When most of us think of boomerangs, we imagine somebody (quite possibly a cartoon character) throwing a banana-shaped stick that eventually turns around and comes right back to the thrower's hand (possibly after hitting another cartoon character in the head). This idea is simply amazing, and as children, our first reaction to such a device was: This stick is obviously possessed with magical powers! Of course, the person or people who discovered the boomerang hadn't actually found a magical stick, but they had come upon an amazing application of some complex laws of physics.
Now, we'll break down the physical principles that make boomerangs work, see what happens as a boomerang flies through the air and find out the proper way to throw a boomerang so that it comes back to you. We'll also delve a little into the history of boomerangs to see how they came about in the first place. Boomeranging is an amazing demonstration of scientific principles as well as a terrific sport you can enjoy all by yourself.
As you can see in the diagram, the two wings are arranged so that the leading edges are facing in the same direction, like the blades of a propeller. At its heart, a boomerang is just a propeller that isn't attached to anything. Propellers, like the ones on the front of an airplane or the top of a helicopter, create a forward force by spinning the blades, which are just little wings, through the air. This force acts on the axis, the central point, of the propeller. To move a vehicle like a plane or helicopter, you just attach it to this axis.
The leading edges of the two wings face in the same direction, like the blades of a propeller.
The classic boomerang's propeller axis is only imaginary, so it obviously isn't attached to anything, but the propeller itself is moved by the forward force of the wings' lift. It would be reasonable to assume, then, that a boomerang would simply fly off in one direction as it spun, just as a plane with spinning propeller will move in one direction. If you held it horizontally when you threw it, as you do with a Frisbee, you would assume that the forward motion would be up because that's the direction the axis is pointing -- the boomerang would fly up into the sky like a helicopter taking off, until it stopped spinning and gravity pulled it down again. If you held it vertically when you threw it, which is the proper way to throw a boomerang, it seems that it would simply fly off to the right or left. But obviously this isn't what happens.
In the next section, we'll see why a boomerang turns and comes back to you.
If you've read How Gyroscopes Work, then you may have already guessed what's going on here. When you push on one point of a spinning object, such as a wheel, airplane propeller or boomerang, the object doesn't react in the way you might expect. When you push a spinning wheel, for example, the wheel reacts to the force as if you pushed it at a point 90 degrees off from when where you actually pushed it. To see this, roll a bicycle wheel along next to you and push on it at the top. The wheel will turn to the left or right, as if there were a force acting on the front of the wheel. This is because with a spinning object, the point you push isn't stationary, it's rotating around an axis! You applied the force to a point at the top of the wheel, but that point immediately moved around to the front of the wheel while it was still feeling the force you applied. There's a sort of delayed reaction, and the force actually has the strongest effect on the object about 90 degrees off from where it was first applied.
In this scenario, the wheel would quickly straighten out after turning slightly because as the point of force rotates around the wheel, it ends up applying force on opposite ends of the wheel, which balances out the effect of the force. But constantly pushing on the top of the wheel would keep a steady force acting on the front of the wheel. This force would be stronger than the counterbalancing forces, so the wheel would keep turning, traveling in a circle.
If you've ever steered a bicycle without using the handlebars, you've experienced this effect. You shift your weight on the bicycle so that the top of the wheel moves to the side, but every bicycle rider knows that the bike doesn't tip over as it would if it were standing still, but turns to the right or left instead.
This is the same thing that is happening in a boomerang. The uneven force caused by the difference in speed between the two wings applies a constant force at the top of the spinning boomerang, which is actually felt at the leading side of the spin. So, like a leaning bicycle wheel, the boomerang is constantly turning to the left or right, so that it travels in a circle and comes back to its starting point.
To keep the wind from forcing the boomerang off course, you should aim the boomerang at a point about 45 to 50 degrees to one side from the direction of the wind (stand facing the wind and rotate about 45 degrees clockwise or counter-clockwise). Adjust the position of the boomerang depending on how much wind there is, as shown in the diagram.
When you have set your grip on the boomerang and you have oriented yourself in relation to the wind, bring the boomerang back behind you and snap it forward as if you were throwing a baseball. It is very important to snap your wrist as you release the boomerang so that it has a good spin to it. Spin is the most important thing in a boomerang throw -- it's what makes the boomerang travel in a curved path.
When you throw the boomerang vertically, the uneven force on the top of the spin tilts the axis down gradually, so it should come back to you lying down horizontally, as a Frisbee would. But don't try to catch it with one hand -- the spinning blades could really hurt you. The safe way to catch a returning boomerang is to clap it between your two hands. Always be careful when playing with a boomerang, especially a heavier model. When you throw the boomerang, you must keep your eye on it at all times or it could hit you on the return. If you lose track of its path, duck and cover your head rather then trying to figure out where it is. Boomerangs move quickly, with a lot of force.
Your first attempt will probably end up on the ground, as will your second and third, so don't try to learn with an expensive hand-carved model -- pick up a cheap plastic design at the toy store. Boomeranging is a difficult skill, but it can be a lot of fun to practice. It's certainly a satisfying accomplishment when the boomerang actually comes right back to you and you catch it perfectly!
A classic boomerang design, hand-crafted by Australian Aborigines
In nature, there are plenty of sticks that are bent in a curve like a boomerang, and people probably threw these sorts of sticks all the time. Because of the stabilizing motion of the two branches of the stick, this sort of stick would have stayed aloft longer and would have been easier to send in the desired direction. Primitive humans noticed this, and so they started specifically seeking out bent sticks when they wanted to throw a club at their target. Then they started selecting the best curved sticks (thinner, longer ones work better) and were soon customizing sticks so they were especially suited for taking down prey. Non-returning boomerangs have been found all over the world. The oldest known non-returning boomerang, an artifact found in Poland, dates from about 20,000 years ago.
The experts aren't really sure when and where people first developed returning boomerangs, but the Aborigines of Australia are generally credited with the invention. Aborigines used non-returning boomerangs, which they call kylies, extensively in hunting, and the theory is that at some point, one or more Aborigines used a kylie with the particular shape of a boomerang and noticed that it traveled in an arc. This might have been pure accident or it might have been the result of design experimentation. One theory is that an Aboriginal hunter fashioned a smaller kylie with a more angled curve because he or she noticed how a bird held its wings in a pronounced V shape while soaring through the air.
The amazing flight pattern of the new discovery didn't really help out much in taking down prey -- it actually made it harder to aim accurately -- but it was, of course, really cool. Evidently, the Aborigines perfected the boomerang design and throwing technique for the simple pleasure of it, and the boomerang has mostly been used as sports equipment ever since then. The standard game is to see who can throw the boomerang the farthest and still catch it on its return. The boomerang did have some limited use in hunting, however. The Aborigines would set up nets in trees and then throw the boomerang into the air while making a hawk call. This would scare flocks of birds so they would fly down into the nets.
The boomerang is actually the first man-made flying machine, and so it is the direct predecessor of the airplane, helicopter, blimp -- even the space shuttle! It's amazing that a hunk of wood can make such effective use of complex principles of physics -- so amazing that it really seems like magic until you understand what's happening. The boomerang is a great learning tool for anyone interested in physics, and it is certainly one of the most remarkable toys in history!