  Invariant conditions for conformable fractional problems of the calculus of
variations under the presence of external forces in the dynamics are studied.
Depending on the type of transformations considered, different necessary
conditions of invariance are obtained. As particular cases, we prove fractional
versions of Noether's symmetry theorem. Invariant conditions for fractional
optimal control problems, using the Hamiltonian formalism, are also
investigated. As an example of potential application in Physics, we show that
with conformable derivatives it is possible to formulate an Action Principle
for particles under frictional forces that is far simpler than the one obtained
with classical fractional derivatives.
