolis effect. Many modern phones have built-in rotation sensors with two small wings that are made to vibrate. Rotation then requires a force or torque which creates an electronic signal for these ‘butterfly gyros’. A carousel gives possibilities to explore physics in rotating systems and to gain first-hand experience of methods to measure rotation, without the need for an external reference, including the classical Foucault pendulum experiment. Amusement rides often involve rotation around more than one axis, and smartphones provide ways to measure rotation and discern new aspects of motion.

Keywords: rotation, butterfly gyro, sensors, smartphone, pitch-yaw-roll, rotation axes, Coriolis effect