Question

In addition to speed, what else we know to predict the position of a moving body ?

Answer 1

To predict the instantaneous position s (t) of a body at any time instant t, we need to know

1) the time instant t
2) s(0) its initial position at t = 0:  s(0)
3) v(t) the instantaneous speed v at every instant of the object.
4) the direction of velocity at every instant of the object:  $\vec{v (t)}/|v(t)|$

(3) and (4) combined is the instantaneous velocity vector.