A mass $m$ is attached to a vertical massless spring or a spring constant $k$. Originally, the spring was relaxed because the mass was held by a clip. Suddenly the clip was released. THe mass dropped down and the maximal elongation of the spring was recorded as $l$. What is the resonant frequency of the system? The gravitational constant is $g = 9.8 \frac{m}{s}$.

I assume (based on instructions from the professor) the intention of the question is that there is some damping going on here (damping constant $b$). What I'm confused about is if the gravitational force here makes this a driven damped oscillator or not. The "driving" force is a constant, so it isn't changing the oscillations at all (I think?). Meaning, is my equation (for $\omega = \sqrt{\frac{k}{m}}$ and $\beta = \frac{b}{2m}$):

$$ \ddot{x} + 2\beta\dot{x} +\omega^2x = 0 $$


$$ \ddot{x} + 2\beta\dot{x} +\omega^2x = g $$

If it is driven, is my resonant frequency $\omega_r = \sqrt{\omega_0^2 - 2\beta^2}$? What else would resonant frequency mean in this case?

  • 1
    $\begingroup$ I don't think any damping is implied. This is a regular SHO and you just need to calculate the force constant $k$ so you can calculate the frequency. You get the force constant from how much the spring extends before the mass is brought to a stop. $\endgroup$ Apr 2 '14 at 6:21
  • $\begingroup$ @JohnRennie That's what it would seem, but the professor who assigned the problem assured me that he intended for there to be implied damping. I can clarify that in the question. $\endgroup$
    – PenciL
    Apr 2 '14 at 6:30
  • $\begingroup$ It's rather odd that your prof would "intend" for "implied" damping without specifying the source of the damping force. Does he typically assign such "open-ended" questions, expecting you to posit something like heat loss in spring deformation or (miniscule) air resistance? $\endgroup$ Apr 2 '14 at 11:44

Your equation is Newton's second law. That's always true for any system in Newtonian mechanics. So it's very straightforward to figure out which equation of motion applies to this system: write out $\sum F = ma$, plug in the forces, and simplify.

Now to address the other issue: does a constant force qualify as a driving force? I'd say there are two ways to think about this:

  • Intuitively, a "driving force" is meant to be some kind of force that will sustain oscillation even without the natural response of the oscillator. Imagine what this system would do without a spring. Gravity wouldn't make it oscillate; it would just fall, and that's not really what most people would consider "driving." Strictly speaking, yes it is the limit of an oscillatory driving force as the frequency goes to zero, but in this case, some of the properties that characterize a driven oscillator, as we normally think of it, don't carry over to zero driving frequency.
  • Mathematically, any simple harmonic oscillator (driven or not, damped or not) has an equilibrium position, and it's conventional to choose a coordinate $q$ such that the equilibrium position is at $q = 0$. With that in mind, consider the coordinate transformation $q = x - \frac{g}{\omega^2}$. I'll let you work out the implications of that. :-) (Fun fact: this is mathematically equivalent to the Higgs mechanism.)

The point to take away from this, either way, is that no, a constant force is not a driving force, but that's really a matter of terminology, specifically what people usually understand "driving force" to mean.

  • $\begingroup$ If it's not driven, then what is the resonant frequency? How can you have resonance on an underdamped spring? Does that mean the frequency for resonance if the system were to be driven? ($\sqrt{\omega^2 - 2\beta^2}$?) $\endgroup$
    – PenciL
    Apr 2 '14 at 19:32
  • $\begingroup$ The resonant frequency is a property of the system, it doesn't depend on the actual driving frequency provided. So yes, you can pretend there is a driving force in order to determine the resonant frequency. Although it's possible that definitions differ; Wikipedia lists a few different things that could be considered the resonant frequency, loosely speaking. $\endgroup$
    – David Z
    Apr 2 '14 at 20:22
  • $\begingroup$ Hmmm. He gave us values for m, k, l, and g and to find the resonance as in my comment above I would need b. Can I find b from m, k, l and g? $\endgroup$
    – PenciL
    Apr 2 '14 at 21:25

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