This result can be understood by noting that the velocity at
the point of contact for this eigenvector is (vx1c)5(2c
1c)50. The point of contact is stationary with respect to the
surface, and therefore there is no exchange of energy between
the translation motion parallel to the surface and therotational motion. This result holds for all values of the tangential
restitution coefficient. The second eigenvalue is l2
52b, and the corresponding eigenvector is