Which named principle rewrites Newton's second law using inertial forces?

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Which named principle rewrites Newton's second law using inertial forces? D'Alembert's principle.

The principle adds an inertial force, usually written as -ma, to the real forces acting on a body. Newton's second law, F = ma, can therefore be rearranged as F - ma = 0, making the moving system resemble one in static equilibrium.

Jean le Rond d'Alembert developed the idea in the 1740s, and presented it in his 1743 work Traité de dynamique. It became especially useful for systems with constraints, such as linked bodies, rods, and rigid mechanisms, because methods from statics could be applied to dynamic problems.

A common mix-up is to treat the inertial force as an ordinary physical interaction. It is a convenient, oppositely directed bookkeeping force, not a separate force-producing object. The related d'Alembert–Lagrange principle extends the idea using virtual displacements and helps lead to Lagrangian mechanics.

Source: Wikipedia · fact-checked Sept. 2026

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