The angle of incidence between the

The angle of incidence between the ball and racket is predicted to be larger than 30 degrees in actual play.
Therefore, we expect that we can estimate the friction when the ball is hit with topspin by measuring the change of
tension in the strings with the strain gauges.
The following features are common to these two figures: the maximum impulse occurs at an angle between 30
and 45 degrees if impulse S2(