Varying radial forces per mass unit from spiral arms (black curve, low frequency variation), GMCs (black curve, overlaid high frequency variations) and the Galactic bar (red/grey curve) as a function of time for 1/10 of the full simulation time range for a particular test particle. The mean distance of this particle from the Galactic centre is 6.8 kpc. Acceleration due to the overall-Galactic potential was about 2 × 10^-11 pc yr^-2. When calculating the comparatively little varying acceleration due to the bar, the mean acceleration from a point mass of corresponding mass in the centre of the Galaxy has been subtracted.