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The work done on the dynamical system when its Cartesian coordinates
change by
is simply

(7.3) 
Here, the
are the Cartesian components of the forces acting on the
various particles making up the system. Thus,
are the
components of the force acting on the first particle,
the components of the force acting on the second particle, and so on.
Using Equation (7.2), we can also write

(7.4) 
The preceding expression can be rearranged to give

(7.5) 
where

(7.6) 
Here, the
are termed generalized forces. A generalized
force does not necessarily have the dimensions of force. However, the
product
must have the dimensions of work. Thus, if
a particular
is a Cartesian coordinate then the associated
is a force.
Conversely, if a particular
is an angle then the associated
is a torque.
Suppose that the dynamical system in question is conservative. It follows that

(7.7) 
for
, where
is the system's potential energy. Hence, according to Equation (7.6),

(7.8) 
for
.
Next: Lagrange's equation
Up: Lagrangian mechanics
Previous: Generalized coordinates
Richard Fitzpatrick
20160331