Adopting Young's modulus of 3.5 MPa, the theoretical Hertzian contact radius aHertz varies between 0.75 mm for W ¼ 0.25 N and
3.21 mm for W ¼ 19.34 N, so that the corresponding ratio aHertz =R
varies between 0.07 and 0.30. The magnitude of the load was thus sufficiently high to cause relatively large deformations of the ball.