How we can think about simple harmonic motion in a diatomic molecule stretching, or any two masses one spring system for that matter.
I've been a bit lax with my labels x1 and x2 for the sake of emphasising the concepts. It should be obvious from context but they are either a displacement or a distance (i.e. magnitude of the displacement). In your own notes, modulus signs could help to differentiate between the usages.
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