Timna asked: Can you explain to us how a boomerang works? What are the physical principles it is based upon?
The boomerang is a weapon and a game that was invented over ten thousand years ago. Despite its simple and ancient design, there is a great deal of interesting physics to learn from its motion.
The boomerang is associated mainly with the native Aboriginal Australians, which used it for hunting and in competitions. However similar tools can be found in other civilizations around the world; from the ancient Egyptians to the Native American tribes. All of this occurred years before Newton proposed the laws of classical physics, which today allows us to comprehend the boomerang’s motion.
|Australian Boomerang | Photograph by: Guillaume Blanchard, Wikipedia
There are a number of different kinds of boomerangs with different shapes – some return to the hand when thrown, and some do not. The Australian boomerang is composed of two wings attached in an obtuse-angled V-shape. The shape of the wings are aerodynamic, similar to those of a plane. And their wide side turns in opposite directions, so while spinning it will always be in the direction of the movement. In order for the boomerang to return to us, we need to throw it vertically and add a twisting motion when releasing it. This self-turn movement is a key factor in the motion since it stabilizes the boomerang in the air – similar to a frisbee – enabling it to return to the thrower.
In order to understand the boomerang’s motion we will first observe the wings. Due to their aerodynamic shape, while the wing moves, the air above it moves faster than the air beneath it, similar to a wing of a plane. According to Bernoulli's principle, this difference in velocities creates a pressure difference between the two sides of the wing, and creates a buoyancy force, which pushes the wing. Since the boomerang is thrown vertically, the buoyancy is pointed sideways – perpendicular to the direction of the throw. However, this in itself is not enough to cause the boomerang to turn back to the thrower. The boomerang turns around itself, meaning it has angular momentum. Changing the angular momentum of an object requires application of a torque. Just as an object moving at a constant speed will not change its velocity as long as no force is applied to it (Newton’s first law of motion), an object moving at a constant angular momentum will not change its angular momentum as long as no torque is applied to it.
Airflow around the boomerang's wings | Illustration: Wikipedia
If we take a close look at the wings we will discover that they also turn and move forward in the direction of the motion of the boomerang. At any given moment, the upper wing turns in the direction of the motion of the boomerang and the lower wing turns in the opposite direction. Therefore, the upper wing moves faster compared to the air, and the buoyancy exerted on it is larger than that exerted on the lower one. This results in a larger torque on the upper wing than on the lower wing resulting in the boomerang being subjected to a torque that turns it and causes it to move in a circular motion back to the thrower.
A similar phenomenon occurs while spinning a dreidel. First the dreidel spins around itself, but gravity applies a torque on the dreidel, causing it to move in larger circles until it falls. This motion is termed precession and it is very common in spinning objects. The precession of the dreidel turns it in wide turns, and eventually causes it to fall; the boomerang’s precession returns it to the thrower.
Boomerang – question back at you
What do you think will happen if we were to throw a boomerang horizontally? To what direction will it go? Will it return to the thrower?
Department of Physics of Complex Systems
Weizmann Institute of Science