The line with marked points shows the flight path of the hammer head during a downstroke. The arrows show selected velocity vectors.
The selction is based on the necessity to choose points for calculting delta-x and delta-y such that the +\-0.6 cm error of measurement doesn't propagate through the velocity calculation and overwhelm the value for V. Even though we have data points at 1 millisecond intervals, for calculating velocity and acceleration we have to choose points about 11 ms. apart.