a=F/m=dv/dt
dt/m=dv/F
∫dt/m=∫dv/F
t=∫dt
=m∫dv/F
=m∫dv/(mg-Fd)
=m∫dv/(mg-kv)
=-(m/k)∫d(kv-mg)/(kv-mg)
=-(m/k)㏑│kv-mg│
=(m/k)[㏑│kv0-mg│-㏑│k(0.02v0)-mg│]
=(m/k)㏑│(kv0-mg)/(0.02kv0-mg)│
=[70/(1.4×10^4)]㏑│(1.4×10^4×7-70×9.8)/(0.02×1.4×10^4×7-70×9.8)│
≒0.022(秒)