Sliding Friction Provides Cues to Judge Object Compliance: Evidence Through Comparison Among Sliding, Rolling, and Pressing Exploration by Using Shaft Bearing

Pressing motions have traditionally been regarded as the most effective method for discerning object compliance. However, recent studies suggest that humans adopt sliding motions just as frequently as pressing motions for this purpose. Sliding exploration inevitably induces friction, which is in part determined by material softness. This study demonstrates that friction provides crucial cues for judging softness. To investigate the role of sliding friction in compliance judgment, this study compares three tactile exploration methods-sliding, rolling, and pressing-using a shaft ball bearing. Participants assessed the softness of seven rubber materials through the bearing under controlled conditions. In the sliding mode, the outer ring of the bearing slid over the materials, generating significant friction. In the rolling mode, the inner ring was held stationary while the outer ring rolled over the material surfaces, minimizing friction. In the pressing mode, participants applied only vertical force using the bearing, with sliding and rolling motions prohibited. Consistent force application was ensured using a balance scale, and participants ranked the materials by perceived compliance for each method. Results indicate that sliding provides the most reliable cues for judging physical compliance, followed by pressing, while rolling produces lower accuracy. These findings highlight the instrumental role of friction in accurately perceiving compliance during tactile exnloration.