Complex sentence: A fundamental result in analytical mechanics is D'Alembert's principle, which states that infinitesimal "virtual work" done by a resultant force F "k" acting on particle "i" in the system, across reversible displacements is zero.
Split into simpler sentences: A fundamental result in analytical mechanics is D'Alembert's principle. This principle states that infinitesimal "virtual work" done by a resultant force F "k" acting on particle "i" in the system, across "reversible displacements", is zero.