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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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.