Halliday, Resnick & Walker — Motion Along a Straight Line: Problem 109

Ship speed in knots. Before modern instrumentation, a ship’s speed was measured with a line that had small knots tied along its length, separated by 47 feet 3 inches. The line was attached by three cords to a wood plate (a clip log) in the shape of a pie slice as shown in Fig. 2.40. One sailor threw the plate overboard and then allowed the force of the water against the plate to pull the line off a reel and through his hand so that he could detect the periodic passage of knots. Another sailor inverted a sandglass so that sand flowed from its upper chamber into the lower chamber in 28 s. During that interval the first sailor counted the number of knots passing through his hand. The result was the ship’s speed in knots (abbreviated as kn). If 17 knots passed, what was the ship’s speed in (a) knots, (b) miles per hour, and (c) kilometers per hour?