You fly a propeller$-$driven model airplane on a $5\mathrm{.}0$m string in a horizontal circle. The $0\mathrm{.}50$kg airplane flies level and at a constant speed, making $1$ revolution every $4$s$.$ a$.$ What is the speed of the plane? $(7\mathrm{.}85$ m$/$s$)$ b$.$ How hard must you pull on the string to keep the plane flying in a circle? $(6\mathrm{.}15$N$)$ c$.$ What is the frequency of the airplane? $(0\mathrm{.}25$ Hz$)$ d$.$ What is the period of the airplane? $(4$s$)$