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I am a grade 12 physics student. My classmates and I are completing a phyiscs assignment based on the following problem.
"Fill a glass with water. place a teaspoon of salt into the water and stir it with a teaspoon. Explain the change in the sound produced by the clicking of the teaspoon against the glass during the dissolving process."
We have completed a number of experiments measuring the change in the frequency during the dissolving process and have found that the frequency of the clicking sound decreases as the salt dissolves. Why is this so?
- Nadia (age 17)
Well, as you dissolve the salt, the water gets more dense. The main
springiness in the oscillation of a tapped glass (that is, where most
of the potential energy is) is in the glass. The glass bends and
changes shape, and the water has to follow. The spring constant
(related to how much force the glass resists with when deformed)
remains the same for the glass but the mass of the water which must
move increases as the salt is dissolved. The standard calculation of
the frequency of an oscillator composed of a spring with a mass
attached gives an angular frequency of sqrt(k/m) where k is a spring
constant and m is the mass that moves. (this is, by the way, how to
measure an astronaut's mass in space when a scale woudn't work).
(republished on 07/29/06)
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