What must be the minimum coefficient of friction `mu` between the tyres of the drive wheels of a car and the road, if the car and the road, if the car of mass `m=2` tons and load `M=4` tons, has an acceleration `a=0.2 ms^(-2)` ? Consider the problem for two cases : (a) all the wheels are driven , (b) only the rear wheels are driven. Assume that the centre of gravity of the car lies at the mid-point between the axes of the wheel and the centre of gravity of the load above the rear axle.