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.
In this edition of HowStuffWorks,
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.
What is It? 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 specially 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 1 to 2 feet (30 to 60 cm) 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 3 feet (1 meter) or
more across. Non-returning boomerangs do not have the light
weight 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.
Why Does It 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 the 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,
stabilizing its motion as it travels through the air.
Non-returning boomerangs are better throwing weapons than
straight sticks because of this stabilizing effect: They
travel farther and you can aim them with much greater
accuracy.
The returning boomerang has specialized 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. If you've read How Airplanes
Work, then you know that this design gives a wing
lift. 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.
A boomerang is two wings combined in one
unit.
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.
Why Does It Come Back? 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.
In the diagram below, you can see that whichever wing is at
the top of the spin at any one time ends up moving in the same
direction as the forward motion of the throw, while whichever
wing is at the bottom of the spin is moving in the opposite
direction of the throw. This means that while the wing at the
top is spinning at the same speed as the wing at the
bottom, it is actually moving through the air at a higher rate
of speed.
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?
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.
How Do I Throw One? As we've seen, there are
several forces acting on a boomerang as it spins through the
air. We know that the boomerang is affected by:
The force of gravity
The force caused by the propeller motion
The force of your throw
The force caused by the uneven speed of the wings
The force of any wind in the area
So there are five variables involved in a boomerang flight.
For a boomerang to actually travel in a circle and come back
to its starting point, all of these forces have to be balanced
in just the right way. To accomplish this, you need a
well-designed boomerang and a correct throw. In cartoons, the
boomerang takes care of all the work and pretty much anyone
can get the boomerang to return on the first try. Any
boomerang enthusiast will tell you, however, that the only way
to consistently make good throws is to practice good
technique. In this section, we'll give you the basics so you
can get started on perfecting your throw.
Your first instinct when you pick up a boomerang may be
to throw it like a Frisbee. If you do this, the force of the
propeller motion will launch the boomerang up into a
vertical arc instead of into a horizontal arc right above
the ground. The correct way to hold a boomerang is at a
slight angle, say 15 to 20 degrees, from vertical. This will
aim the force of the propeller up just enough to balance the
force of gravity so that the boomerang isn't pulled to the
ground before it can make a complete circle.
Hold the boomerang as shown in the diagram above, with
the V-point, called the elbow, pointing toward you,
and with the flat side facing out. Hold the boomerang at the
end of the bottom wing, with a light pinch-like grip. This
boomerang is designed for a right-handed person -- when you
hold it correctly with your right hand, the curved edge is
on the left and the top wing's leading edge is facing away
from you. It probably won't travel back to you if you throw
it with your left hand. If you are left-handed, make sure
you get a left-handed boomerang -- one that is a mirror
image of the boomerang in this illustration. Colorado
Boomerangs sells a variety of boomerang styles, and the
company says that every model is available in a left-handed
version. If you are throwing with your left hand, hold the
boomerang so that it is tilted to the left, with the curved
side facing to the right. A right-handed boomerang will
travel in a counter-clockwise circle and a left-handed
boomerang will travel in a clockwise circle.
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!
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.
First let's consider how a primitive hunter might have come
up with a non-returning boomerang. We know that at some point
people started using the rocks and sticks they found around
them as crude tools. One very early invention was the club,
which is just a stick that you hit something or somebody with.
Hurling a club to hit somebody is just a slight extension of
this basic tool, so it's not a stretch to suppose that this
was a common use of the club.
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!
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