Answer
(a) $140~m/s=310~mi/h$
(b) $0.70~m$
Work Step by Step
See conversion factors (inside front cover).
$1~mi=1609~m$ and $1~h=3600~s$
(a) $140~m/s=(1.4\times10^{2}~m/s)(\frac{1~mi}{1609~m})(\frac{3600~s}{1~h})=3.1\times10^{2}~m/s=310~mi/h$.
Notice that $140~m/s=1.40\times10^{2}~m/s=1.4\times10^{2}~m/s$. We considered $140~m/s=1.4\times10^{2}~m/s$ (two significant figures) because we are not sure whether the zero is a significant figure or not.
(b) $1~ms=10^{-3}~s$
$distance=(speed)(time)=(140~m/s)(5.0\times10^{-3}~s)=0.70~m$
