Bristle Tips Model for Demonstrating the Direction of the Frictional Force between Surfaces

The bristle tips model is an apparatus designed to help students visualize the frictional force direction acting upon one object by the other object. The apparatus is made from the bristles of scrubbing brushes glued on the acted-upon object. At contact areas between two objects, the direction of the inclined bristle tips corresponds to the direction of the frictional force. This observation can be confirmed by drawing the free body diagram of the system under study.