How Does a Boomerang Move Through the Air?
Boomerangs are one of Australia's most famous pieces of history. Boomerangs have been used for more than 10,000 years by Indigenous Australians for hunting, fighting and ceremonies. These days, returning boomerangs are more of a fun toy than a serious tool, and science incursions are a great way to learn more about how they work!
Despite the fact they date back to ancient history, boomerangs are actually a sophisticated tool that take advantage of advanced physics. In this article we're going to explore how boomerangs move through the air and discuss the three things that cause boomerangs to come back when thrown.
The Shape of a Boomerang's Wing
The first component of a returning boomerang is the shape of the arms. Classic Australian boomerangs have just two arms joined together at a 60 to 80 degree angle that creates a C-shape object.
Each of these arms actually has the same shape as the wing of an aeroplane. If you were to cut straight through a boomerang's arm, you'd discover that the cross section was in the shape of an "aerofoil". An aerofoil is like an elongated oval, with the bottom being relatively flat, and the top being more curved. Both sides of an aerofoil work together to create "lift", which is how planes and boomerangs are both
able to fly through the air.
As a boomerang flies its wings split the air, and the aerofoil shape generates lift in two ways:
1. The air passing over the top surface of the wing speeds up, lowering its density and allowing the wing to rise upwards from the ground
2. The air passing under the wing is deflected downwards, putting upwards pressure on the wing and causing it to rise
These two things generate lift, but they don't explain why a boomerang comes back.
The Way the Boomerang is Thrown
The second part of making a boomerang return is the way it's thrown.
You see, boomerangs take advantage of a physics convention called "gyroscopic precession", but this effect only happens when they're thrown properly. Throwing a boomerang can take a bit of practice. There are a few basic steps involved:
1. Hold the boomerang vertically in your right hand, gripping the tip of the wing that's closer to the ground
2. Make sure the outer convex edge of the boomerang is facing backwards, with the tips of both wings pointing forwards
3. Tilt your arm at the shoulder until the boomerang is at an angle of about 20 degrees from vertical
4. Swing your arm to throw the boomerang forwards
5. As you throw, flick your wrist to create a snapping motion that makes the boomerang spin
Done right, your boomerang should have two separate but simultaneous motions going on. The first is forward motion from the throw - the same as if you threw a tennis ball. The second motion is the spin. Because the boomerang is spinning and moving forwards at the same time, each arm of the boomerang is actually moving at different speeds. As the boomerang spins around, the top wing is moving faster than the bottom wing. This means the top wing generates more lift and causes the boomerang to tilt as it flies through the air, and this is the first ingredient of making a boomerang fly back to your hands.
The Effect of Gyroscopic Precession
The third and final component of boomerang flight is gyroscopic precession. Gyroscopic precession is a force that's generated by spinning objects. It's usually demonstrated using a bicycle wheel during science incursions to make the explanation simple.
Imagine we have a bike wheel that has been hung from the ceiling from just one side of its axle. If it's left to hang there, the bike wheel will fall to one side and face down at the ground. But, if the bike wheel is spun, it should be able to hold itself upright in the air even though it's only suspended from one side of the axle.
This is due to the "torque" created by the spinning bike wheel. In this case, torque is a type of force that points along the length of the axle and keeps the wheel from falling over. Combined with the weight of the wheel, the torque will also cause the spinning wheel to rotate around its vertical axis - this rotation is an example of gyroscopic precession.
It's easiest to see the effects of gyroscopic precession with a video demonstration.
Boomerangs work a lot like the spinning bicycle wheel in our example above, so they also generate gyroscopic precession.
So, as a boomerang flies, the wing-shaped arms generate lift and cause the boomerang to tilt. At the same time, the boomerang is affected by gyroscopic precession and rotates back towards you slowly. Put together, these two things cause boomerangs to fly in a circle, and with a bit of practice you should be able to get a boomerang to come right back to your hands!
.. Guest post by Street Science
