# The quality factor

1. Oct 11, 2013

### mbigras

1. The problem statement, all variables and given/known data
Show that the fractional change in the resonant frequency ($\Delta \omega/ \omega_{0}$) of a lightly damped simple harmonic oscillator is ≈ $(8Q^{2})^{-1}$.

2. Relevant equations
Is this a formula for the resonant frequency?
$$\omega_{m} = \omega_{0}\left(1 - \frac{1}{2Q^{2}}\right)^{1/2}$$
How would I use this formula?

3. The attempt at a solution
Right now I'm in a place where I'm not sure what this question is asking. I'm trying to move to a situation where I understand what is being asked. What does fractional change in the resonant frequency mean? What parameter is changing that causes the resonant frequency to change?

2. Oct 11, 2013

### Staff: Mentor

When Q is infinite, the resonant frequency =ω0

At any practical value of Q, the resonant frequency changes from this a little, and becomes
resonant frequency =ωm as given in that equation.

I'm speculating that they want you to use the first term of a Taylor Series expansion to approx a power of ½ by something to a power of 1.

3. Oct 11, 2013

### haruspex

That formula will not lead to the desired answer. Are you sure it isn't $\omega_{m} = \omega_{0}\left(1 - \frac{1}{(2Q)^{2}}\right)^{1/2}$? That's what seems to be implied by http://en.wikipedia.org/wiki/Harmonic_oscillator.

4. Oct 11, 2013

### mbigras

so the fractional change means: how far away from $\omega_{0}$ is $\omega_{m}$? How do I apply the taylor series expansion to this question?

5. Oct 11, 2013

### Staff: Mentor

Fractional change would be ( ωm - ω0 ) / ω0

See how much you can simplify that expression.

6. Oct 11, 2013

### mbigras

I'm getting
$$\left(1-\frac{1}{2Q^{2}}\right)^{1/2}-1$$

7. Oct 11, 2013

### haruspex

... where Q is large, right? So expand the square root expression according to binomial / Taylor and take the first few terms as an approximation. You'll have to figure out how many terms to take.

8. Oct 11, 2013

### mbigras

thank you very kindly for your help. I see what you mean about how my formula needs parenthesis.

9. Oct 11, 2013

### haruspex

... where Q is large, right? So expand the square root expression according to binomial / Taylor and take the first few terms as an approximation. You'll have to figure out how many terms to take.

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