You have been called to testify as an expert witness in a trial involving a head

Question

You have been called to testify as an expert witness in a trial involving a head-on collision. Car A weighs 1515 lb and was traveling eastward. Car B weighs 1125 lb and was traveling westward at 43.0 mph. The cars locked bumpers and slid eastward with their wheels locked for 21.5 ft before stopping. You have measured the coefficient of kinetic friction between the tires and the pavement to be 0.750. How fast (in miles per hour) was car A traveling just before the collision? (This problem uses English units because they would be used in a U.S. legal proceeding.)

Explanation / Answer

1/2 (Me + Mw) V^2 = u (Me + Mw) g d     energy after collision

V^2 = 2 * .75 * 32 * 18.5

V = 29.8 ft/sec     speed of cars after collision

Me Ve - Mw Vw = (Me + Mw) V        conservation of momentum

Ve = [Me + Mw) V + Mw Vw] / Me

Ve = [(1515 + 1125) 29.8 + 1125 * 63.1] / 1515

since 43 mph = 63.1 ft/sec

Ve = 99 ft/sec = 67.4 mph

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