  We use the Newtonian limit of a general scalar-tensor theory around a
background field to study astrophysical effects. The gravitational theory
modifies the standard Newtonian potential by adding a Yukawa term to it, which
is quantified by two theoretical parameters: $\lambda$, the lenghtscale of the
gravitational interaction and its strength, $\alpha$. Within this formalism we
firstly present a numerical study on the formation of bars in isolated
galaxies. We have found for positive $\alpha$ that the modified gravity
destabilizes the galactic discs and leads to rapid bar formation in isolated
galaxies. Values of $\lambda$ in the range $\approx 8$ -- 14 kpc produce
strongest bars in isolated models. Then, we extent this work by considering
tidal effects due to interacting galaxies. We send two spirals to collide and
study the bar properties of the remnant. We characterize the bar kinematical
properties in terms of our parameters ($\lambda, \alpha$).
