解：?$(1)v=\frac {s_1}{t_1}=\frac {{ 15 }\ \mathrm {m}}{{ 0.75 }\ \mathrm {s}}={20 }\mathrm {m/s}$?
?$ (2)s_2=vt_2={ 20 }\mathrm {m/s}×{ 4×0.75 }\ \mathrm {s}={ 60 }\ \mathrm {m}$?
?$ 60\ \mathrm {m}+30\ \mathrm {m}=90\ \mathrm {m}>80\ \mathrm {m}$?，所以會(huì)撞上
