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In some texts (e.g. Taylor's Classical Mechanics), the generalized force is defined to be (I'll simplify to one particle in one dimension for ease of notation): $Q \equiv \frac{\partial{L}}{\partial{q}}$.

While other texts (e.g. Goldstein's Classical Mechanics) define the generalized force to be: $Q \equiv F\frac{\partial{x}}{\partial{q}}$.

These two definitions are equivalent if the kinetic energy is independent of generalized coordinates $q$, but this is not the case in general. Adopting the former definition makes sense because then Lagrange's equation can be written as: $\dot{p} = Q$, which maintains the form of Newton's Second Law, where $p$ is the generalized momentum. On the other hand the latter definition makes sense because then it is the case that: $dW = Q dq$, which maintains the form of the definition of work in the generalized coordinates.

Are one of these two definitions ``correct''? Nowhere can I find this distinction discussed in my available texts; if one adopts the latter definition (generalized force = $F\frac{\partial{x}}{\partial{q}}$), then what is $\frac{\partial{L}}{\partial{q}}$ called?

Qmechanic
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user1247
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1 Answers1

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TL;DR: The settings/frameworks and the definitions of generalized force in Refs. 1 and 2 are different.

  1. Ref. 1 defines the generalized force as $$Q_j~:=~\frac{\partial L}{\partial q^j}\tag{7.15}$$ such that Euler-Lagrange (EL) equation looks like Newton's 2nd law: $\frac{dp_j}{dt}=Q_j$. This mostly$^1$ seems useful in the context of a stationary action principle (SAP), but possibly beyond point mechanics.

  2. Ref. 2 defines the generalized force as $$Q_j~:=~\sum_{i=1}^N{\bf F}_i\cdot \frac{\partial {\bf r}_i}{\partial q^j}.\tag{1.49}$$ We can then derive Lagrange equations from d'Alembert's principle. This makes sense in the context of point mechanics even without a SAP and even with non-conservative and semi-holonomic constraints.

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$^1$ In applications beyond a SAP, definition (1.49) appears naturally, so that it seems confusing to also introduce definition (7.15). For the record we note that Ref. 1 assumes a SAP.

References:

  1. J.R. Taylor, Classical Mechanics, 2005; eq. (7.15).

  2. H. Goldstein, Classical Mechanics; eq. (1.49).

Qmechanic
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