where U ¼ Uη=ðE0 R0 Þ and W ¼ W =ðE0 R02 Þ. Considering that the disc is flat and rigid, the reduced radius R0 is here equal to R and the reduced modulus E0 is equal to 2E=ð1 ν2 Þ, where, assuming incompressibility, the Poisson's ratio ν is equal to 0.5. The first term in Eq. (1) describes the Couette contribution to friction, and
SRR denotes the slide-roll ratio equal to 2 for pure sliding. The second term describes the Poiseuille contribution, the sign of which is negative for the ball and positive for the disc. In our experimental setup, the total friction force acting on the ball is measured, hence the minus sign in Eq. (1) applies.