# Transverse movement of N masses

1. Feb 6, 2014

### mahler1

The problem statement, all variables and given/known data.

Consider the system of $N$ masses shown in the figure (see picture 1).

a) Using the approximation for small angles, write the equation of transverse movement for the n-th particle.

b) Propose a solution of the form:
${ψ_n}^{p}(t)=A^{p}\cos(nk^{p}+α^{p})cos(w^{p}t+\phi^{p})$ ($p$ is in parenthesis in the original statement but I couldn't write it correctly here). Find the dispersion relation and decide whether it depends on the contour conditions.

The attempt at a solution.

For part a), I am making the assumption that $x_n-x_{n-1}=d$, $l_0$ and $k$ are the natural length and spring constant for all the springs and $m$ is the mass of all the particles. Taking these things into consideration, the equation for the $n$-th mass is (see picture 2)

(1)$\hat{y})$ $m\ddot{y_n}=-k(a_n-l_0)\sinθ_n+k(a_{n+1}-l_0)\sin_{n+1}$, with

$a_n=\sqrt{(y_n-y_{n-1})^2-d^2}$, $a_{n+1}=\sqrt{(y_{n+1}-y_n)^2+d^2}$. Now, for small oscillations, I can approximate $a_n$ and $a_{n+1}$ by $d$, i.e., $a_n≈d≈a_{n+1}$

so (1) reduces to

(2) $\hat{y})$ $\ddot{y_n}=-\dfrac{k}{m}(d-l_0)\dfrac{y_n-y_{n-1}}{d}+\dfrac{k}{m}(d-l_0)\dfrac{y_{n+1}-y_n}{d}$

if I call ${w_0}^2=\dfrac{k}{m}\dfrac{d-l_0}{d}$, then the equation (2) equals to

(3) $\ddot{y_n}={w_0}^2(y_{n+1}-2y_n+y_{n-1})$

I would like to know if I've done correctly part a). With regard to part b), I don't understand the notation of the proposed solution, to be more specific, I don't understand what $p$ is, is it an index? Could someone explain me what $\ddot{y_n}$ would be just to see what $p$ stands by?

Sorry if this isn't the adequate section for my thread, I didn't know whether to put it here or in another section.

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Last edited: Feb 6, 2014
2. Feb 6, 2014

### TSny

Hello mahler1. Your equation of motion looks good to me.

The problem wants you to find solutions of your equation of motion in the particular form given in the statement of the problem. These types of solutions are called "normal modes". In a normal mode each particle oscillates with the same frequency $\omega^{(p)}$ but the individual particles will have amplitudes that differ from one particle to the next. Each normal mode is like a "standing wave". In solving the problem, you will see that more than one normal mode is possible. The index $(p)$ denotes a particular normal mode: $p = 1,2,...$. You will discover how many normal modes there are and what their frequencies are.

$\psi_n^{(p)}(t)$ is just another notation for your $y_n(t)$. That is, it represents the displacement of the nth particle as a function of time for the $p^{th}$ normal mode.

So, see if you can make the given form satisfy your equation of motion. You will need to think about the boundary conditions.

Last edited: Feb 7, 2014
3. Feb 8, 2014

### mahler1

Very clear answer, thanks. I've supposed that the two extremes are fixed, i.e., $y_0(t)=0=y_{N+1}(t)$.

$y_{N+1}(t)=0$ if and only if $0=A^{(p)}\cos((N+1)k^{(p)}+α^{(p)})\cos(\phi^{(p)})$. From here, I could set $\phi^{(p)}=\dfrac{\pi}{2}$.

Now, using that $y_0(t)=0$ and that $\phi^{(p)}=\dfrac{\pi}{2}$, I get that $0=0=A^{(p)}\cos(α^{(p)})\cos(\dfrac{\pi}{2})$, so I would also conclude that $α^{(p)}=\dfrac{\pi}{2}$.

As you've said, I am going to have $N$ different frequencies corresponding to each normal mode. I am having trouble to find the form of each $\omega^{(p)}$, I think I must use equation (3) but I don't know how to. Could you help me with that?

4. Feb 8, 2014

### TSny

OK. Good. Note that these boundary conditions must be satisfied at all times t, not just t = 0. If you think about it, the phase constant $\phi^{(p)}$ determines the phase of the normal mode vibration at time $t = 0$. But there is no restriction on this initial phase. So, you will not be able to determine a value of $\phi^{(p)}$.

I suggest starting with the boundary condition at n = 0. Make sure this condition holds at all times t.

Then move on to the boundary condition at n = N+1.