Coupled oscillator

Homework Statement

Two identical undamped oscillators, A and B, each of mass m and natural (angular) frequency $\omega_0$, are coupled in such a way that the coupling force exerted on A is $$\alpha m (\frac{d^2 x_A}{dt^2})$$, and the coupling force exerted on B is $$\alpha m (\frac{d^2 x_B}{dt^2})$$, where $$\alpha$$ is a coupling constant of magnitude less than 1. Describe the normal modes of the coupled system and find their frequencies.

I just need someone to explain to me what is the form of the differential equation with respect to each mass. The rest I can continue.

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Could someone explain to me what is exactly is the "coupling force" ?

Simon Bridge