Boomerang:

Flying right back to you

When most of us think of boomerangs, we imagine somebody throwing a banana-shaped stick that eventually turns around and comes right back to the thrower's hand.

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.

Boomeranging is an amazing demonstration of scientific principles as well as a terrific sport you can enjoy all by yourself. When we talk about boomerangs, we usually mean the curved devices that return to you when you throw them, but there are actually two different kinds of boomerangs. The kind we're all familiar with, returning boomerangs, are especially crafted, lightweight pieces of wood, plastic or other material.

Traditionally, these are basically two wings connected together in one banana-shaped unit, but you can find a number of different boomerang designs available these days, some with three or more wings. Most returning boomerangs measure one to two feet across, but there are larger and smaller varieties. When thrown correctly, a returning boomerang flies through the air in a circular path and arrives back at its starting point.

Returning boomerangs are not suited for hunting - they are very hard to aim, and actually hitting a target would stop them from returning to the thrower, pretty much defeating the purpose of the design.

Returning boomerangs evolved out of non-returning boomerangs. These are also curved pieces of wood, but they are usually heavier and longer, typically three feet or more across.

Non-returning boomerangs do not have the lightweight and special wing design that causes returning boomerangs to travel back to the thrower, but their curved shape does cause them to fly easily through the air.

Non-returning boomerangs are effective hunting weapons because they are easy to aim and they travel a good distance at a high rate of speed. There is also such a thing as a battle boomerang, which is basically a non-returning boomerang used in hand-to-hand combat.

How a boomerang works

How does a boomerang fly? If you throw a straight piece of wood that's about the same size as a boomerang, it will simply keep going in one direction, turning end over end, until gravity pulls it to the ground. So the question is, why does changing the shape of that piece of wood make it stay in the air longer and travel back to you?

The first thing that makes a boomerang different from a regular piece of wood is that it has at least two component parts, whereas a straight piece of wood is only one unit.

This makes the boomerang spin about a central point, stabilising its motion as it travels through the air. Non-returning boomerangs are better throwing weapons than straight sticks because of this stabilising effect: they travel farther and you can aim them with much greater accuracy.

The returning boomerang has specialised components that make it behave a little differently than an ordinary bent stick. A classic banana-shaped boomerang is simply two wings joined together in a single unit. This is the key to its odd flight path.

The wings are set at a slight tilt and they have an airfoil design - they are rounded on one side and flat on the other, just like an airplane wing. The air particles move more quickly over the top of the wing than they do along the bottom of the wing, which creates a difference in air pressure. The wing has lift when it moves because there is greater pressure below it than above it.

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.

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 a spinning propeller will move in one direction. If you held it horizontally when you threw it, 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.

Unlike an airplane or helicopter propeller, which starts spinning while the vehicle is completely still, you throw the boomerang, so that in addition to its spinning propeller motion, it also has the motion of flying through the air.

When a wing moves through the air more quickly, more air passes under it. This translates into more lift because the wing has to exert more force to push down the increased mass. So, it's as if somebody were constantly pushing the whole spinning propeller of the boomerang at the top of the spin.

But everybody knows that when you push something from the top, say a chair, you tip the thing over and it falls to the ground. Why doesn't this happen when you push on the top of a spinning boomerang?

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 where you actually pushed it.

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.

How was it invented?

Boomerangs make perfect sense once you understand all of the physical forces at work, but it doesn't seem like something early man would suddenly come up with, out of the blue.

So, how on earth did this amazing invention come about? Anthropologists believe it was mostly a matter of trial and error.

The boomerang is actually the first man-made flying machine, and so it is the direct predecessor (ancestor) of the airplane, helicopter, blimp - even the space shuttle! It's amazing that a chunk 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!