7.68) You have been hired as an expert witness in a court case involvingan autom

Question

7.68)
You have been hired as an expert witness in a court case involvingan automobile accident. The accident involved car A of mass 1900 kgwhich crashed into stationary car B of mass 1100 kg. The driver ofcar A applied his brakes 15 m before he crashed into car B. Afterthe collision, car A slid 18 m while car B slid 30 m. Thecoefficient of kinetic friction between the locked wheels and theroad was measured to be 0.60. Show that the driver of car A wasexceeding the 55-mph (90 km/h) speed limit before applying thebrakes.

Explanation / Answer

According to law of conservation of momentum m1 * u1 + m2 * u2 = m1 * v1 + m2 * v2 u1 = (2k* S)^(1/2),u2 = 0 and v2 =(2k* S1)^(1/2) k= 0.60,S = (15 + 18) m = 33 m and S1 = 30m or v1 = [m1 * u1 + m2 * u2 - m2 * v2/m1] m1 = 1900 kg and m2 = 1100 kg

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