Solid State Physics: Atomic chain oscillations

[skrat]

Homework Statement

Ok, don't get angry with me. The original problem is from Solid State Physics but my problem is very well in "Introductory physics". Here is the problem:

The chain consists of molecules, which has three atoms, each with mass $M$. Spring constant between the atoms inside the molecule is $K$ and between the molecules $Q \ll K$ as shown in the picture.

Now
part a) wants me to do "something" and use approximation that the molecules are rigid.
part b) wants me to do "something else" and use approximation that the molecules are not connected with a spring.

Homework Equations

The Attempt at a Solution

Now both of these approximations in part a) and b) should tell me something about the spring constants $Q$ and $K$. But I can't figure it out.

I am assuming that part b) wants me to set $Q\rightarrow 0$ while I have absolutely no idea what part a) is supposed to say. Maybe that $K\rightarrow \infty$?

 

[tech99]

Homework Statement

Ok, don't get angry with me. The original problem is from Solid State Physics but my problem is very well in "Introductory physics". Here is the problem:

The chain consists of molecules, which has three atoms, each with mass $M$. Spring constant between the atoms inside the molecule is $K$ and between the molecules $Q \ll K$ as shown in the picture.
View attachment 84228
Now
part a) wants me to do "something" and use approximation that the molecules are rigid.
part b) wants me to do "something else" and use approximation that the molecules are not connected with a spring.

Homework Equations

The Attempt at a Solution

Now both of these approximations in part a) and b) should tell me something about the spring constants $Q$ and $K$. But I can't figure it out.

I am assuming that part b) wants me to set $Q\rightarrow 0$ while I have absolutely no idea what part a) is supposed to say. Maybe that $K\rightarrow \infty$?

It looks as if the system has two resonant frequencies, and the two are independent.

 

[haruspex]

I have absolutely no idea what part a) is supposed to say. Maybe that K→∞K\rightarrow \infty?

Sounds right.