The requirement that the virtual work be zero for an arbitrary variation δ r (t) = ε h (t) is equivalent to the set of requirements Q i = F ⋅ ⋅ ∂ ∂ h ∂ ∂ q i = 0 , i = 1 , … … , n . The terms Q i are called the generalized forces associated with the virtual displacement δ r.