Using the equation for Kepler's constant for Earth $K$:
$$K = \frac{GM}{4\pi^2}$$ $$K = 1.01 \times 10^{13} \;\rm m^3/s^2$$

Finding time ($r = 6.4 \times 10^{6} \;\rm m$ (radius of Earth)):

$$K = \frac{r^3}{T^2}$$ $$T = \sqrt{\frac{r^3}{K}}$$ $$T = 5095 \;\rm s = 85\; minutes$$

So 85 minutes is approximately the minimum time for a satellite to orbit the Earth. Why can't it just go faster to take less time? Would that mean it would fall out of orbit?