Interesing physics...helpp?

Question

1. A 722kg satellite is in circular orbit 7380km above the surface of earth(ME = 5.98 * 10^24kg). The gravitational force acting on the satellite is?

ANS- 1521N

2. The mass of Neptune is 1.03 * 10^26 Kg. If g= 13.80 N/kg on the surface of Neptune, The radius of neptune is?

ANS- 2.23 * 10^7m

Answer 1

f = GMm/r^2

M = mass of the earth = 5.98x(10^24)kg

m = mass of satellite = 722 kg

G = constant = 6.7 x (10-11) Nm^2/kg^2

r = distance from center of earth to the satellite

r = (radius of earth) + (altitude of satellite)

radius of earth = 6378 km

altitude od satellite = 7380 km

r = 6378 + 7380 = 13758 km = 1.3758x(10^7) meters

F = [6.7x(10-11)][5.98x(10^24)][722] / [1.3758x(10^7)]^2

F = 15282x[(10^-11)(10^24) / 10^14]

F = 1528 N .... my value of G... should have used 6.67 I guess

2. Neptune

Mass = 1.03x(10^26) Kg

g = 13.80 N/kg on the surface

g = GM/r^2

13.8 = [6.7 x (10-11)][1.03x(10^26)] / r^2

r^2 =  [6.7 x (10-11)][1.03x(10^26)] / 13.8

r^2 = 0.50007x(10^15) = 5.0007x(10^14)

r = 2.236x(10^7) meters