A frigate bird is soaring in a horizontal circular path. Its bank angle is estimated to be $25^{\circ}$ and it takes $13 \mathrm{~s}$ for the bird to complete one circle. ( $a$ ) How fast is the bird flying? ( $b$ ) What is the radius of the circle? (See "The Amateur Scientist" by Jearl Walker, Scientific American, March 1985, p. $122 .$.)