Phantastic Physics: The Boom-A-Rang
A Boom-A-Rang is intended to come back to the thrower. With a little practice and some understanding, it can be made to return to you. However, when starting out, be prepared to do a lot of running after it. The Boom-A-Rang is a complex application of several physics principles all rolled into one toy. This lesson covers each of the principles in the order they are needed.
- How is it thrown?
- The airfoil shape - Where's the lift? (Bernoulli's)
- How does the spin create different forces of lift? (more Bernoulli's)
- How does spinning give it stability and make it come back? (Gyroscopic precession)
How is it thrown?
The Boom-A-Rang must be thrown with the flat side of it against the palm of your throwing hand. It is held so that it is almost vertical but tilted about 15 degrees clockwise from vertical. It should be thrown straight in front of you as though you are aiming for the ground about 20 meters in front of you. It must be released with a good snap of the wrist to give it enough spin.
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The airfoil shape - Where's the lift? (Bernoulli's)
One side of the Boom-A-Rang is flat while the other side has a very traditional airfoil curve. Just like the airplane wing, this creates lift by making the air flow faster over the curved surface than it does over the flat service. The faster moving air exerts less force than the slower moving air so a greater force is created perpendicular to the surface of the flat side of the wing. Since the Boom-A-Rang is thrown with a vertical alignment the force known as lift is actually going to be pushing on the flat surface toward the left.
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How does the spin create different forces of lift?
Once the Boom-A-Rang is thrown, it will see a self induced wind as it moves (the same way you feel wind in your hair when you run). If the Boom-A-Rang is thrown to the left as pictured below, then as it spins it will actually "see" two different amounts of wind.
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As the blades of the Boom-A-Rang spin into the wind they will actually
"see" a faster wind. When the blades are at the bottom of the spin they
will see a slower wind because they are spinning with the wind. Remember from Bernoulli's principle that the faster the overall airspeed the greater the difference between the amount of force created by the slower air and faster air. |
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As a result of the faster airflow at the top half of the spin, a greater force will be found at the top half of the Boom-A-Rang trying to tip it over than will be found at the bottom half of the Boom-A-Rang |
How does spinning give it stability and make it come back? (Gyroscopic precession)
To understand the final phase of the Boom-A-Rang explanation, you must understand the effect of spinning on an objects stability. Spinning objects develop a great deal of stability as a result of something known as angular (rotational) momentum and its conservation. Spinning objects become so stable they will resist minor attempts to change their direction or alignment. This is why a football is spiraled when thrown, bullets are fired down a rifled barrel (the barrel has spiral shaped grooves called rifling), spinning tops or gyroscopes don't fall over, and it also describes why it is harder to balance on a bicycle whose wheels are not spinning. When we throw the Boom-A-Rang with a spin, it also becomes more stable. When the larger force on the top half of the arc tries to tip the Boom-A-Rang over, it resists and responds by turning toward the left. (The same way the spinning bicycle tire turns to the person's left as it is tipped like the boom-a-rang would be.) This is known as gyroscopic precession. This continual leftward response eventually redirects the Boom-A-Rang back to its thrower.
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