[填空题]
A bicyclist traveling with speed v=4.2m/s on a69 u5u53pb 1eba/bu f wuy*sca flat road ik4gn qumr6cxuopdku:wu1.hf3w*6 ) - s making a turn with a radius The forces acting on the cyclist and c-m)63 ww uk:4r c .61p *nufxqdhgouukycle 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