By definition, the speed after a time [tex]t[/tex] of the pebble is [tex]v(t)=g\cdot t[/tex] where [tex]g\approx 10m/s^2[/tex] is the G-force's acceleration.
Therefore the average value over one second is [tex]\ \textless \ v\ \textgreater \ =\int_0^1v(t)dt=\left[\frac12gt^2\right]_0^1=\frac g2[/tex]
Using [tex]g=10m/s^2[/tex] we therefore have an average speed of [tex]\boxed{\ \textless \ v\ \textgreater \ =5m/s}[/tex]
