From analyzing the video of the ball trajectories taken by using the high-speed camera, the impact angle Tin is
9.56°(SD=2.26°) and the reflection angle Tout is 13.3°(SD=3.96°). Fig. 4 displays the pressure waveform of the
collision between the ball and the latex foam material. (a) displays the collision impact waveform for Tango 12 and
the soft material (hereinafter, material S), (b) for Tango 12 and the embossed material (hereinafter, material E), (c)
for Vantaggio and material S, and (d) for Vantaggio and material E. As shown in Fig. 1, the direction
perpendicular to the force plate was defined as the positive direction of the Z-axis, and the direction in which the
ball progressed horizontally with respect to the force plate was defined as the positive direction of the Y-axis, with
the Z-axis component of the collision pressure as Fz (see the left-hand side of Fig. 4) and the Y-axis component as
Fy (see the right-hand side of Fig. 4). The collision speeds were 10 (red), 15 (yellow), 20 (green), 25 (blue), and 30
(purple) m/s. Ten measurements were performed at each speed, and the average curve at each speed was calculated
for reduction of the electrical noise. As shown in Fig. 4, the average ball contact time at all speeds was 0.01
seconds, which is a value close to that in prior studies (Razaei et al. (2011)). It is known that Fz depends on the
reflection coefficient between the ball and the material, and not on their friction coefficient. However, it is
understood that the surface friction force largely contributes to Fy, and it is considered that a large Fy results in a
large surface friction coefficient. In addition, it is considered that when the impulse of Fy is large, the angular
impulse that causes the ball to rotate becomes large, and so, the angular velocity of the ball after the collision
becomes large. Fig. 5 shows the maximum values of velocity dependence for Fz (left) and Fy (right). The solid
blue line is the combination of Tango 12 and the material S, the dashed blue line is that of Tango 12 and the
material E, the solid red line is that of Vantaggio and the material S, and the dashed red line is that of Vantaggio
and the material E. When we compared the maximum values of velocity dependence for Fz and Fy, Fz had a
greater velocity dependence than Fy, and the change due to the difference in material was greater for Fy than for
Fz. In particular, the results for Vantaggio had higher values of Fy, at all speeds, for the material S than for the
material E. In addition, the results for the maximum value of Fy for Tango 12 showed it to be higher for the
material S when the ball velocity was 25 m/s or lower, and higher for the material E when the ball velocity was 30 m/s. Although we consider that these phenomena are largely related to the surface structure of the material of the
ball, we cannot specify the cause at this stage. We cannot rule out the possibility that, as the ball velocity increases,
a force greater than or equal to the maximum static friction force acts in the Y-direction, and the ball slips during
the collision and the friction force decreases. In this case, the maximum static friction force for Tango 12 may be
considered to be higher for the material E than for the material S. In the next paragraph, we set forth the crosssectional
structure of the surface of each material, measured by the laser displacement meter, and consider the
relation between Fy and velocity dependence.
