Using the equation:

(Kepler's constant for earth):
K = GM/4π^2
K = 1.01 x 10^13

Finding time (r = 6.4 x 10^6 m (radius of earth)):

C = r^3/T^2
T = sqrt(r^3/C)
T = 5095 s
T = 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?

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?