The ends of a spring of stiffness k are moved in the longitudinal direction according to the harmonic law: x₁ = A₁ cos(ωt+φ₁), x₂ = A₂ cos(ωt+φ₂); the average force of the spring tension over the period is zero. How does this force change with time? Determine the maximum and average energy of the spring over a long period. At which phase difference φ₂−φ₁ is the average energy of the spring the greatest?