vertical force for an incident angle of 20° was around 200N. The larger the angle of
incidence, the larger the force, growing to around 1000N at the incident angle of 75°. In Fig. 3 the
horizontal axis shows time; the ball hit the racket at the time labeled “impact”, and it separated from the
racket at the time indicated by “release”. The tangential (fictional) force on the racket face, shown in the
figure by the solid line, was exerted on the string plane for a duration of about 5[ms] as the ball came in
contact with the strings. At incidence angles of 20°, this tangential force was in phase with the normal
force. This can be interpreted as the ball rolling along the string bed while it also experiences some slip.
On the other hand, at incident angles of 45° or more, the exertion time of tangential forces was decreased
about 3 ms. This shows that 3 ms after impact, the ball velocity parallel to the contact surface and the
velocity at which the ball rolls have become the same. At incident angles of 30°, two phenomena of
slipping and rolling are mixed. Just after impact the ball slips on the string-bed and around 4ms later, it
starts rolling.
For a ball coming into contact with the string surface, we integrate the forces in the vertical and
tangential directions over time to calculate the impulse components S1f, S2f that we showed as equation (4)