Sunday, 30 January 2011
Does the sculpture defy the laws of physics?
Set the boat spinning first anticlockwise and then clockwise. Watch closely the direction in which the boat rocks.
When you spin the boat anticlockwise, it spins for a long time and eventually stops. When you spin the boat clockwise, however, it begins to rock longitudinally, the spinning slows rapidly and it changes its direction to rotate in an anticlockwise direction.
When an object is spun around an axis whose rotational inertia is other than the maximum or the minimum, it becomes subject to torque and begins to rock. Our stubborn boat is slightly asymmetrical and, on its base, it spins around an unstable axis. For this reason, it begins to rock as it spins both longitudinally and laterally. The rotating direction determines with of these two unstable motions will be dominant. Here, spinning the boat clockwise will cause more rocking in a longitudinal direction, while lateral rocking is greater when the boat is spun in an anticlockwise direction. Longitudinal rocking affects the spinning motion more and, in the end, when combined with friction, it will alter the spinning direction. At the same time, the rocking motion changes to a crosswise motion. The first attempts to explain the physics of the stubborn boat were conducted more than a century ago, but the behaviour of a spinning boat still remains without a definitive physical explanation.
The phenomenon is similar to that of a poorly balanced car tyre. It also begins to wobble if the direction of the spinning axle is slightly different that the natural direction of the spinning axle of an asymmetrical tyre.