WE=WI would imply 8= 219, which contradicts equilibrium:8 rium. How can you explain this? teached the system is not in equilib- before the final displacement d is Strain energy and strain energy density Figure 3: Strain energy density Strain energy and strain energy density(see also unit on first law of thermodynamics U=UdV 10) From first law at˙ � � • WE = WI would imply δ = 2mg k , which contradicts equilibrium: δ = mg k , • before the final displacement δ is reached the system is not in equilib￾rium. How can you explain this? Strain energy and strain energy density Figure 3: Strain energy density Strain energy and strain energy density (see also unit on first law of thermodynamics): U = UdV (10) V From first law: ∂U� = σij �ij ∂t 4