The interaction between a nanopatterned polymer brush and a rigid pyramidal body representing an AFM tip has been investigated using molecular dynamics ( MD) simulation. The computed forces for varying position and penetration depth are systematically contrasted with the density and pressure tensor profiles of the unperturbed brush. For weak penetration of the AFM tip in the brush, we find that the force can quantitatively be computed from the properties of the unperturbed brush after folding with the geometry of the AFM tip. This steric effect leads to a force profile that is significantly wider than the physical brush. The structure of the perturbed brush has also been examined, and we show that for deep penetration of the AFM tip more than half of the force originates from the reorganization of the brush.