Q.
Ten small planes are flying at a speed of 150kmph in total darkness in an air space that is 20×20×1.5km3 in volume. You are in one of the planes, flying at random within this space, with no way of knowing where the other planes are. On an average, about 25×N hours time will elapse between collision of a plane with your plane. Assume a safety region as a sphere of radius 10 m around every plane. Find N.
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answer is 9.
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Detailed Solution
Using kinetic theory of ideal gases,Mean collision time = t=mean free pathaverage velocity=λv=12πd2nv Where, n = number density of planes and d = safe-region diameter=2R=2(10)m=20m=20×10-3kmHere, n=number of planesvolume=1020×20×1.5km3=0.0167 km−3⇒t=12×3.14×202×10−60.0167×150=225 h .
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