This week’s question was asked by a friend.
QUESTION: What makes a boomerang come back when you throw it?
ANSWER: A typical boomerang is made of wood or plastic and varies from 15 to 30 inches in length. They weigh around three or four ounces (84 to 112 grams). Boomerangs come in many different shapes, but the banana shape is the most common.
A Dutch physicist, Felix Hess, wrote a 555-page book, Boomerangs, Aerodynamics, and Motion, to explain why and how boomerangs return. It’s a bit overdone. We’ll try to keep it simple.
The two arms of the boomerang act as wings. The underside is flat and the top is curved, just like wings on an airplane. Air going over the top of the wing must travel faster because it has a greater distance to go. Wherever there is a fast flow of air, there is low pressure. The low pressure on top and higher pressure on the bottom of the wing gives the wing its lift. It’s classical Bernoulli’s Principle.
But the wings on a boomerang are different than the wings on an airplane. On one arm, the curved top points into the wind. On the opposite arm, the curved top points away from the wind. Comparing it to an airplane, one of the wings is made backwards but mounted on the plane the normal way. Such an airplane would have a hard time flying straight. So does the boomerang.
The boomerang is held vertically when thrown, not like a frisbee that is thrown horizontally. The flat side is held away from the thrower’s body and thrown with a snap of the wrist to give it rotation.
The common right-handed boomerang will start to lean to the left. Why so? When the boomerang is flying forward, the top wing is moving faster through the air, while the bottom wing is moving slower through the air. Because of the spin, the bottom arm is moving backward compared to the top arm, which is moving forward. The lift on the top arm is greater than the lift on the bottom arm.
This combination creates pressure on the top arm. It’s similar to riding a bicycle with no hands. If you lean to the left, the spinning front wheel of the bike will begin to turn left, even though you didn’t touch the handlebars.
What is described thus far would have the boomerang just tip over, but it doesn’t. The spinning boomerang also acts like a gyroscope. When you push a spinning wheel, the wheel reacts to the force as if you pushed it at a point 90 degrees off from where you actually pushed it.
You can try this yourself. Roll a bicycle wheel or old car tire along next to you. Give it a push at the top at right angles to the direction of travel. The wheel or tire will turn left or right as if there were a force acting on the front of the wheel or tire. The wheel or tire is reacting 90 degrees to the direction of your force. Strange but true! Bit by bit the boomerang turns to the left, tracing a big circle, flattens out and hopefully lands at your feet.
The Aborigines of Australia are given credit with the invention of the boomerang. It’s believed they used non-returning clubs, called kylies, for hunting. One theory is that an Aboriginal hunter fashioned a smaller kylie with an angled curve and noticed how it arced in the sky. Perhaps an accidental discovery that turned out to be a fascinating toy.
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