4.1.3 Lack of System Equilibrium

As illustrated in (4.1.2 System Response Time Versus Timing of Load Change), system response loads (reactions, internal forces and moments, etc.) are

not necessarily equal and opposite to the applied dynamic loads, so the net sum of the forces and moments acting on the system are something other than zero. This means that the system does not meet the laws of static equilibrium, and cannot be solved using traditional static solution methods.

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Introduction to Dynamic Theory