VII. EFFECTS OF BALL SPEED
It is well known that the coefficient of restitution decreases,
the impulsive force increases, and the ball contact
time decreases as the ball speed increases. The Hertz
model2,9 for colliding solid spheres indicates that F0
}(v1)1.2 and t }(v1)20.2, where F0 is the force amplitude
and t is the duration of the impact. These relations were
checked for the superball and the tennis ball, colliding with
the 50 mm piezo disk/brass rod structure, for incident ball
speeds in the range 1 – 8 ms21. For the superball, it was
found that F0}v1 n and t }v1
m , where n51.1560.05 and m
520.2260.01. For the tennis ball, it was found that n
51.1060.05 and m520.0760.01. The superball therefore
behaves in a manner that is close to Hertzian, but the tennis
ball behaved more like a simple spring where F0}v1 and t is
independent of v1 . The force law for a golf ball has been
measured by Jones,9 who found that a golf ball is close to
Hertzian over a wide range of ball speeds up to 80 ms21.