[填空题]
A bicyclist traveling wmg)rods.cd,;7s ler 1 ith speed v=4.2m/s on a flat road is making al79d:i5ixj5r(kceqsx ja b5jf; 7 f6c turn with ak7qlbc dsacjf5x: r9ix; (57ijej5f6 radius The forces acting on the cyclist and cycle are the normal force $\left(\mathbf{\vec{F}}_{\mathrm{N}}\right)$ and friction force $\left(\mathbf{\vec{F}}_{\mathbf{fr}}\right)$ exerted by the road on the tires, and $m\vec{\mathbf{g}}$ the total weight of the cyclist and cycle (see Fig. 8–56). (a) Explain carefully why the angle $\theta$ the bicycle makes with the vertical (Fig. 8–56) must be given by tan $\tan\theta=F_{\mathrm{fr}}/F_{\mathrm{N}}$ if the cyclist is to maintain balance.(round to the nearest integer) (b) Calculate $\theta$ for the values given. $^{\circ} $ (c) If the coefficient of static friction between tires and road is $\mu_s=0.70$ what is the minimum turning radius? m