If anybody is good wit Kepler's Law or Gravity problems can somebody message me... I seriously need help...

post here maybe...

A planet is found that has a moon with an orbit of 20.3x107 m. The moon's period is 20.3 hours. What is the mass of the planet?

Our solar system rotates around the center of the galaxy at a distance of about 30,000 light years (1 light year = 9.5x1015 m). It takes about 207.9 million years for the solar system to make one rotation around the center of the galaxy. If all of the stars in the Galaxy have the same mass as our Sun (2x1030 kg), how many stars are there in our Galaxy?
Don't use units.
(Our sun is actually kind of small, but this gives you a good estimate.)

During a total solar eclipse, the sun, the moon and the Earth form a straight line with the moon in the middle. Calculate the magnitude of the net force on the Earth at this moment.

What would a 69.0 kg object weigh at a height of 214.2 km above the Earth's surface?

refer to :
http://forums.d2jsp.org/topic.php?t=67521922&f=257

M = 4.pi².a^3 / (G.T²)

M = sum of masses ( = Mass of planet, since usually mass of moon << mass of planet)
a = 20.3.10^7 m (circular orbit)
G = 6.67.10^(-11) (standard units)
T = 20.3 hr = 1753920 sec



Same formulae, divide the result by mass of Sun to find out the number of stars (I found 1.6.10^11 stars = 160 billions).

Back later on this topic, hope it helps so far...