How to find when the particle changes direction?

As time increases, if the particle's velocity changes sign from positive to negative, or negative to positive, then it must have changed (opposite) direction on its linear path. As time increases on a graph of the particle's position versus time, it changes directions when position changes from increasing to decreasing, or from decreasing to increasing.

Efficiency is an action of doing something. for example when you take a test. good efficiency means you did well but bad effeciency or no effiency means you did bad.