[填空题]
A bicyclist traveling wit sfj9g06+a7 , exjfpt:upt,j eh speed v=4.2m/s on a e;m4 ok; uvc14ce(iiy flat road is making a turn with a radius The forces acting on the cyclist and cycle are the normal forcecc4uk;;oi1(y ei mve4 $\left(\mathbf{\vec{F}}_{\mathrm{N}}\right)$ 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