If the generalized forces Q j are derivable from a potential energy V (q 1,..., q m), then these equations of motion take the form d d t ∂ ∂ T ∂ ∂ q ˙ ˙ j − − ∂ ∂ T ∂ ∂ q j = − − ∂ ∂ V ∂ ∂ q j , j = 1 , … … , m .