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"Answer This" Questions

Chapter 11

12-B. Lever.

Explain how a simple bottle opener, such as the one on a Swiss army knife, acts as a lever. Where is the fulcrum in this case?



The figure below shows the situation. The fulcrum is at the top of the bottle cap, and the long arm and the short arm are on the same side of the fulcrum. Nevertheless, the long arm is still the distance of the small force (exerted by one's hand) from the fulcrum and the short arm is the distance of the large force (exerted by the opener on the cap edge) from the fulcrum.



12-B. Trumpet bell.

Explain how the bell on the end of a trumpet, trombone, or other brass instrument acts as an impedance matching device?


The trumpet bell gradually broadens the instrument opening from a small tube to a large opening. Without it, the sound meeting the end of the small tube would be so strongly reflected, because of the discontinuity between a narrow tube and the open air, that very little sound would get out. By having a bell that gradually opens to the outside air, the impedance difference between the trumpet tube and the open air occurs gradually over the whole length of the bell rather than suddenly. The gradual change in the diameter of the tube means there is less sudden impedance difference, so that less reflection occurs overall and more of the sound gets out of the trumpet into open air. A louder sound results to the hearer. Of course, reflection still does occur at the open end or else standing waves resonances would not form. The exact shape of the bell also affects the frequencies of the resonances.


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© William J. Mullin