# Coupled oscillator; frequency?

philnow

## Homework Statement

Two identical undamped oscillators are coupled in such a way that the coupling force exerted on oscillator A is $$\alpha\frac{d^2x_a}{dt^2}$$ and the coupling force exerted on oscillator B is $$\alpha\frac{d^2x_b}{dt^2}$$ where $$\alpha$$ is a coupling constant with magnitude less than 1. Describe the normal modes of the coupled system and find their frequencies.

## The Attempt at a Solution

I know this isn't much of an attempt, but I've searched online and in the text... what am I supposed to do with this coupling constant?

## Answers and Replies

Staff Emeritus
Homework Helper
Start by writing the equation of motion for both oscillators.

mathman44
That's where I'm stuck...

$$m\frac{d^2x_a}{dt^2}=\alpha\frac{d^2x_a}{dt^2}$$
$$m\frac{d^2x_b}{dt^2}=\alpha\frac{d^2x_b}{dt^2}$$

?

mathman44
I'd be glad to show more work if I knew what to do with this coupling constant!

Staff Emeritus
Homework Helper
That's where I'm stuck...

$$m\frac{d^2x_a}{dt^2}=\alpha\frac{d^2x_a}{dt^2}$$
$$m\frac{d^2x_b}{dt^2}=\alpha\frac{d^2x_b}{dt^2}$$

?
Those equations say the only force on the masses is the coupling force. What about the restoring force?

mathman44
$$m\frac{d^2x_a}{dt^2}=\alpha\frac{d^2x_a}{dt^2} - k(x_a)$$
$$m\frac{d^2x_b}{dt^2}=\alpha\frac{d^2x_b}{dt^2} - k(x_b)$$