In the same manner as for collision pressure, the direction perpendicular to the surface was defined as the Z-axis,
and the direction parallel in-plane was defined as the Y-axis. When measuring for material E, in order to measure
the size of the embossed hole, a cross-section traversing the hole was measured. Although the Y-axis scales for the
two graphs in Fig. 6 match, please be aware that the Z-axes differ by as much as 1 order. That is, we found that the
depth of the unevenness of the surface is ten times greater for material E than for material S. From this
measurement result, we were able to estimate that the diameter of a hole embossed in the face of material E was
approximately 1 mm, the depth was about 0.6 mm, and the holes were evenly spread with a spacing of
approximately 2 mm. If we assume the use of this kind of surface structure and consider that the holes in material
E will not contact the ball during collision, the contact area of material E is approximately 40% lesser than that of
material S. In general, when the contact area decreases, the friction force applied on a body decreases. The change
in the maximum value of Fy due to the material, as expressed in Fig. 5, is at most 10%, but it may be considered
that it is caused by the difference in contact area. But, in this study we assumed the deformation of the ball during
the impact is same value at each speed. So, the surface roughness of the latex foam material is anything more than
one factor
From analyzing the video of the ball trajectories taken by using the high-speed camera, the impact angle Tin is9.56°(SD=2.26°) and the reflection angle Tout is 13.3°(SD=3.96°). Fig. 4 displays the pressure waveform of thecollision between the ball and the latex foam material. (a) displays the collision impact waveform for Tango 12 andthe soft material (hereinafter, material S), (b) for Tango 12 and the embossed material (hereinafter, material E), (c)for Vantaggio and material S, and (d) for Vantaggio and material E. As shown in Fig. 1, the directionperpendicular to the force plate was defined as the positive direction of the Z-axis, and the direction in which theball progressed horizontally with respect to the force plate was defined as the positive direction of the Y-axis, withthe Z-axis component of the collision pressure as Fz (see the left-hand side of Fig. 4) and the Y-axis component asFy (see the right-hand side of Fig. 4). The collision speeds were 10 (red), 15 (yellow), 20 (green), 25 (blue), and 30(purple) m/s. Ten measurements were performed at each speed, and the average curve at each speed was calculatedfor reduction of the electrical noise. As shown in Fig. 4, the average ball contact time at all speeds was 0.01seconds, which is a value close to that in prior studies (Razaei et al. (2011)). It is known that Fz depends on thereflection coefficient between the ball and the material, and not on their friction coefficient. However, it isunderstood that the surface friction force largely contributes to Fy, and it is considered that a large Fy results in a
large surface friction coefficient. In addition, it is considered that when the impulse of Fy is large, the angular
impulse that causes the ball to rotate becomes large, and so, the angular velocity of the ball after the collision
becomes large. Fig. 5 shows the maximum values of velocity dependence for Fz (left) and Fy (right). The solid
blue line is the combination of Tango 12 and the material S, the dashed blue line is that of Tango 12 and the
material E, the solid red line is that of Vantaggio and the material S, and the dashed red line is that of Vantaggio
and the material E. When we compared the maximum values of velocity dependence for Fz and Fy, Fz had a
greater velocity dependence than Fy, and the change due to the difference in material was greater for Fy than for
Fz. In particular, the results for Vantaggio had higher values of Fy, at all speeds, for the material S than for the
material E. In addition, the results for the maximum value of Fy for Tango 12 showed it to be higher for the
material S when the ball velocity was 25 m/s or lower, and higher for the material E when the ball velocity was 30 m/s. Although we consider that these phenomena are largely related to the surface structure of the material of the
ball, we cannot specify the cause at this stage. We cannot rule out the possibility that, as the ball velocity increases,
a force greater than or equal to the maximum static friction force acts in the Y-direction, and the ball slips during
the collision and the friction force decreases. In this case, the maximum static friction force for Tango 12 may be
considered to be higher for the material E than for the material S. In the next paragraph, we set forth the crosssectional
structure of the surface of each material, measured by the laser displacement meter, and consider the
relation between Fy and velocity dependence.
In the same manner as for collision pressure, the direction perpendicular to the surface was defined as the Z-axis,
and the direction parallel in-plane was defined as the Y-axis. When measuring for material E, in order to measure
the size of the embossed hole, a cross-section traversing the hole was measured. Although the Y-axis scales for the
two graphs in Fig. 6 match, please be aware that the Z-axes differ by as much as 1 order. That is, we found that the
depth of the unevenness of the surface is ten times greater for material E than for material S. From this
measurement result, we were able to estimate that the diameter of a hole embossed in the face of material E was
approximately 1 mm, the depth was about 0.6 mm, and the holes were evenly spread with a spacing of
approximately 2 mm. If we assume the use of this kind of surface structure and consider that the holes in material
E will not contact the ball during collision, the contact area of material E is approximately 40% lesser than that of
material S. In general, when the contact area decreases, the friction force applied on a body decreases. The change
in the maximum value of Fy due to the material, as expressed in Fig. 5, is at most 10%, but it may be considered
that it is caused by the difference in contact area. But, in this study we assumed the deformation of the ball during
the impact is same value at each speed. So, the surface roughness of the latex foam material is anything more than
one factor
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