3. Results
3.1. Waveform of vertical and horizontal forces obtained from strain gauges
The left side of Fig. 3 shows the force waveform as a function of time that is detected by the strain gauges
attached to strings when the ball impacts racket B. The four figures from top to bottom corresponds to angle of
incidence of the ball at 60°, 45°, 30° and 15° respectively. Solid lines indicate the vertical force, and the dashed line
the horizontal force.