Q.A 747 Boeing plane is landing at a speed of $70\ \text{m s}^{-1}$. Before touching the ground, the wheels of the plane are not rotating. Taking the wheel to be a uniform disc of mass 100 kg, radius 0.7 m, and the coefficient of friction with the ground to be 0.5, how long a skid mark do the wheels leave before they begin pure rolling? [Note: the source text for this problem appears to have lost a data point during extraction (its printed answer, 2.1 m, is not reproducible from the given numbers under any standard rolling-friction model) so it is held rather than answered with an unverifiable derivation.]