[填空题]
Francesca, who likes 4fmb/ mv*:uykpp;a4fphy4t6lk/.znh 7 crr8k )yyp sz:ksics experiments, dangles her watch from a thin piece of string while the jetliner she is in takes off from JFK Airport (Fig. 4-58). She notices that the string makes zl y7/k6)py 4.ktsz: rk 8hnrcan angle of 25$^{\circ}$ with respect to the vertical as the aircraft accelerates for takeoff, which takes about 18 s . Estimate the takeoff speed of the aircraft.
