In class we discussed a force exerted on a soft ball against a rigid surface. We were told that the strain (of the order: we are doing dimensional analysis) h/a where h is the height difference from the undeformed ball, and a is the radius of the circle intersection of the ball with the surface. However, on wikipedia it says that strain is the "measure of how much a given displacement differs locally from a rigid-body displacement." Wouldn't this be h/R (Radius of the undeformed ball.) Can someone give a more clear definition of strain? Thank you in advance.