Experiments were performed to reveal how humans acquire information about the shape and mechanical properties of surfaces through touch and how this information affects the execution of trajectories over the surface. Subjects were instructed to make reaching movements between points lying on the boundary of a virtual planar disk object of varying stiffness. It was found that subjects' trajectory adaptation was dependent on the stiffness of the object. When the virtual boundary exceeded a threshold stiffness, subjects adapted by learning to produce a smooth trajectory on the object boundary, while at lower stiffness they adapted by recovering their original kinematic pattern of movement in free space. This adaptation suggests the internal representation of two distinct categories through a continuum of force fields: force perturbations and object boundaries. In the first case, the interaction forces are opposed and the trajectory is restored. In the second case, the trajectory is modified so as to reduce the interaction forces.