When a heavy, bulky object such as a chest must be moved across a roughly level floor, placing "rollers" between the floor and the object makes the task much easier. As the diagram shows, several rollers—A, B, C…—are needed. As the chest moves in the direction of the arrow, roller C slips out behind it: someone must retrieve it and place it in front of the chest, near D.
Rollers that do not roll
Rollers reduce the effort needed to move the chest, but they have a drawback: they roll very easily if the floor is sloping… Could we devise rollers that would move a chest with minimal effort yet would not roll away by themselves on a slope? The next figure suggests a "roller with a triangular cross-section" that certainly will not roll away on its own. The trouble is that if a chest is placed on point C and pushed, either it must slide over point C (making the task just as hard as if the chest were resting directly on the floor), or the "roller" must tip by pivoting about B. But this is very difficult, because the chest must "climb" from C to C'.
In fact, it does not "roll" at all! An equilateral triangle therefore cannot serve as a roller. But nothing prevents us from trying to improve it.
