Question:
How do these “fortune telling” sticks work?
Answer:
The sticks are two inverted pendulums, with different natural (and resonance) frequencies. The NO stick has a much lower natural frequency than the YES sticks because of its larger mass and inertia.
When the base is moved to-and-fro subtlely but periodically, both sticks experience a periodic driving force.
When the base is shaken slowly at the natural frequency of the NO stick, energy is transfered efficiently to the NO stick. The NO stick goes into resonance and attain a large amplitude of oscillation. The YES stick, being too agile, easily moves in phase with the base and attains the same amplitude of oscilation as the base.
When the base is shaken quickly at the natural frequency of the YES stick, energy is transfered efficiently to the YES stick. The YES stick goes into resonance and attain a large amplitude of oscillation. The NO stick, being too retarded, is unable to track the rapidly changing driving force. It ends up oscillating in anti-phase with the base and basically remains motionless.
xmPhysics 