The viscoelastic property of a polymer is studied by dynamic mechanical analysis where a sinusoidal force (stress σ) is applied to a material and the resulting displacement (strain) is measured. For a perfectly elastic solid, the resulting strain and the stress will be perfectly in phase. For a purely viscous fluid, there will be a 90 degree phase lag of strain with respect to stress.[3] Viscoelastic polymers have the characteristics in between where some phase lag will occur during DMA tests.[3] When the strain is applied and the stress lags behind, the following equations hold:[3]