I am not understanding what I am doing wrong here. Please see the question below:
Two harmonic oscillators are made using identical springs with identical masses and are set up side by side. You notice that they cross (i.e., have the same position) when they are moving in opposite directions and both are at half of their maximum amplitude. What is the phase difference between the two oscillators?
Note: $\phi \in [\pi,2\pi].$
Now, I defined the phase difference to simply be the difference in phase offset between the two oscillators.
I made a picture where they were both at half of their maximum amplitude, and I was quite convinced that if the first oscillator (call it oscillator A) is moving down (say) from the top of the motion, it would have a phase offset of $\phi_1 = \frac{\pi}{4}$. Similarly, the second oscillator (call it oscillator B) moving upwards towards the top of the motion would have a phase offset of $\phi_2 = \frac{7\pi}{4}$ and so the phase difference between the two would simply be $\phi_2 - \phi_1 = \frac{3\pi}{2}$, which is wrong.
Any hints for this? I really don't understand what I'm doing wrong.