The peak 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)
and equation (5).
3.2. Result by the image analysis
Table 1 shows the incident velocity, incident angle, reflection velocity, reflection angle, and angular
velocity of the ball, as well as the velocity and angle of the racket after the collision, that were
determined from analysis of the images obtained with the high speed camera. These results are applied to
equations (1)-(3) which yield the calculated impulses S1, S2 and S2’ from the change of the momentum.
The reduced mass used for this racket, 0.05[kg] was assumed to be one fourth of the total racket mass.
The radius of the ball r was taken to be 0.0325[m] with the assumption that it did not deform.
3.3. Comparison of Impulses
Fig. 4 shows the vertical component of impulse on the string plane obtained by the two methods using
racket B. The impulse S1 obtained from analyzing images is denoted with (Ⴠ), while that obtained from
measurements of string tension, S1f is denoted with (Ⴄ). The horizontal axis shows incident angle and the