Analytical solution
ω²_cm = k/m
X(t) = X₀ cos(ω_cm t) + (Ẋ₀/ω_cm) sin(ω_cm t)
Y(t) = Y₀ cos(ω_cm t) + (Ẏ₀/ω_cm) sin(ω_cm t)
ω²_rel = (1+2α)k/m
x(t) = x₀ cos(ω_rel t) + (ẋ₀/ω_rel) sin(ω_rel t)
y(t) = y₀ cos(ω_rel t) + (ẏ₀/ω_rel) sin(ω_rel t)
r₁ = R + r/2
r₂ = R - r/2