Abstract
Our students like using the covers of their TI graphing calculators in an inquiry-based extension of a traditional exercise1 that challenges their preconceived ideas about friction. Biology major Fiona McGraw (Fig. 1) is obviously excited about the large coefficient of static friction (μs = 1.3) for the four little rubber feet on her TI calculator cover. The angle of repose of 53° is three times the value she just obtained for a wood block on the same plane. That block fits nicely inside her calculator cover. By using the string already attached to the block, the cover with the block inside can be pulled along the board at a constant speed to obtain the coefficient of kinetic friction μk for rubber on wood (Fig. 2). Results vary with the age and condition of the rubber feet. Observations with only three rubber feet on some covers help to reinforce the idea that the friction force is independent of the area of the surface in contact. (Students first study the effect of area by working with the wood block on its wide side and then on its narrow side.)
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