Take a stroll in a nearby park. Chances are, there will be a family playing with the boomerang - a past hunting tool, now more commonly found as a toy which has never ceased to amaze children and adults alike. The slim boomerang, when thrown at an angle, has the ability to return to the thrower’s hands. The physics theory behind this, however, may perhaps be just as fascinating as the toy itself.
The phenomenon is coined “Gyroscopic Precession”. In concise words, torque increases the angular momentum in the direction of the torque, and causes objects to follow a surprising direction of movement. In the boomerang’s case, air flows over its beveled edges, and as the top part of the boomerang spins faster than the bottom part, more lift is generated at the top. This causes a twisting force - torque, which you might think would cause the boomerang to flip over. But it doesn’t.
This is because the boomerang also rotates, generating a significant amount of angular momentum. The angular momentum originally points largely upwards, but the torque causes it to change direction and point back towards the thrower. Thus, the boomerang follows a curved path and returns to where it first started.
Another famous example of “Gyroscopic Precession” is the bicycle wheel. When spinning, it resists gravity making it fall to the side and rotates to its left. This is because its angular momentum points outwards, whereas the torque produced due to gravity points to its left. The angular momentum follows the direction of the torque, so the wheel moves in this surprising way. The movement of the wheel can also be reimagined as orbits of satellites around planets, as introduced in the video.
Gyroscopic Precession has shown its face in various areas in daily life, from the miniscule gyroscope in our mobile phones, to the magnificent gyrocompasses on airplanes and ships. In this technological age, perhaps it can too, by extended to Artificial Intelligence - Human Activity Recognition. A thought is that gyroscopes can be incorporated into smart wristbands and ankle-bands, and after a series of machine learning, be able to detect when an accident has occurred. For the solitary elderly living in Hong Kong, these smart bands may even notify family members or younger neighbours, so that these elderly receive the help they need in time, making a positive impact to our society.
“I forgot how a boomerang works. But it came back to me.”
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