How Do You Calculate Gravitational Force Between Earth and an Asteroid?

[lusterjester]

Homework Statement

We just have learned about Gravitational Attraction. The formula (for those who don't know is Fg = G (m1m2/s Squared)
M meaning Mass
G meaning universital gravity
S meaning distance between

I had to research the mass/radius of the Earth. I was given the mass/radius of an unknown asteroid.

Earth mass = 6.0 * 10 to the 24th Kg
Earth Radius = 6.3 * 10 to the 3rd Km

Unknown Asteroid mass = 7.18 * 10 to the 7th
Radius = 5.00 x 10 to the -1.

We were given this chart to fill out that had the fallowing Catagories within it (i'll give a sample of the first row to give you all an idea).
----------------------------------------------------
Asteroid's Distance from the Earth's Surface
1.00 x 10 to the 3rd

Displacment between their centers (km)

Force of gravitational Attraction (N)
--------------------------------------------------------

Homework Equations

G = 6.667 x 10 to the -11
S squared = Square root of ( G(m1m2)/Fg)

The Attempt at a Solution

Now I know my information looks like this (at least I think it does)

m1 = 7.18 * 10 to the 7th
m2 = 6.0 * 10 to the 24th Kg
s squared = ?
G = 6.667 8 10 to the -11
Fg = ?

I've been totally lost ever since. I know I can't figure something out without having S or Fg all ready found. Any one have an answer for me?

 

[Andrew Mason]

Draw a picture.

In Newton's formula, S is the distance between the centres of mass of the two masses. You are given the distance between the surfaces. What is the distance between their centres? Just plug that into the formula.

AM