Answer
$7.49\times 10^3 \ m/s$
Work Step by Step
Given:
The height above the earth surface $h=640\ km$
Time period $t=98\ min = 98\times 60\ s = 5880\ s$
The radius of the earth is $R_e=6370\ km$
Mean radius $r = R_e+h =6370\ km + 640\ km = 7010\ km = 7010\times 10^3\ m$
The speed of earth is $v = \frac{2\pi r}{t}$. Using this formula to find $v$:
$v = \frac{2\pi (7010\times 10^3\ m)}{5880\ s}$
$v=7.49\times 10^3 \ m/s$
