Sea-floor age-depth and age-trench-depth relations suggest that the oceanic lithosphere continues to thicken and subside with age beyond 80 Ma. If so, slab-pull (FS) and ridge push (FR) forces can be calculated from the age of the sea floor at trenches (t), and if the shear stress acting on the base of the plate increases with absolute velocity (&ngr;a), the approximate force balance equation is Dv2a=Svat3/2+Rt where D, S and R are constants related to drag, slab pull and ridge push, respectively. The relation between velocity and age is v2a=S'vat1/2+R' We have tested this model by linear regression using rate and age data for 15 trenches va(mm/a)2=(9.20¿.32)vat1/2+(26¿250) The strength of this correlation (r2=0.98) is strong evidence in favor of the validity of this simple model. These results suggest that ridge push is just sufficient to overcome drag at the base of the plate, and does not contribute significantly to the motions of oceanic plates, though the value of R' is consistent with the motions of plates not attached to subducted slabs.