Gravitational field strength problem- help needed

[schroedinger4]

gravitational field strength problem-urgent help needed

Homework Statement

Moon has a radius of 1740km, Mass of 7.35 x10^ 22kg, grav field strength of moon= 1.62Nkg^-1, distance from Earth to Moon is 380000km G=6.67x10^-11Nm^2kg^-2

The Moon's gravitational pull on the Earth causes the ocean tides. Show that the gravitational pull of the Moon on the Earth's oceans is approximately 3 millionths of the gravitational pull of the Earth on its oceans.

Homework Equations

g = GM / r^2 g = gs R^2 / r^2 where R= radius of planet, r = distance

The Attempt at a Solution

g = gs R^2 / r^2
1.62 = gs 1740x10^3 / 380000 x 10^3
(1.62 x 380000x10^3) / 1740 x 10^3 = gs

1/3000000= 3 x10^-6
3 x10^-6 x 9.81 = gs ?
Please help!

 

[BruceW]

I don't think you've done it quite right, but you're not far off. You equations are a little confusing also.

I would first try to calculate the gravitational field at Earth due to the mass of the moon (as an equation). And then calculate the gravitational field at Earth's surface due to the mass of the Earth (as another equation). Then divide one by the other to get the ratio.