# [SOLVED]concept question frequency

#### dwsmith

##### Well-known member
Given $\sin\left[\frac{\pi ct}{L}\left(n + \frac{1}{2}\right)\right]$.

The period is $\tau = \frac{2L}{c\left(n + \frac{1}{2}\right)}$ so the frequency is $\frac{1}{\tau}$, correct?

Last edited:

#### MarkFL

Staff member
Yes:

$\displaystyle f=\frac{1}{\tau}$

#### dwsmith

##### Well-known member
Yes:

$\displaystyle f=\frac{1}{\tau}$
Is the period I obtained correct? Is that the yes or is the yes it is the reciprocal?

#### Ackbach

##### Indicium Physicus
Staff member
If you have a function $\sin(kt)$, find the period $\tau$ by setting $k\tau=2\pi$.

#### MarkFL

Staff member
Sorry for being vague.

I agree with the period you found and with the relationship between frequency and period that you stated.

$\displaystyle \tau=\frac{2\pi}{\omega}$

In your case $\displaystyle \omega=\frac{\pi e}{L}\left(n+\frac{1}{2} \right)$

And so:

$\displaystyle \tau=\frac{2\pi}{ \frac{\pi e}{L} \left(n+ \frac{1}{2} \right)}= \frac{2L}{e\left(n+\frac{1}{2} \right)}$

#### dwsmith

##### Well-known member
If you have a function $\sin(kt)$, find the period $\tau$ by setting $k\tau=2\pi$.

Is there a difference between frequency and natural frequency (eigenfrequencies)?

#### Ackbach

##### Indicium Physicus
Staff member
Is there a difference between frequency and natural frequency (eigenfrequencies)?
The term "natural frequency" refers to resonance. So if you have a forced mass-spring system, e.g., and you tune the forcing function to the same frequency as a term in the homogeneous solution, you end up with unstable behavior.

The term "frequency" just refers to what's going on in this thread.

There's also the term "angular frequency", which is represented by $\omega=2\pi f$.

Hope that's as clear as mud.

#### dwsmith

##### Well-known member
The term "natural frequency" refers to resonance. So if you have a forced mass-spring system, e.g., and you tune the forcing function to the same frequency as a term in the homogeneous solution, you end up with unstable behavior.

The term "frequency" just refers to what's going on in this thread.

There's also the term "angular frequency", which is represented by $\omega=2\pi f$.

Hope that's as clear as mud.
I am trying to find the natural frequency (eigenfrequency) of $u$.
How would I do that then?
$$u(x,t) = \sum_{n = 1}^{\infty}\sin\left[\frac{\pi x}{L}\left(n + \frac{1}{2}\right)\right]\left\{A_n\cos\left[\frac{\pi ct}{L}\left(n + \frac{1}{2}\right)\right] + B_n\sin\left[\frac{\pi ct}{L}\left(n + \frac{1}{2}\right)\right]\right\}$$

#### Ackbach

##### Indicium Physicus
Staff member
I am trying to find the natural frequency (eigenfrequency) of $u$.
How would I do that then?
$$u(x,t) = \sum_{n = 1}^{\infty}\sin\left[\frac{\pi x}{L}\left(n + \frac{1}{2}\right)\right]\left\{A_n\cos\left[\frac{\pi ct}{L}\left(n + \frac{1}{2}\right)\right] + B_n\sin\left[\frac{\pi ct}{L}\left(n + \frac{1}{2}\right)\right]\right\}$$
Well, I could be wrong, but I would say that all of your
$$f_{n}=\frac{c\left(n + \frac{1}{2}\right)}{2L}$$
are the eigenfrequencies. I don't think there's one single eigenfrequency.