[填空题]
Francesca, who likes physics experiments, dangles her watch froc vn7w hvkx ey1u7:1z+gmfay i v1)f5i7t(vz5m a thin piece of string while the jetliner she is(5vfa) 7fmiizt g v15y in takes off from JFK Airport (Fig. 4-58). She notices that the string makes an 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.
